Front and rear tire choice and weight distribution [Archive]

Claude Rouelle

11-04-2015, 09:51 AM

I realized today that I had dozens of PM in my Inbox. I never looked at this Inbox. Sorry guys I will not answer those. Simply too busy. Just post your question on the forum for the benefit of the whole community.

One caught my attention tough: it was about the front and rear tire choice and the weight distribution, and a statement I made during recent OptimumG seminars and in informal conversation at the FSAE / FS competitions.

If I read in a design specification sheet that the students have the same front and rear tire (I am fine with that) but the weight distribution is 42 % front I will probably not read the rest of the spec. sheet with a lot of attention The car is already hill born and the only thing they could do to have a decent handling will be to create a patch on patch.

There are clear vehicle dynamics principles that show that your front and rear tire cornering stiffness ratio should not be far away, just a few %, from your weight distribution. And that your front and rear roll stiffness distribution ratio should be very close to your suspended weight distribution. If you don't then you must have tons of compliance and you won't get good performance. You could maybe get a balanced car with a patch on patch but you won't get a lot of grip. Or you would get a lot o potential grip but with a car very difficult to drive.

There is a reason why even a cheap car like the Smart do not have the same front and rear tires.

Food for thoughts.

Tim.Wright

11-04-2015, 10:48 AM

If I read in a design specification sheet that the students have the same front and rear tire (I am fine with that) but the weight distribution is 42 % front I will probably not read the rest of the spec. sheet with a lot of attention The car is already hill born and the only thing they could do to have a decent handling will be to create a patch on patch.

There are clear vehicle dynamics principles that show that your front and rear tire cornering stiffness ratio should not be far away, just a few %, from your weight distribution...

I'd counter that and say that putting the same tyre front and rear doesn't mean you have the same cornering stiffness front and rear. Something that practically EVERYONE seem to miss is that cornering stiffness is roughly proportional to the vertical load (e.g. your static weight loads!!!).

So if you have 42%F mass distribution and the same tyres front and rear, then (leaving out other K&C effects) your cornering stiffness distribution is going to be more or less 42%F too - save for a for percentage points change due to load sensitivity.

Tim.Wright

11-04-2015, 12:31 PM

Further to this - there is no real reason why roll stiffness or LLTD distributions should be similar to the mass distribution. Its just a rule of thumb and like all rules of thumb that I have ever come across in this field it's wrong.

There are 3 main (first order) effects in this discussion: tyre behaviour (cornering stiffness and load sensitivity), weight distribution and lateral load transfer distribution. Theoretically you can control all of them (especially in FSAE) but in reality there is a much smaller practically acheivable range of these 3 things. It's important that working together - these 3 effects give you the desired reseponse (objectively measured - e.g. using the static margin or similar) in the complete range of operation of the vehicle (linear, transition and saturation/limit ranges).

For example - here is a very common setup for front engined road vehicles (e.g. the not so cheap Audi TT):
1: Equal tyres front and rear for cost reduction
2: Large front mass distribution (60%F) which give a baseline understeer response on the equally sized tyres (due to tyre load sensitivity)
3: Roll distribution significantly more rearward than the mass distribution to reduce the level of understeer in the transition and limit ranges.

The 3 things work together to give a response which has a large static margin in the linear range which then reduces to a lower, but still positive level in the transition and limit range. If you look at the vertical loads on the highly loaded external wheels, you will see that their front:rear bias starts equal to the mass distribution and then moves rearward to be closer to 50% (matching the tyre size distribution) near the limit. If you would have left the LLTD equal to the mass distribution (like the books say) the car would be an understeering pig at the limit.

By the way, this particular rule of thumb regarding the roll stiffness relation to the suspended mass distribution took me approximately 3 years to unlearn.

Claude Rouelle

11-04-2015, 07:19 PM

Tim,

I do agree with you that there are some shortcuts or simplification (especially in the tire linear range – low slip angle) made between on one hand weight distribution and front and rear tire cornering stiffness distribution and on the other hand weight distribution and roll stiffness distribution. The three should be look at together.

I also agree with you that all other things being equal or ignored (aero, camber, compliance for instance) if you move your CG forward you need either to soften your front suspension or stiffen your rear one to keep the initial balance.

However, let me know if you have similar perspectives or not…

1. Do you agree that the ideal car performance occurs when each tire is used at the maximum available grip (if we only think lateral grip that means peak slip angle) AND we have the yaw moment you want when you want.
The ideal yaw moment you want is zero on the skid pad (or for a fraction of a second at or near the corner apex)
But in transient (corner entry, exit) the amount of ideal yaw moment depends on the variation of the speed and the variation of radius. In simple words too much yaw moment (or yaw acceleration) = oversteer and too little yaw moment (or yaw acceleration) = understeer.

