Tire/Wheel combo.

[Daves]

What is most important:

1. Lowest unsprung mass?
2. Smallest radius of gyration?
3. Lowest cost?

I actually think most 13" x 6.5" wheel/tire combos. weigh the same or less than their 10" x 6.5" counterparts.

However, using a 13" wheel/tire combo. will yield a higher radius of gyration (if the materials are the same) than a 10" wheel/tire combo. This would raise the effort needed to start or stop the axle (only really important with rear axle).

Finally, 13" wheels cost about 3 times as much as 10" wheels.

It sounds as if many students are satisfied to say, "All the best universities use 13" so let's just go with that." However, if you can get 10" wheels for 33% of the cost and lower your moment of inertia (I hope that's the correct term), then why go with 13" wheels? And not all of the best universities (with respect to FSAE) do use 13" wheels -- N.C. State's 2003 car (one of my favorite cars) uses 10" wheels and tires.

I'm not sure of the cost of 10" tires compared to 13" tires, though. I'd suppose they are less expensive.

So far, I think I would use 13" wheels and tires up front (for bigger brakes) and 10" wheels and tires on the back (for lower moment of inertia & less cost). As far as unsprung mass goes, I don't think it matters much whether 10" or 13" are used.

[gug]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by dave_s:
I actually think most 13" x 6.5" wheel/tire combos. weigh the same or less than their 10" x 6.5" counterparts.
<HR></BLOCKQUOTE>

qweh? that does sound interesting. can anyone explain the reason to me?

on that topic, how hard is it to manufacture your own lightweight wheels? anyone care to give stories/recommendations in particular relating to carbon-fibre ones? wheels seem to be a majorly under-engineered part of our car. people will spend days working towards 100grams off their muffler then ignore the 20kgs of unsprung mass that is the wheel.

13" wheels would give much better feedback to the driver than 10", right? that would have to be one of the reasons so many go with 13".

TIA

- if it isnt coming, you need a bigger tool.

[Frank]

13" easier to package

decision made

http://fsae.com/groupee_common/emoticons/icon_smile.gif

[woollymoof]

13" wheels bigger contact patch, give better feel because of diameter to width ratio.

Cheers,

Kirk Veitch
Swinburne University of Technology

[Daves]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by gug:
qweh? that does sound interesting. can anyone explain the reason to me?

on that topic, how hard is it to manufacture your own lightweight wheels? anyone care to give stories/recommendations in particular relating to carbon-fibre ones? wheels seem to be a majorly under-engineered part of our car. people will spend days working towards 100grams off their muffler then ignore the 20kgs of unsprung mass that is the wheel.

13" wheels would give much better feedback to the driver than 10", right? that would have to be one of the reasons so many go with 13". <HR></BLOCKQUOTE>


The reason a 13" wheel/tire combo is lighter than a 10", in my opinion, is that the wheel is a single piece of metal (or other material), whereas the tire has material on both sides (twice the material). Here is an attempt at a diagram:

|-tire--| <---- tire (double wall)
|_____|
----|----
----|---<-----wheel (single wall)
----|----
----|----
|-------| <-this piece of metal has big influence
|-tire--| on the radius of gyration

Also, I think the density of the aluminum alloy used in the wheels is close to the density of the rubber used in the tire. Keep in mind that these are fairly weak wheels because of their low weight -- we could not even use a normal tire-mounting machine without it bending our wheels.

So by increasing the wheel diameter, the single wall is being increased while the double wall is being decreased.

I'll assume that a 10" wheel/tire combo. weighs the same as a 13" wheel/tire combo. for the following. The pros of larger and smaller diameter wheels:

larger wheels(13"):
1. easier packaging (A-arms, spindle, kingpin)
2. allow larger brakes on front (better stopping)
3. better driver feedback (also depends on springs) and less weight transfer

smaller wheels(10"):
1. can mount muffler below A-arms in back
2. force use of smaller suspension components, saving weight
3. cost 1/3 as much
4. lower rotational inertia
5. smoother ride (also depends on springs)


Sometimes I think 13" is kind of overboard for FSAE because Formula 1 uses 13" wheels, yet an F1 car is much larger than an FSAE car. However, I recently read in Road & Track that F1 cars would be much faster (lap times) if they had 18" wheels.

