scrub radius poll

[bird2]

Hey guys im taking a poll of what your scrub radii are. of the people running an outboard hub, how are you keeping scrub to a minimum?

[Dash]

Less than an inch, and I use suspension geometry for the most part.

[sbrenaman]

~2" on 2010, ~1" on 2011.

Need a lot of positive wheel offset and if you're still having issues, you can use Kingpin Angle to get it to 0" easily. However, I was able to get ~1" with only ~1deg of KPA.

[Fyhr]

We've also run 25mm for a long time, with 0 deg KPA.

A recent video of the tyre under load makes me question the accuracy and effect of our static scrub though: http://www.youtube.com/watch?v=ZnzXk-A7Avw (please ignore the off-tarmac event and focus on tyre deformation and contact patch movement).

[DougMilliken]

A recent video of the tyre under load makes me question the accuracy and effect of our static scrub...
If you have tire force & moment data, you could look at the overturning moment data (Mx) to put some numbers on the tire deflection seen in the video.

[BillCobb]

Finally, method in the madness.

Here's what we are talking about:

http://www.youtube.com/watch?v=nmo_dkNZIHM

Tires can have a large or small positive or negative Mx just going down the road straight. Then load, slip and camber can knock around all the geometric settings. This means you design for it rather that pick a number out of your ... Since these cars are known to attempt high lateral force levels. you ought to factor in this key player. Your driver will appreciate the effort (hey, that's a pun).

[DougMilliken]

Originally posted by BillCobb:
http://www.youtube.com/watch?v=nmo_dkNZIHM
(turn volume down before playing!!)

The TTC DVD includes videos on Calspan TIRF...but with no sound track.

[swong46]

There is a lot of talk about front scrub radius, but what about the rear? does it even matter?

[PatClarke]

Of course it matters!!! Think about it.

Pat

[Fyhr]

Originally posted by PatClarke:
Of course it matters!!! Think about it.

Pat

The only trick you need to convince Pat is: Large toe-base (say 100mm), large mechanical trail (mmm, self aligning goodness! (100mm) and give it a horribly large rear scrub (100mm?) for an acceptable solution. Then extrapolate this down to, hey we run zero rear scrub (we did) a smaller mechanical trail (why not reversed?) and you can easily allow yourself a nice easy to package 10 mm toe base! http://fsae.com/groupee_common/emoticons/icon_wink.gif

But in all seriousness, refer to the videos above, calculate how your forces are affected by changing scrub and what you need to do to cope with those changes and you will be close to both a solution and answers to keep design judges happy. Remeber to read this too: http://www.formulastudent.de/d...ness-and-compliance/ (http://www.formulastudent.de/de/academy/pats-corner/advice-details/article/chassis-stiffness-and-compliance/)

[Vittorio]

Originally posted by Fyhr:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PatClarke:
Of course it matters!!! Think about it.

Pat

The only trick you need to convince Pat is: Large toe-base (say 100mm), large mechanical trail (mmm, self aligning goodness! (100mm) and give it a horribly large rear scrub (100mm?) for an acceptable solution. Then extrapolate this down to, hey we run zero rear scrub (we did) a smaller mechanical trail (why not reversed?) and you can easily allow yourself a nice easy to package 10 mm toe base! http://fsae.com/groupee_common/emoticons/icon_wink.gif

But in all seriousness, refer to the videos above, calculate how your forces are affected by changing scrub and what you need to do to cope with those changes and you will be close to both a solution and answers to keep design judges happy. Remeber to read this too: http://www.formulastudent.de/d...ness-and-compliance/ (http://www.formulastudent.de/de/academy/pats-corner/advice-details/article/chassis-stiffness-and-compliance/) </div></BLOCKQUOTE>

1) Large toe base, to maintain the desired toe while coping with compliance.

2) Why? Self aligning is desirable? It means both rear wheels will turn out in cornering, generating less lateral force. I can't see any beneficial effect, since in turn-in you won't have lateral force, then no turn-out (which could be useful in this phase). In mid-corner and exit under lateral load the behaviour will tend towards oversteer, beneficial? It depends, generally I would say not. Am I missing something?

3) Again, why? Positive scrub radius? It causes toe-out under braking and toe-in under acceleration. These are beneficial in some cases (e.g. may help in turn-in provided you don't spin and reduce power oversteer in corner exit), but how much? Is it important to implement a large rear scrub? And what about instability under braking?