2. If we exclude the effect of camber and pressure and we agree the front/rear tire width ratio (same front and rear compound, let’s not complicate things) is very similar to front/rear tire cornering stiffness ratio….
And if I take your Audi TT example….
And you have the same front and rear tire…..
And we look only at the front and rear outside tires…..
If the car is at the limit (that means each front and rear tire is close to its peak slip angle… that means that the front and rear cornering stiffness is zero…ouch ratio of zero Vs zero… how do we go about that? A bit of academic here…)
And you manage, despite the front weight bias distribution, and thanks to your TLLTD to get the same vertical load…
Well ….same front and rear tire, same (peak) slip angle and same vertical load … that means same front and rear lateral force… (let's exclude the cosine of the steering angle)
But a and b are different so the yaw moment around the CG is not zero. If you are at the limit we should have 1000 Nm of yaw moment one way or the other

What is right or wrong with my perspectives?

Claude

Claude Rouelle

11-04-2015, 07:29 PM

To go in the same direction as Tim first reply... maybe in other words
When we change our weight distribution we change FOUR parameters; your front and rear tire cornering stiffness - at least in the linear range = small slip angle (and I agree with Tim; it follows pretty much the same ratio change as the weight distribution change) but also a (distance front axle to CG) and b (distance CG to rear axle). a and b will change in a much higher percentage tan the cornering stiffness ratio. Not only the tire front an rear lateral grip Fyf and Fy b change but as a and b are the leverage arms of the front and rear grip... that changes the Yaw moment about the CG

JulianH

11-05-2015, 04:23 AM

Menisk

11-05-2015, 05:20 AM

I think the secret was that Claude has no idea if there is or not, but hopes to inspire people to look beyond the solutions everyone else is using.

JulianH

11-05-2015, 05:55 AM

Well of course, that's what we are thinking as well.

But calling his "bluff" two years ago resulted in a "no, no, you just did not look at the right place". So I'll try again ;)

theTTshark

11-05-2015, 07:17 AM

Claude,

I don't want to hijack this thread, but coming from "front and rear tire choice":

A couple of years ago, you made the hypothesis that there is a "tire out there, which noone uses but could be the perfect one for FSAE".
Zurich is searching since the fall of 2012 for a tire to replace the Hoosier and was not able to find one so far. As a team that normally finds stuff, they are still wondering.

Care to reveal your secret?

Yup same here. We searched for awhile and I think the only company we came across that had something suitable that no one is running was American Racer. Many hours were spent on Google trying to find the magic tire. haha

Claude Rouelle

11-05-2015, 07:52 AM

Claude Rouelle

11-05-2015, 07:56 AM

In any case I was told it could be difficult to find any Hoosier tires for FSAE / FS competition next year. I heard the company has been bought by Yokohama and they have merged together to create a new company with a new name....

JulianH

11-05-2015, 08:22 AM

Still.... That tire does exist and to my knowledge (and I attend at least 6 or 7 FS competitions each yea) it has never have been used in any FSAE / FS competition. One team was close to the answer and then gave up a few years ago. Just think about the type of car and the type of race duration that are similar to FS in the racing world. The search is not that difficult. Well it could depends on how many languages you know. Think outside the box. It is there.

Languages in AMZ:
German (including the famous Swiss, Austrian and Liechtenstein dialects)
English
French
Italian
Spanish
Luxembourgish
Croatian
Romanian
Serbian
Greek
Russian
Portugues
Dutch
Afrikaans
Chinese
Japanese

Still, we were not able to find it.

"The team that gave up" could only be Berkeley with the American Racer tire, but this one did not show any performance out there...

So we are back at "nope, there is none".

Silente

11-05-2015, 08:53 AM

hillclimb tires? :)

Anyway, regarding the front weight distribution, i guess one thing which still deserve to be mention is the differential.

In my experience, it could change dramatically how a car handle.

What i have seen, for example, is that big/heavy GT cars with pretty good power and very high CG requires something completely different than, say, a small aero single seater with not too much power.

It turns out that they also use completely different TLLTD to Weight distribution settings, still having different front and rear tires.

Claude Rouelle

11-05-2015, 09:34 AM

Julian H,

Finding the "right" tire.... well....how do you know if it is the "right" or "better" tire if you do not have any information of, for example, measurements of forces and moments tested (lab and ideally also track) in similar test conditions than the "reference" testing conditions you are used to?

You also need to design and tune your car around those specific tires. If you have something like 5 times the critical damping on your rear dampers because that is the only remedy (I mean patch on patch) you found to cope with your rear axle longitudinal compliance and your chain jumping out of the sprocket then you won't get a lot of grip and consistency and driver confidence for that tire whether it is a bad, good or magic tire.

Claude

JulianH

11-05-2015, 09:49 AM

Julian H,

Finding the "right" tire.... well....how do you know if it is the "right" or "better" tire if you do not have any information of, for example, measurements of forces and moments tested (lab and ideally also track) in similar test conditions than the "reference" testing conditions you are used to?