The best idea would be to test 1 car with the same driver with 10" and then 13" and see which gives faster lap times. Also, a combination of 10" and 13" could be tried. Of course, the car would need to be able to fit 10" or 13" wheels.

[This message was edited by dave_s on July 29, 2003 at 01:45 PM.]

[This message was edited by dave_s on July 29, 2003 at 02:03 PM.]

[Travis Garrison]

Our 10"x6.5" wheel tire combo weighs 12.7 lbs without brakes...they also cost us ~$30 per wheel...

Lets get some weights for 13" combos and prices...enough guessing eh? Or you can look up manufacturers specs...


Travis Garrison
WWU

[dr47watson]

Dave_S,

What issue of Road & Track did you see the bit about F1 and 18" wheels? Was it a statement from somebody in F1?


Thanks,

Rich

[Big Bird]

For anyone who is interested.

Last year RMIT ran 13 inch three-piece rims, locally (Oz) made, 4mm thick 6061, with 6 inch wide Hoosier tyres. Weight of wheel without centre approx 7kg.

This year we have douglas 10 inch rims, 4mm 6061 again, with Dunlop 6 inch wide rubber. Weight of wheel without centre just over 5kg.

Both measurements taken on a dodgy spring balance, so can't give accuracy to the gram. But that gives an indication anyway.

Cheers all,

Geoff Pearson
RMIT FSAE 2003

Design it. Build it. Break it.

[woollymoof]

Dave_s F1 uses 13" wheels because of regulations, not because they think the cars will be faster. The regulations are there to slow the cars down, smaller brakes etc. So, you can't really compare us using 13" wheels to F1 using 13" wheels.

Cheers,

Kirk Veitch
Swinburne University of Technology

[Daves]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>
dr47watson originally posted:
What issue of Road & Track did you see the bit about F1 and 18" wheels? Was it a statement from somebody in F1?
<HR></BLOCKQUOTE>

Rich, July 2003 (With Ferrari Enzo on cover). The article is "Groovin' on a Sunday afternoon."

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>p. 122, by Dennis Simanaitis: What if wheels and tires were formula libre? In his anaylyis of the ultimate F1 car (see 'Racing into the Future,' June 1999), Peter Wright postulates wheels as large as 18-in. diameter, as is the case with current GT cars. Optimal rear tires would be as wide as 20 in., with fronts around 16 in.<HR></BLOCKQUOTE>

When saying that 13" wheels for FSAE were "overboard," I was not saying they are not optimal. However, when considering all the factors (especially cost -- we spent $300 per wheel last year, but they only massed 3 kg (weighed 6.6 lbs) each), 10"� wheels seem much more viable, especially when buying 2 sets of wheels (or more). Any other opinions?

[Daves]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Brian Walby originally stated:
You would want to make sure that you equivalenced out your ride rate and natural frequencies - 2 tires at 2 sizes will probably have 2 different effective tire spring rates. Not to mention trying to compare the lateral stiffness of the tire, what their lateral force vs. slip angle curves look like, operating temperatures...You change your tires and you open up a Pandora's box of issues. <HR></BLOCKQUOTE>

Brian, what I meant was take a set of 13"� wheels and put them on a car that normally accepts 10"� wheels. They should fit readily. Make sure the tire/wheel combos. are the same overall width and overall diameter. Then compare lap times. This would be a fair measurement.

Open up Pandora's box? The whole point of this competition is to build an autocross car. The first thing many people would do (after purchasing your autocross car) would be to change the wheels. You have no idea what size they are going to be, but as long as they keep the same overall diameter and width, your design better accept them. Maybe that is a valid reason for using 13"� wheels and tires -- so that if someone hypothetically bought your car (designed for smaller wheels), they would not screw it up by putting 13"� wheels on.