Thank you for your contribution!

Vittorio

[Fyhr]

Please excuse my attempt at being funny, I'll try to provide serious input.

Originally posted by Vittorio:

1) Large toe base, to maintain the desired toe while coping with compliance.

Yes, very desirable, are there negative effects?
A larger toe base may lead to a smaller distance between wheel center and the lower a-arm mount giving higher forces.


2) Why? Self aligning is desirable? It means both rear wheels will turn out in cornering, generating less lateral force. I can't see any beneficial effect, since in turn-in you won't have lateral force, then no turn-out (which could be useful in this phase). In mid-corner and exit under lateral load the behaviour will tend towards oversteer, beneficial? It depends, generally I would say not. Am I missing something?

I don't think self aligning in the rear is desirable, assuming we are not trying to achieve any elasticity-steer effects. But again, it may be beneficial for packaging reasons and the effects of these decisions must be handled by the same reasoning you provided above.



3) Again, why? Positive scrub radius? It causes toe-out under braking and toe-in under acceleration. These are beneficial in some cases (e.g. may help in turn-in provided you don't spin and reduce power oversteer in corner exit), but how much? Is it important to implement a large rear scrub? And what about instability under braking?

http://www.britishracecar.com/KurtDelBene-BRP/KurtDelBene-BRP-DC.jpg
Implementing a rear scrub may prove beneficial as it could allow a taller upright, feeding forces to a stiffer part of the spaceframe or monocoque and a less constrained suspension geometry. But again, what are the effects?


Thank you for your contribution!

Vittorio

[mech5496]

About rear scrub, yes, ridiculous large scrub might allow you an easier packaging (essentially the pickup points are not constrained by the rim size) but think about the force at the toe link/fifth member....

[Z]

Problems with rear scrub radius?

Use beam-axle, or swing-arms (lat, long, or semi) - no problems! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Z

[Viv]

Hi Everyone. My name is Viv from University of Toronto. I figured as my question relates to a few comments in this thread, I would start this thread again.

I am looking to use tire data to determine the ballpark scrub radius I should be looking at. Above it was mentioned that Mx will give u an idea of tire deflection. So I took the Mx and divided by the normal load (I think the primary force causing the Mx) and this provided me the lateral tire deflection. Unsurprisingly, the tire deflects towards the direction of turn (by about 30 mm). The more interesting thing was the variation in this with braking and accelerating. The tire deflection varies from -10 to -70 mm in accelerating and braking respectively (as per the SAE J670 sign convention). What I was trying to figure out is what this would mean when the tires are on the car considering u don't know which way the tire is on the car in comparison to the testing (i.e. the tires aren't asymmetrical by design). I might be looking at some value here that is meaningless not sure though.

The second question is related to the Mz result from tire deflection. Am I correct to assume that the Mz resulting from this tire deflection and the drag would be included in the tire data Mz?

Thanks in advance.

[Tom Wettenhall]

To my mind, I'd think that contact patch movement under acceleration or braking would be the result of static toe meaning that 'car longitudinal' (x) forces (accel, brake) have a 'tyre lateral' component which causes some contact patch movement. Failing that, I suppose with some static camber applied your tyre won't really behave in a symmetrical way anymore under longitudinal load. Failing that, I dunno, sorry!

I've got a couple of semi-unrelated questions; who all approaches front scrub radius from a steering feedback perspective? I would think that scrub radius has a fairly strong effect on steering load under brakes - does anyone care? Why/not?

[theTTshark]

It's a major contributor to steering loads, so it's very important to analysis it from that perspective as well. We made a spreadsheet that used scaled tire data to calculate steering loads for old vehicles of ours and then compare that to our new designs.

[Westly]

It's a major contributor to steering loads, so it's very important to analysis it from that perspective as well. We made a spreadsheet that used scaled tire data to calculate steering loads for old vehicles of ours and then compare that to our new designs.

Have you physically tested it?

We did physical testing by changing the construction of some 3 piece wheels from 50mm scrub to 100mm scrub and found it had very little impact on steering input force required. (It did also adjust vehicle track at the same time though so wasn't a completely isolated test).

Our Adams simulation also supported this - that there would be little increase in steering force.