Claude

Luckily, we have the possibility to test all tires that we get our hands on with a tire test truck on real tarmac, not on unrealistic ground like Calspan. Sure on the car it is sometimes difficult if e.g. a tire needs more camber than the Hoosier (which determines the design of the suspension).

So far, there was no tire out there that showed any promisse compared to the known ones (Hoosier, Goodyear (sad to see them go) and Conti).

But if you tell the world which option you see out there, we will of course try them out...

JT A.

11-05-2015, 10:10 AM

Yeah the American Racer tires for midget oval cars were the closest thing I could find when I was digging around. Then Berkeley tried them on their ultra-light car and they apparently had no grip. So the search continues...

Claude Rouelle

11-05-2015, 10:18 AM

Yes, you are and your team is lucky Julian . In fact not "lucky" just smart, curious and resourceful. You use you R&I (Resources and Imagination)

We have done test on a trailer too as well as with WFT (Wheel Force Transducers) on a vehicle. These testing methods have their limitation too.

No simulation or simulator are perfect but some are useful.

At the end it depends more of the experience and skills of the user / operator / test program definition and data analyst. I think the important skills of such person(s) are non emotional objectivity, open mind and curiosity, more than just education.

Understand the sensor and making sure you have reliability is important too.. We have a rule of minimum 7 ideally 10 test sessions of the same tire in the same conditions to have enough data base to build useful, applicable, reliable trends and conclusions that will allow you to use that tire.

Unfortunately and ideally you need a lot of tires to do that job: you cannot restart a test with a tire that has a different temperature and went through a different number and type of heat cycles.

Claude Rouelle

11-05-2015, 10:41 AM

Silente,

You have 12 +1 causes for the yaw moment: 4 tires Fy, 4 tires Fx, 4 tires Mz and the aerodynamic Mz Vs Beta (and if you are picky the roll and steering angle) of the car. In some conditions (part of the corner, brake and throttle application) the tire Fx plays a big role in the Grip, Balance, Stability and Control. That is what ABS, Traction control and mainly ESP is about.

You do not need to have an heavy car to make the tuning of the differential an important part of the car performance. The reality is that most FS/ FSAE teams (much lighter car than a GT) do not have a differential model, do not take the time to test different differential settings

***

As far as weight distribution and roll stiffness distribution of, for example, an LMP2 (I think you play in that area) you will have about something like 46 % weight front and 70 % antiroll stiffness front (K&C or 7 post rig measurements, not Excel spreadsheet). There are are reasons for that... The tires, the aero, the compliance and the transient response have to be taken into account. An LMP2 is rigid - surely compared to a FSAE - but "more rigid" front than rear: that is why a small change of FARB stiffness will have more effect than the same % change of RARB stiffness: a stiff spring on a soft spring is still a soft spring.

maxay1

11-05-2015, 05:24 PM

"2. If we exclude the effect of camber and pressure...." ???????

And what are students to do if they are unable to procure this mythical tire that only Claude knows about? Will they have to actually 'engineer' their car at this point? There are probably a couple of racing series in which the tire is specified, and the front to rear mass
distribution is not suited to the cornering stiffness of the tire. And certainly most mass-produced vehicles available commercially have the same tire front to rear, are we to believe that only the designers of the SMART car have the requisite knowledge to
properly design a vehicle?
Some foods for thought.....

Claude Rouelle

11-05-2015, 06:06 PM

MaxAy1,

It is all about perspective and the way the reader(s) want to interpret my posts .....or the productive debate (or controvesry) that we want to create.

1. The "no camber no pressure" is a simplification of the question to Tim. No need to remind me or the audience of the importance of these factors

2. Of course you need to "cope" with the tire(s) you have and sometimes with the weight distribution you have. In racing and in passenger car. A Nascar weight distribution will change close to 1.5 % from full to empty fuel tank. Same (but lesser) issue with a Porsche GT3 wit their front fuel tank (compared to a central fuel tank of a Ferrari 458 for example). On a passenger car .....with or without passengers and luggage? And yet, some engineers and students will do a better job than others. What is your point?

3 I never said that the Smart was a smart car. Just pointing that there is some limitation in Tim approach; that car was initially designed with the same tires front and rear and it made sense on the economic point of view. The "Smart" guys tried everything to get that car work; kinematics, suspension stiffness and damping....you name it. Until they put larger rear tire they could not find a good and even a safe balance. GM went through the same issue on a other car a bit more than a decade ago.

4. I am not in favor of students getting performance with "just" a better tire. FSAE / FS is an engineering competition not an apply the recipe challenge. I am in favor of students finding a better tire, AND knowing WHY it is a better tire and AND designing a better car around that tire. And when possible working the other way around by designing also a tire around the car. That is what Delft is doing and doing pretty well.

By the way it is not about the Max Ay, neither the Max Ax. It is about having the best possible acceleration in the appropriated combined X, Y, Z direction at any time. It is called the Optimum Acceleration: the OptimumG.
I could not help! :)

Claude

BillCobb

11-05-2015, 06:34 PM

Further to this - there is no real reason why roll stiffness or LLTD distributions should be similar to the mass distribution. Its just a rule of thumb and like all rules of thumb that I have ever come across in this field it's wrong....