[Charlie]

That is way too much hypothetical for me. I seriously doubt any racing vehicle should be designed so people can put different size wheels and tires on the if they want. And even the notion that we would delve into a hyptohetical situation that would evolve from an already hypothetical situation (production of these cars) just makes my head hurt. http://fsae.com/groupee_common/emoticons/icon_wink.gif

I think Brian's point is, the 10" and 13" tires both have benefits, but these benefits are inherent in the design. You have to design for one or the other. A direct test is sketchy at best, and IMO would not yield useful results.

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Scott Wordley]

We've used both size wheels (on different car) and as bigbird states at 10" wheel and tyre combo was considerably lighter, both the 10"s and 13" wheels were homemade as well.

We had 2 main problems with the 10s, being packaging (steering particularly but now also unsprung mounts for the wings, a problem most teams don't have) and also the fact that it is becoming increasingly difficult to find a good 10" tyre.

These factors made the decision to switch to 13" pretty easy for us.

Regards,

Scott Wordley

http://www-personal.monash.edu.au/~fsae

[Eric Wort]

We ran 10 inch wheels with Goodyear tires and 13 inch wheels with Hoosier tires back-to-back one day last summer on the 2002 car. The suspension had been tuned for the 10 inch wheel and tire combination. Just bolting on the Hoosiers on gave us .1G more of lateral acceleration on the skidpad vs our best time with the 10's and 2-3 seconds off of our best autocross course time. Additional re-tuning of the suspension(and driver training with the 13's) resulted in approximately .05 more G's along with a second and a half more off of the autocross time.

The 520lb 2002 car was able to pull better skid pad times on the 13's than our "somewhat infamous" 1998-1999 398lb-415lb(depending on who you ask) turbocharged monster of a car fitted with super-soft 10 inch Goodyear prototype tires (which they never made, we might have stuck with 10 inch tires if they had made them).

Eric Wort
UIUC Formula SAE (http://dilbert.cen.uiuc.edu/soc/sae/formula/)

[Engelbert]

Are 10"s better than 13"s ?

To find out (in regards to competition results) I would:

1. Get results for last 10 years of FSAE.
2. Figure out ratio of 10"s run, to 10"s in the top 10 places (dynamic events only).
3. Do same for 13"s.
4. Take away outliers (perhaps top and bottom 10% ?).
5. Compare.

I'm going to do the above to find out for myself. I'm sure you could just have a guess and get a good enough answer, but I wouldnt mind knowing the actual statistics.

Then do you go with the statistics ? Or do you ignore the fact one or the other tyre wins many more events ?

My opinion is:
If you consider your team to have a good chance of winning, then I spose you could disregard the statistics (as you are considered an outlier, an unusual competitor), but if you consider your team to be middle-of-the-road, 95th percentile, then statistics apply to a greater extent...go with the flow. (Unless you have other factors influencing your choice, of course.)

I'll post my results when I could be bothered!

Sam.
2001 UQ FSAE

[Frank "Ruska"]

Brakesystem Packaging is one of our arguments to use 13" wheels.

Frank Röske
Student Racing Team
University of Applied Sciences Stralsund/Germany

www.student-racing.de (http://www.student-racing.de)

[awhittle]

13'S in the front 10's in the rear ???

Just thinking outloud

[Daves]

Why not? A little over half the cost of (4) 13's. The back brake(s) is inboard anyway.

[MikeWaggoner at UW]

The spring rate of the tires is the only deterrent against ten's IMHO. One of our experienced drivers claimed 10's felt a lot squishier, and the sidewall is bigger so it makes sense. Anyone seen any numbers on this?
I think packaging problems are fun.

UW FSAE
The views of Mike Waggoner are not necessarily the views of the UW FSAE team.

[Charlie]

Is it something that can't be compensated for with spring rate? I imagine laterally, it would feel less precise.

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Denny Trimble]

Actually, I'm the one who made that "squishy" comment to Mike, and it was based on driving our '98 car with Goodyear 19.5" diameter, 10"-rim tires. No matter what you did with the springs, the tire spring rate was so soft and relatively unadjustable, that it dominated the behavior of the car. In our bumpy parking lot, there was just too much oscillation!

Napkin Analysis:
19.5" x 10" = 4.75" sidewall height
18 x 10" = 4" sidewall height
20 x 13 = 3.5" sidewall height

I haven't actually tried the Hoosier 18x10's, we switched to 20x13's in '99 and haven't looked back.