Based on this we ran 70mm of scrub radius.
You mechanical and pneumatic trail will have a significantly greater inpact of steering force and

[BillCobb]

What test procedure did you use for the steering effort? With any kind of kpi, caster and rear roll stiffness, you should have picked up a tierod load gradient (slope of Force per SWA which transcribes to Force per g using a little math).

There is a nice test procedure for a direct measure of scrub radius if you happen to be clever and can think inside of a box. [No electrons need be harmed in the pursuit of this information]. Make sure you measure BOTH wheels because ... [ well let's just say that things can get interesting when more than one of something is assembled by 'SelectCreature _Callback'.]

[BillCobb]

Hey Trent: Say "Hello" to Dave Rogers from Terri & me down there in Huntersville. Been a while...

[theTTshark]

Have you physically tested it?

We did physical testing by changing the construction of some 3 piece wheels from 50mm scrub to 100mm scrub and found it had very little impact on steering input force required. (It did also adjust vehicle track at the same time though so wasn't a completely isolated test).

Our Adams simulation also supported this - that there would be little increase in steering force.

Based on this we ran 70mm of scrub radius.
You mechanical and pneumatic trail will have a significantly greater inpact of steering force and

I should have clarified more, but wanted people to think about it a little. Think about times when only one wheel experiences a bump or one corner locks up while the other isn't. That's where it can really screw with your steering forces. So not as big of a factor as long as everything is going normally, but we know how often race cars are in that position...


Hey Trent: Say "Hello" to Dave Rogers from Terri & me down there in Huntersville. Been a while...

Will do Bill.

[J-rob68]

There is some good info in here in regards to using the scrub to find the MX and using that to calculate steering force at the end of the system. Things you need to determine and validate are what steering forces are acceptable? How does steering force change as grip changes?
Will braking force, especially trail braking change this force with large scrub radii? TTC data and matlab simulation are great places to start.

One thing that I don't think has been mentioned is that scrub radius can also contribute substantially to the amount of energy that you are applying to the tire. AKA: It can and will affect the heat up rate and magnitude of your tire temperatures.

Its really not a simple matter of how much scrub do you want. There's a lot that plays into it but mostly steering force, tire temp and steering linearity.

[Claude Rouelle]

Jason

"One thing that I don't think has been mentioned is that scrub radius can also contribute substantially to the amount of energy that you are applying to the tire. AKA: It can and will affect the heat up rate and magnitude of your tire temperatures. "

Can you explain this?

Claude

[Claude Rouelle]

Jason, Let me refine / add to my question... do you have in mind both the front and rear suspension? Claude

[J-rob68]

Claude,

In context to tire temperatures I was strictly referring to the front suspension geometry, or any end that has significant steering rates (in the case of some silly rear wheel steer cars).

As with any given scrub radius you are dragging the contact patch around the steering axis if the value is other than zero. If you have a large scrub radius and have a very rapid or continuously changing steering angle, such as any typical FSAE track demands you will be heating the tire up quite significantly.

This could be beneficial in events such as autocross, or it could be harmful in events such as endurance depending on how you design and use the system. I'm simply stating that its something to be aware of.

[Claude Rouelle]

Jason.

We are on the same wave length qualitatively: the scrub radius has an influence on the energy that is "stored" in the tire and therefore the tire temperature and the tire temperature rate of change.
Unless you do not steer.

However
1. The scrub radius is not the only parameter that influence the tire temperature and the tire temperature speed.
Track changes (mostly a factor of the roll center height) and damper setting for example are very important.
By experience the damper is by far the number 1 setup parameter that influence the tire temperature. But by experience only and that come to my second point.
2. It is possible to calculate the energy going in each tire from lateral and longitudinal slip (by simulation with a decent tire model of even better from real track data especially if you have a slip angle sensor)
and learn something about the tire thermal properties thanks to this energy calculation and measurement from IR temperature sensors (from the outside - thread- or in the inside - we use IR sensors attached to the rim and "looking" at the carcass)
However I never saw any study that comparatively quantifies the effect of, for example, the car kinematics, stiffness, damping, pressure on tire temperature...

The tire is still a black hole

[BillCobb]

That's why you need one of these. The Ford Firestone tire debacle produced mountains of tire temp quantitation by figmosity. That's me with glasses. The horns have been removed surgically.

[J-rob68]

This type of procedure could be done in the ttc testing if wheel spacers where provided no?