---

For example - here is a very common setup for front engined road vehicles (e.g. the not so cheap Audi TT):
1: Equal tyres front and rear for cost reduction
2: Large front mass distribution (60%F) which give a baseline understeer response on the equally sized tyres (due to tyre load sensitivity)
3: Roll distribution significantly more rearward than the mass distribution to reduce the level of understeer in the transition and limit ranges...

Audi TT 2wd: Equal tires front to rear, except front pressure is 36 and rear is 33. Not sure of the wheel rim width and not sure of the spare tire (5th tire or high pressure compact).
Front roll steer is 2% understeer (.02 deg/deg), rear roll steer is 12% understeer.
Front roll stiffness is 1070 Nm/deg, rear is 675,
Front roll center (force based) is 8 mm, rear is 150 mm.

BRI Potemza RE040 P225/45R17 at 39 hot has cornering coefficient of .335, rear at 36 hot is .365. (YOU convert to cornering stiffness).

AVG WF = 466 kg, rear = 315 kg (that's PER WHEEL).

This is all at 2 passenger + full gas tank loads. Now someone besides me do the maths so I don't loose you...

Believe me, the TT has a lot of more understeering stuff from kinematic and elastic items, but that's not the current topic.

craigorydean

11-05-2015, 07:14 PM

~As a response to Claude on this topic I have always question why we don't see cars with 15 in wheels. Yes I know there seem to be some fairly clear reason, most of all everyone's worst nightmare un-sprung weight. If we actually do some creative things, how much more un-sprung mass would it be. I haven't looked in a while but I don't think any 8in or 15in tires have been tested in the TTC. Hoosier does make both of these sizes with some different compoundsand they seem to be fairly easy to get for TTC purposes.

~I will agree to one point that gets made on here by some of the older guys that contribute quite a bit and that is that most FSAE, not all, but most FSAE teams have a heavy tradition of coping/ carrying over designs for no reason but the old adage of well it has worked for us and everyone else so its good to go. Even I am guilty of this 100000 times over. But I think what it would take is a really good car that does something very different and do really well. Maybe CAL-Berkeley has started something like that buy having the lightest car,( I saw the car at comp and it was sad to see them have a DNF due to the chain falling off) or GFR still dominating with direct actuated dampers. Texas A&M had a really good car going in the so called "Wrong Direction" by going to a 4cyl on 13in wheels. Most of us use lost of opinions rather than pure fact to make points here cause we really don't have DATA from other competitors cars just what we see as general knowledge that we use to connect the dots of other cars. One thing I do know is that the stopwatch doesn't lie only the person using it. So maybe its time for us to go back to the trial and error instead of trying to hone in for the one bulls eye kill shot.

~Next is maybe this needs to be looked at harder but at first glance I feel the FSAE-M tracks are tighter than the FSAE-L. Yes I know this is mostly to do with the amount of space available but also having been to each one that also the surfaces are much different. This leds me to question how we really design out cars for FSAE competition. I know the really well prepared/designed car would fare well in any situation cause they would be able to be tuned for any given circumstance. But it seems the courses in Lincoln are just better suited for speed rather than the courses at Michigan which seem to be for the agile cars.

Claude Rouelle

11-05-2015, 08:29 PM

Craig,

- 15" wheel? in FS / FSAE ? Not necessarily a bad idea.... Pirelli made a 15" for Darmstadt university few years ago...But about mass and rotating inertia? What about the influence of the yaw inertia?

- Yep a lot of monkey see, monkey do

- "Wrong" design direction. A part of the decision in FS / FSAE, including from judges, is emotional. It shouldn't but it is. We are human being. That being said in the case you mentioned simulation and data has proven that low weight and high aero downforce are more important than power in FS/ FSAE.

- Design and tune a car per circuit? You mean different input will need different transfer function is the output is the same: minimal lap time and best consistency and drive-ability? Ins't that what engineering is about?

Claude Rouelle

11-05-2015, 08:40 PM

Impressive data base Bill!!!! You did not mention the driver age though ....

Tim,

Based on Bill's numbers

Weight distribution = 59.6 % Antiroll stiffness distribution = 61.3 % (all other things being equal - especially front and rear tire being the same) that is towards understeer in simplified analysis

So these 2 numbers are not too far away...

Comments?

Sincere question, no trap because honestly I am not 100 % sure of the answer.

Claude

Swiftus

11-05-2015, 08:56 PM

I would be interested in seeing if someone could find a 19" diameter tire for a 15" rim. That would be a R15 185/30 - 195/25 - 205/25 for 7" to 8" wide tires. If you Z it up in the rear weight idea, the widest 15s I found for a 15" wheel were 325/50. To make that 19" in diameter, that tire would need to be a R15 435/10 Slam that sweet ride! :D

The Hoosier type tires are pretty darn light and going up to a bigger wheel and wider tire is going to add quite a bit of unsprung and rotating mass... But! if you could get a tire that wide made from rubber like Violet Beauregarde's chewing gum, man that would be fun to drive!