I think the advantages of shorter sidewalls (up to a point where the tire is too stiff; we're not there yet) are:
1) Shorter tire lag time (development of lateral force)
2) Stiffer, more damped vertical compliance
3) Smaller slip angles (debatable whether you want this or not)

I also love the other benefits of 13's (suspension geometry, brake packaging), but the weight, rotational inertia, and yaw inertia penalties are huge by our standards.

University of Washington Formula SAE ('98, '99, '03)

[karter]

Has anyone looked at different rim widths? Karts useto use something called a G ring which snaped over the rim and supported the tire sidewall. More recently the trend is to put a 8.1" X 6" tire on a 10" rim. Makes the kart very responsive. I wonder what this would do transition in a slolam???

[Sleeque]

Does anyone know anything about LeCont tyres?
They seem to have tyres suitable for Formula Student, the prices seem okay, with discounts offered, and the company is probably small enough to make special compounds if needed.
Their website is http://www.leconttyres.com

Slan leat
Sleeque

[NJM.]

This is a topic that we at NC State have pondered in detail for some time now. Thanks dave_s for your complement. This year we were the highest ranking team with 10s at 8th place.

We have been running a 10" wheel for 3 years now. We chose to run the 10" based on weight. At that time there was a large weight difference between the two wheel tire combos. We dropped 40 lbs. by going to 10s. Now the difference is much smaller although I believe there is still a small weight advantage for the 10s. Our wheel tire combo weighs approx. 14 lbs. We certainly see some wheel deflection and we are forced to life our wheels to fairly low hours. We had one wheel destroy itself after being punished with 1.8 g spikes on our test track. The cost is better and that makes purchasing multiple sets of wheels nice. Mounting the 10s is one of the worst things I have ever done. We made ourselves a little rig to keep the rim from bending in half while it is being mounted. It is still ridiculous to mount. As for radius of gyration we have never looked at it in depth.

Having looked at some tire data the 10s we run have an advantage in overall traction versus the 13s at the relatively low wheel loads these cars see. Where the 13s seem to have an advantage in feedback at the limit. This makes the 10s harder to control and take to their ultimate traction. We have worked to incorporate suspension geometry that gives the driver more feedback through the steering. The 10s also require a slightly higher slip angle to reach their maximum cornering force. Like Denny said this leads to cornering lag time. This makes a car that doesn't necessarily dive into corners but rotates well in the middle on tight courses.

As for tire heating, important in autocross and skidpad, the 18's should heat up quicker as they spread the energy over less area. Like karter mentioned, we have done a lot of testing with different tires sizes, mated to different size rims and found a few interesting combos that give the tire better response and more even tire temps.

Packaging with a 10" wheel is a real pain but can be done with few or no compromises. In order to get kingpin and scrub values we have to shove our uprights well into our wheels. This becomes a problem at the high wheel angles needed for formula sae courses. In order to achieve these we had to make more complex a-arms and really watch the tie rods for rubbing on the wheels. At lock there is less than a milimeter between rim and tie rod. As for brakes, if you are using adequate brake rotor materials and brake pads for the elevated temps., they are no problem to make work. We continually brake at the limit of the tires with good modulation and no fade.

Looking statistically over the past 5 years there have been 10 schools with 10 in. wheels in the top ten.

RIT 2nd-99
Cal Poly Pomona 3rd-00
Leeds 5th-00
Brown 6th-00, 10th-02
Dartmouth 7th-01
Michigan Ann Arbor 4th-02
UNC Charlotte 5th-02
Purdue 9th-02
NC State 8th-02

I am very sorry if I left anybody out. That is 10 out of a possible 50 spots with the highest finish being a 2nd. I am not sure of the number of 10s versus 13s but I think that less than 1/5 of the teams run 10s. That said, to be in the top 10 it is better to run 10s. So are the teams with 13s good because they run 13s or would they be just as good or better on 10s? I wonder if anyone has successfully answered that question. I am sure they would never tell if they did.