Claude Rouelle

11-06-2015, 07:53 AM

Tim,

Yep me again...sorry

So just to understand what you say you ideally want with a numerical example...

Let's say your LF and RF static corner weight are both 465 kg and your LR and RR static corner weights are both 315 kg. That gives you a weight distribution of 59.6 % front

Let's say also that your weight transfer is 150 kg (1500 N) front and 300 Kg (3000 N) rear. That gives you a TLLTD of 1500 / (1500 + 3000) = 33.3 %. Really rear biased.

That gives you dynamic load of (left hand corner)
LF = 356 - 150 = 315
RF = 365 + 150 = 615
LR = 315 - 300 = 15
RR = 315 + 300 = 615

I have ignored compliance, bump steer roll steer, camber change, Ackermann and the fact that tire are in fact different because of the pressure just for the sake of the example.

Now you RF is 615 an your RR is also 615 so your outside tire front / total vertical load distribution is 1000 * 6150 N / (6150 N + 6150 N) = 50 % If I understood you well that is what you try to achieve, correct? Or is there something I did not get?

With outside wheel vertical loads equal (6150 N RF and RR) and still 3150 N of vertical load on the inside front and only 150 N on the inside rear that will be a big time oversteering car.....

Your perspective will be well appreciated.

Claude

BillCobb

11-06-2015, 01:43 PM

More food for thoughts:

Porsche Cayman:
wf (avg per wheel again) = 352 kg, wr= 438
front roll steer = .14 deg/deg, rear = .02
front roll stiffness (Nm/deg) = 1314, rear = 1293
front roll center (FY based) = 22 mm, rear =88
fr= Conti P235/35ZR19 32 psi, rear = P265/35 ZR19 37 psi

Porsche Cayman:
wf= 680, wr =630
ef= .037, er = .04
kf = 1957, kr =1606
hf=58, hr = 180
P255/55R18 BRI ER30 Turanza 38 psi, rear = P255/55R18 42 psi.

Porsche 911 Turbo:
wf= 335, wr = 522
ef = .037, er =0.0
kf = 1027, kr =1575
hf = 84, hr = 108
225/40ZR18 PIR PZERO 36 psi, 295/30ZR18 PIR PZERO 44 psi.

Every Porch (Proof only rich suckers have everything, and VW/AUDIO (Virtual Warranties ...) has different chassis recipies. Now
Honda (Historically only never drove automobiles) has pretty much the same chassis parameters on all their FWD sleds, except one year when they flipped the roll axis inclination angle. Sup wit dat ?

I believe I know what the tire magic can/could be. I could try it on my golf cart. Its not a specific tire, just an alternate configuration that works with any good tire. It could free up a very high %rear weight distribution constraint. Just like my golf carts (both of them).

11-06-2015, 09:05 PM

Wow, three pages in one day!

Claude,

If I read in a design specification sheet that the students have the same front and rear tire ... but the weight distribution is 42 % front I will probably not read the rest of the spec. sheet...
The car is already hill born and the only thing they could do to have a decent handling will be to create a patch on patch.
... you won't get a lot of grip ... you would get ... a car very difficult to drive.

No.

I reckon a statistical survey of all circuit racecars of last 50+ years would have the vast majority of winners as follows.

* Front-drive cars lift their inside-REAR wheel during hard cornering.

* Rear-drive cars lift their inside-FRONT wheel during hard cornering.

For obvious reasons the FWD cars are front-heavy, and RWD cars are mostly rear-heavy. (Teams with Front-Engine-Rear-Drive cars put great effort into shifting weight rearwards, but might still only get to ~50:50. Nevertheless, they still lift the inside-FRONT wheel. I don't think I have ever seen a REFD car!).

So the assertion that the "optimum" TLLTD should be similar to weight-distribution is most certainly NOT seen in practice.

The most obvious reason for the above set-ups is the usefulness of being able to "put the power down" when exiting the corner, together with the desirability of having the resultant forward-thrust-vector close to the centreline of the car. You need both driving wheels well planted to do this. Note that a spool-diff with one of its wheels OFF the ground gives a highly offset Fx force, so not good.

Another reason for above set-ups is that the path to "a lot of grip" for the last ~50 years has been, quite simply, to fit the biggest, stickiest, tyres you can find.

For an example of how this works, consider a 33%F:67%R car, with RWD and four of the biggest, stickiest, tyres in the world. This car has;
1. Great acceleration (because 2 x big tyres with ~90+% of car dynamic weight on them),
2. Fantastic braking (because mega-tyre-area, with all 4 tyres ~equally loaded), and,
3. Great cornering grip, which is also well balanced (because you lift the inside-front (!), ie. 1 x big-front-tyre carries 1/3 of total-Fy, and 2 x big-rear-tyres carry 2/3 of total-Fy).

The numerical FWD example you, Claude, gave a few posts up would be similarly well balanced, and could also put power down on exit quite well (though nowhere near as well as a RWD car).

However, IF the car has 4WD, AND it has a system that gives full torque control at each of the four wheels, then it might become preferable to distribute all the loads equally, on a per-area basis of the tyre treads. This is mainly because having unnecessarily large tyres at the lighter end of the car increases total mass, yaw inertia, aero drag, and plain old cost. But not many 4WD fully torque-vectoring cars like that in FSAE (other than Delft, Zurich, and ...?).

So your prejudiced view, emboldened in above quote, does not reflect well on your suitability as a Design Judge. On the other hand, maybe that is what the students should expect!
~o0o~

Julian,

Have you tested any of these Avon 10"s (http://www.avonmotorsport.com/motorsport/crossply/10-crossply-slicks)? Their smallish outer diameter of ~40 cm (16") looks attractive to me. They are also apparently available in multiple compounds, from hillclimb to historic-F1, but I have heard perhaps not very easily available?

And what about something like these Maxxis Razrs (http://www.maxxis.com/catalog/tire-191-109-razr-tt)? I know for sure there are countless almost-never-heard-of brands for this sort of off-road racing, in many sizes suitable for FS. Quad-bikes/ATVs have wheels in 8", 9", 10", 12", and many different widths... It is a big market, and I guess quite easy to get into because there is no red-tape required to approve the tyres for main roads. Quality will vary, but you can only know by trying them.
~o0o~

Bill,

"Porsche Cayman: ... hr = 180". Rear RC-height =180 mm?

Is that right, or a typo? Or are they resurrecting Beetle suspension!? (Well, Beetle was more like 350 mm. Much more fun! :))

Z

BillCobb

11-06-2015, 09:48 PM

Good eyes, Z. The Cayman roll centers measured were hf=22 mm, hr = 88 mm.

The second car is a CAYENNE: hf = 58mm, hr = 180.mm

911s are 84 mm and 108.

My wireless keyboard battery stb'd and I was called away for dinner as it launched.

Thank you for the notification. I wouldn't want a copy cat car to appear as flat as that layout .

I personally am more amused by the levels of roll steer in these performance cars. As you get out in lateral la-la land, this effect disappears and the car is stuck with other traits, usually ones not familiar or friendly to a rich driver and his hot date.

BillCobb

11-06-2015, 09:54 PM

But speaking of 10" Avons, do you remember ... ? and my golf car teaser !

mech5496

11-07-2015, 05:33 AM

Pretty sure ETH has tested Avons when trying their WR attempt. We have also going back and forth quite a few times with Avon during the last 3 years, but have not actually tested yet due to budget constraints.

JulianH

11-07-2015, 10:12 AM

Exactly, we tried out the Avons last year: https://scontent.xx.fbcdn.net/hphotos-frc3/t31.0-8/10694379_10152595309151107_3967418641402872132_o.j pg

We had them on the car and on the tire test truck. They are quite good but if I remember correctly, they are only in the same region as the Hoosiers and have a "life-span" problem. While you can run 3-4 Endurances on a Hoosier R25B, the Avon is done after the first driver or so.
Of course you can play with the suspension to make it less painful for the tire but it did not manage to surpass the Hoosier.

We did not try the Maxxis one's as they do not seem to have a real Slick option and maybe focus more on mud than asphalt. But maybe someone should look into that ;)

11-08-2015, 06:48 PM

Bill,

I believe I know what the tire magic can/could be. I could try it on my golf cart. Its not a specific tire, just an alternate configuration that works with any good tire. It could free up a very high %rear weight distribution constraint. Just like my golf carts (both of them)

Hmmm... "...alternate configuration..." ???

I have been thinking about this for a while now, but had to go back to the Rule Book to make sure. Didn't check all the rules, but this seems to be the only relevant part.

"T2.1 Vehicle Configuration
The vehicle must be open-wheeled and open-cockpit (a formula style body) with four (4) wheels that are not in a straight line."

So four wheels, ... but no limit on the number of tyres fitted?

So make up two special 12" wide rear-wheels, each with a "creative and imaginative" central rib... Then, on each wheel, fit TWO 6" wide tyres, side-by-side!

Or 16" wide rear-wheels, each fitted with two 8" wide tyres. Or 22" wide wheels, each with two of the 11" section-width Maxxis Razrs! (Err, slow down Z. Maybe better leave a gap in the middle for the aero-undertray.)

Of course, this C&I is not all that new...

http://possumjimandelizabeth.com/adjusted_images/pinole_8n_dual.jpg

http://www.winntyres.co.uk/wp-content/uploads/2012-Fendt-724-Fitted-with-trelleborg-Tyres-and-Stocks-Duals-FR.jpg

I even recall a "supercar" going this route not so long ago (10 - 20 years ago?), but couldn't find it with quick googling.