I am fairly confident that we were getting the most out of our tires this year. Our drivers ran as fast as 65 seconds on this years endurance track with a flat left rear tire (due to a rim that leaked once it reached operating temps). We consistently see greater than 1.5 gs sustained cornering on data aq. At 458 lbs. our car was fairly light, had good power and decent suspension. One car that I thought was blindingly fast and were 4 seconds faster than us on the autocross was Missouri-Columbia. They run 13s. Whether there is more to be gained in our suspension or if we are hindered by the tire will hopefully be answered by time competition comes around. \

Noah Mckay
NCSU Formula SAE 2003
www.wolfpackmotorsports.com (http://www.wolfpackmotorsports.com)

[John Gregor]

A good 13 inch wheel/tire combo weighs about 15 lbs.

[Charlie]

Good post Noah. You bring up wheel deflection. Is this something you are willing to have to save a little weight? Surely it affects handling in ways you can't accurately predict.

For example, on turn in, is your car less responsive because of tire deflection or wheel deflection? We have some good photos of tire deflection on our car on turn-in. Is that the highest load spike these cars see?

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[This message was edited by Charlie on August 09, 2003 at 09:49 AM.]

[Charlie]

We don't have a good picture on the web but we have seen significantly more tire deflection than below. Our wheels are heavy compared to the lightest out there but very stiff.

http://www.eng.auburn.edu/organizations/SAE/AUFSAE/2003charlieendurance1.jpg

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Daves]

Thanks for the information, Noah. However, I am confused about one statement you made:

"That said, to be in the top 10 it is better to run 10s."

I am confused because 20% of the top cars have used 10" wheels, and 80% of the top cars have used 13" wheels. Did you mean "to be in the top 10 it is better to run 13's?"

[NJM.]

Dave, sorry for the confusion and my bad wording. I did mean to say what I said however. What I am saying is that 20% of the top 10 teams run 10 inch wheels and I believe (no real statistics on this) that less than 20% of the total teams run 10 inch wheels. So there is an over representation of teams with 10 inch wheels in the top 10. This may be bad use of statistics.

Charlie, the wheel deflection is something that concerned us this year. We are looking at custom wheels in the future to help with this. The turn in response is something that we would be looking to gain. I am not sure how much of a factor it really is because in testing something I would expect to see is a tire that wants lots of camber because camber is reduced due to deflection. Looking at tire temps and wear the tires never wanted excessive camber. These tires really do deflect a lot. I am not sure if it is the construction or the low pressures. We have tried running higher pressures but what you gain in stiffness and response of the tire you more than lose in effective contact patch. Charlie, did you guys find anything in wheel deflection? Is that why you run stiff but heavy wheels?

Noah Mckay
NCSU Formula SAE 2003
www.wolfpackmotorsports.com (http://www.wolfpackmotorsports.com)

[MikeWaggoner at UW]

"We certainly see some wheel deflection and we are forced to life our wheels to fairly low hours. We had one wheel destroy itself after being punished with 1.8 g spikes on our test track."

Are you just using the Al rims without stiffeners? At WWU they use stiffeners, each one weighed very little (don't remember if it was 1 lb or .5 lbs). With FEA the stiffness can be made comparable (or better) to 13's with a long service life.

http://dot.etec.wwu.edu/fsae/v35/Pictures/Wheels/DSC01617.JPG

[Charlie]

We haven't done any extensive testing on wheel stiffness, but we did conciously trade weight for stiffness in 2003, mainly on the wheel halves.

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[NJM.]

Mike,

We do use a wheel stiffener but not nearly as trick or well designed as that on the WWU car. We just have used an aluminum plate. Although failures like this one are making us have a rethink. Maybe we'll take some hints from the WWU car. This crack developed after a day of 1.8g punishment on a test track. It cracked through both rim halves and the stiffener.

Charlie, by your result, I would assume that the weight/ stiffness tradeoff you made this year was well worth it.

http://www4.ncsu.edu:8030/~njmckay/wheel1.jpg

Noah Mckay
NCSU Formula SAE 2003
www.wolfpackmotorsports.com (http://www.wolfpackmotorsports.com)

[Frank]

i'm very curious as to what thickness, and specific alloy that is