But what would the Anti-C&I Officials say? :D

Z

11-08-2015, 07:19 PM

Further to above, it may be feasible to take two, say, 8" wide tyres, and glue+rivet them together at their sidewalls, then cut off the redundant pair of (now inner) bead-wires, and thus make a single 16" w-i-i-i-de tyre.

This should be just as legal as Delft's bespoke tyres. But maybe with even better performance?

Z

Swiftus

11-08-2015, 09:32 PM

... it may be feasible to take two, say, 8" wide tyres, and glue+rivet them together at their sidewalls, then cut off the redundant pair of (now inner) bead-wires...
Z

Z - I think this would not be allowed by the note in rule T6.4.1 which says:

NOTE: Hand cutting, grooving or modification of the tires by the teams is specifically prohibited.

BillCobb

11-08-2015, 10:26 PM

I remember testing on a car and on a Flat-Trac tire tester, a "Twin Tire" concept that Avon was working to develop. It was two tires mounted on 1 rim. The concept fit the perfect scenario for a multi-tread and multi service tire and wheel configuration. (Search Google for Avon Twin Tire).

For example, you could run a low drag compound slightly larger diameter tire on the inner tire to get the best fuel economy and ride. When you cornered or lay on the brakes or throttle, you would engage the outer tire having higher cornering stiffness and higher mu.

The wheel size was conventional with TWO separate bead retention rails. (See attached photo).

Now, to solve the high percentage rear weight bias and high power needs of your race cars, a clever team could shoe-horn a dual tire concept using either the same or two slightly different tires onto a single wheel. You are essentially doubling the load capacity of the tire and greatly increasing the load transfer sensitivity. (Yes, in the industry, the LT sensitivity means that for every increment of FZ you get a corresponding high output of FY.

The way I can easily show how well this solves the "tighty-loose" FSAE car having a 30% (OR LESS) front weight distribution is to simulate it behavior using a traditional tire model but with a slightly altered input/output function. A reasonable dual tire model for concept investigations sends 1/2 the FZ to a single tire and doubles the FY etc. output(s). Yes there are some camber change implications but that is easily solved by the use of solid (beam) axles of the same architechture as the farm tractor, semi-truck, dually 1-ton pickup and aircraft landing gear configurations.

This notion allows the use of appropriately sized tires on each end of the car that are usually the same exact tire construction with the load fraction these tires carry being the same. Thus it would be entirely possible to direct the %front distribution to ANY reasonable fraction, including duals on the front if you wanted to try that scenario. Note that the dual tires love load on them, so they respond well to extra load transfer and the single tire axle would also be readily tunable with a stab bar or springs that are not cast iron sewer pipe.

As I recall, there have been several F1 cars with more that 4 'wheel's (Ferrari T12T6, Tyrell P34, and also March and Williams cars. But they were on 6 WHEELS, and thus banned (they were apparently pretty good in spite of crappy tires). All you really need to do is have 4 wheels with 6 tires on the car!

This immediately solves the limit traction oversteer 'problems' and GREATLY improves the cars' transient reponse because of the muchly reduced rear cornering compliance. (Oh, OK, we won't talk about it's most important contribution to a high rear weight fraction automobile because the students will forever be bambozzled and probably wind up killing themselves).

Works for me ! I'll bet a couple of motorcycle tires would stick and sidebite pretty well in such a configuration. Yes I know they are camber feeders, but what the H.

BillCobb

11-08-2015, 10:56 PM

I suppose it's worth mentionaing that NASCAR requires the use of "inner liners" on many of its tracks as a safety precaution in the event of a tread sep or blowout. Well, kiddies, the "inner liner" is not a tube or a baloney skin, but a full fledged actual real treadless tire running on its own rim area on a steel wheel. (I happen to have two still in my garage). So, you have 4 wheels and 8 tires running on the car. Sound familiar ???

For the curious and intellectually stimulated participants in this forum, (The rest can doze off now), the ORDER OF FILLING the air chambers in these wheels has a big effect on the properties of the tire (Think molecular counts here). Fill inner one first you get property set #1. Fill outer one first, you get property set #2. Even though the pressures for each can be designated, the filling order allows an almost infinite variation in properties based on how many molecules are stashed in each of the inner and outer tires. (That's because the inner one crowds the outer tire's air chamber if its filled first). And what does the "air" in a tire really 'do' ? Am I getting through here ???

If you are not cheating, you are not winning

Pawel Roszyk

11-09-2015, 03:41 AM

Hi. My name is Pawel Roszyk and I am the member of PUT Motorsport from Poznan, Poland. Sorry for PM, Claude.

Anyway, we are buliding our second car and want to move our CG a little bit to the rear, to improve acceleration. We've started with 50:50 weight distribution last year (with different front-rear tires!) and want to make it around 47.5:52.5.

In order to have enough and comparable data, we're fixed to the TTC tested tires. (We've also checked Avon data, but we don't have any possibility to test tires by ourselves so we've stayed with TTC.) Out of the tested 10" tires it was very difficult to achieve cornering stiffness close to the mass distribution and one of the tires seemed to offer significantly more grip than any other. My conclusions are:

1) It's better to have a patch on a patch with better tires than perfectly neutral car with tires giving you much less grip.
2) We probably won't be winning on a Skid Pad.
3) The car will be slightly understeering (which is not necessarily a crime, when the car will be driven by an amateur driver).
4) That will increase our turn-in potential (when the car is turning and decelerating) which, I believe, is not a terrible idea to improve the performance of an amateur driver. (None of our drivers is Lewis Hamilton.)

Do you think it's acceptable reasoning? I'd be glad to see any feedback from more experienced engineers.

Menisk

11-10-2015, 03:28 AM

I've never really heard this term "patch on patch" before. Can someone please clue me in as to what is meant by it?

Pat Clarke

11-10-2015, 03:41 AM

Like putting a bandaid on a broken arm...and then taking an aspirin!

Pat Clarke

dynatune

11-10-2015, 03:31 PM

Instead of talking of front and rear tire cornering stiffness, I personally think that it is far more useful to talk about front and rear axle cornering stiffness which includes of course the effects of tire cornering stiffness and weight AND on top of that all the effects of suspension kinematics and compliance's. My knight in shining armor is R. T. Bundorf who did find a pretty clever way of describing and weighing the various parameters that at the end of the day define the balance of a vehicle. Of course weight and tires define quite heavily the axle cornering stiffness but all the other parameters Mr. Bundorf identified are most certainly not to be neglected.

Cheers,
dynatune

BillCobb

11-10-2015, 05:31 PM

You are absolutely correct, DT. However you look at it, ignoring front aligning moment steer compliance, front (and rear) lateral force steer compliance, front and rear roll steer and even the rigid body net Mz terms in any form of analysis and/or simulation makes the predicted results completely ridiculous and unproductive. Even in 'racecars' (that term sounds so haute doesn't it ??). These kinematic and compliance terms are substantial, hence fundamentally critical elements of a vehicle static and dynamics recipe. Predicting them via analysis or lab measurements is a straight-forward process. To ignore them is simply ludicrous.

FYI, when I hired into GM in the last Century, Tom Bundorf was my boss. Actually, Ron Leffert and Dick Rasmussen also carried forth the analysis to include their influences and effects on transient response (ya know, like vehicle DYNAMICS). It's THIS subject that connects the human factors part of driving a vehicle to the synthesis of appropriate tire, chassis and steering parameters in ANY vehicle. A co-worker of mine, Richard Topping, wrote several papers on the Chassis Synthesis procedures and order of operations for it. SAE and Tire Association Papers by Bundorf, Rasmussen, Leffert, Pete Riede (and me), et al.are plentiful and show the range and magnitudes of non-tire cornering compliance values. Funny how tire properties are the only ones which have greatly improved over the years (because of radial tires, weight reduction, and driver expectations). The rest seem to have the same distributions.

I wrote a GM Engineering report on how to apply this from a systems engineering perspective (ya know Bode forms, complex transfer function representations, solution of couple differential equations, blah, blah, blah) that might make for interesting reading for those who have survived Freshman Calculus. For the rest, it might be way too troubling and demeaning. It does show, for example, why the kprime term in yaw radius of gyration is so important and why the rear of the car is the place to put your best engineering.

A MMM plot constructed from realistic cornering compliance values will look VERY different than one made up from just weights, wheelbase, speed and tire cornering stiffnesses. Besides, if you could/should ever drive such a car in the real world, you would soon hate it.

But, C'est la vie in University !

11-10-2015, 07:19 PM

Jay (Swiftus),

I think this would not be allowed by the note in rule T6.4.1 which says:
NOTE: Hand cutting, grooving or modification of the tires by the teams is specifically prohibited.

Thinking, thinking..... Got it!

You get Joe, who works down the road at "All-Auto-Mods-R-Us", to do the work. Joe is not part of "the team". :)

However, that Rule does bring into question the whole issue of chemical enhancement of tyres...
~o0o~

Anyway, I am warming to this idea of dualies on the rear-axle.

The practicalities are quite easy. Just make up a special wheel-centre (ie. the "spokes" part) and bolt on two spun aluminium shells, one each side. Two 6" wide tyres per wheel would be a significant jump up on the currently widest readily available ~8"s. Then go a-testing!

I suggest a "Request for Clarification" letter, to be sent to the Rules Committee, be drafted ASAP.
~~~o0o~~~

Pawel,

As I pointed out at bottom page 3, you can have much more rear weight% than you are proposing, with the biggest, stickiest, equal sized tyres you can find fitted at all corners, and you will have a very fast and well-balanced car (... you just have to lift the inside-front-wheel in corners).

Watch the FSAE-Australasia comp in one month (mid-December) for some examples.

Z