Tire Deformation

[Tom Duncan]

See below a photograph of Denny Renfrow's San Francisco Region Spec Ford (Club Ford to the rest of the country) Crossle shod with the American Racer spec tire. The photo was taken in the fast downhill semi-blind lefthand turn 9 at Laguna Seca (which admittedly loads the right side, esp the front, tires more than normal) and probably just after Denny turned into the corner.

Notice the degree of deformation of the contact patch of the right front tire. Yikes! From the looks of the lines on the pavement trailing the car I think the rear tire is probably doing the same thing.



Seems to me this merits some discussion. I have often wondered exactly where the 'cupping' comes from on the inside edge of the tires after they are used for a while, and I think this might be it.

This also gives new attention to the possible need of warming up the tire before hard use, as the flexibility of the sidewall seems fairly important here.

It should be said that Denny is among the fastest drivers in the class and that he is usually wringing all he can out of his car, as would seem to be the case here.

I will try to photograph a Formula Ford (with a different tire) sometime in the future at the same point on the track to see what they have going on.

The photo was shot with a vertically moving shutter, but it was at a very high speed (likely about 1/2000) and I was panning with the car, so I don't think the camera has added any distortion.

As I mention in the caption of this photo as it is shown on my website (this and other photos of FF racing here: www.oddog.com ) I think it is a good thing we can't see this when we are in the car!

Any and all comments welcomed.

Tom Duncan

[James Hakewill]

Looks like Denny has a bit of an understeer issue in turn 9! Hmm.. must start forming evil plans..

You can see the trail from the RF tyre clearly in the picture - the car is certainly travelling sideways to some extent, and the front wheels are turned in relative to the centreline.

I'd always associated the cupping with the inside edge of the tyre being scrubbed off - when cornering with some healthy slip angle and negative dynamic camber.

There were some articles recently in Sportscar taken from Paul Haney's tyre book. The book certainly includes some diagrams showing the weird shape the tyre assumes when under load, and the pressure distribution across the contact patch. As I recall, it also talks about the cupping phenomenon.

Until I really understand what's going on with the tyres, I'll stick to fiddling the static camber to get even tyre temps, and tuning handling on the roll bars!

During qualifying I also noticed some radical tyre deformation on the RR of David Stillwell's VD car, on GY tyres. As he went inside me on the entry, the rear very briefly looked similar to your picture - almost rolling off the rim it seemed.

Turn 9 has the highest G loading anywhere at Laguna - at least for my car - 1.9g @ 91/93mph.

Is your picture of Denny and me together also turn 9?

James

p.s. great pictures..

[size="1"][ June 01, 2004, 05:49 PM: Message edited by: James Hakewill ][/size]

[James Hakewill]

Hi Tom,

Some more info..

Here's some interesting reading - an extract from Paul Haney's book about tires - he spells it like that, but that doesn't mean that I have to .
http://www.insideracingtechnology.com/tirebkexerpt2.htm

Anyway, having looked through a bunch of books which don't really cover the camber case, and pondered it some more, what I reckon is this..

At a big slip angle (e.g. in understeer), as the tread rotates into the contact patch, the inside edge of the contact patch is not 'stuck' to the road but is actually sliding - resulting in overheating of the inside edge, and the rubber going soft and getting pulled off.

Not sure how to account for exactly why this is the case - other than the obvious statement there is a big load transfer to the outside front wheel, and there's negative camber.

Both Haney's book and Milliken and Milliken say that 'it's complicated' and 'no complete theory exists'... so the field is open for speculation!

Testing Mike B's Ralt at Thunderhill the other day I could actually see this in action in turn 2 (a pretty much constant radius 110mph 180 degree left-hander) - when the front end was understeering I could see the rubber forming into great globs on the inside edge and flying off.

Maybe someone with more of an idea of what's going on can chime in..

James

Some villians in a Jag doin' a runner after a blag.. (From Stirling Moss' "How to Watch Motor Racing")



A 60's or 70's tyre testing machine..



[size="1"][ June 02, 2004, 01:55 AM: Message edited by: James Hakewill ][/size]

[Avon Racing Technical]

Aha,

This brings up the subject of static camber and general suspension geometry:- when the car pitches into the corner the desired effect is to have the largest amount of footprint on the ground when you need it most. Therefore, you should not be looking for an even tyre temperature spread because the tyre will spend more time unloaded than loaded. The inside of the tyre should be hotter (when measured with a needle-type probe) than the outside. Front to rear spread should be pretty even, certainly on the loaded side of the car.


Pete.

[Dave SanF 50]

Pete
Thanks for the expert opinion. Can you clarify the "front to rear spead". Are you referring to front to rear tires?
dave jalen

[James Hakewill]

Pete - Good point about the lateral G.

So you're saying that the inner edge of the tyre is being heated an extra amount because the negative camber causes the inner sidewall to flex more than the outer sidewall?

Do you attribute Tom's inner-edge wear pattern to negative camber whilst running, or from scrubbing the inner edge during cornering?

The histogram below is from a Spec Ford lap at Laguna Seca (1:37.0):



By far the largest amount of time is when when the lateral G is zero or very low. The other big peak is up to 1.4g which is the usual peak G for a non-cambered corner.

p.s. now I've discovered how to post pictures there'll be no stopping me blathering on!

[TimW]

Originally posted by Avon Racing Technical:
This brings up the subject of static camber and general suspension geometry:- when the car pitches into the corner the desired effect is to have the largest amount of footprint on the ground when you need it most.

How does the camber thrust impact the lateral capabilities on tires the size for FFs?

[Avon Racing Technical]

There are a number of misnomers regarding tyre temperature, but the most common is that the sidewall flexing creates temperature. Tyre temperature comes from the cyclic loading (in compression as well as shear) of the tread. If a tyre is very worn and has little tread depth, there is little rubber to compress so as to get it hot, and there is no chance for the lateral/longitudinal loading to produce any shear ‘stretch’.

Therefore the inside edge gets hot(ter) because it spends longer being sheared against the ground as a result of spending more time in contact with the ground.

Very (read extremely) simplistically then, the suspension geometry, tyre size, tyre pressure, spring rates, roll-stiffness, damping and everything else are all there to allow the tyre to spend the most amount of time in contact with the ground. Where a compromise is required, then let the contact patch be greater when there is a greater load on the tyre, say mid corner. Of course, when you change one thing, everything else changes and you need to account for that. For example, if you find you are deflecting the sidewall too much in cornering, you could stiffen the lateral stiffness by increasing tyre pressure, but that will increase the vertical spring rate of the tyre, and the system as a whole. (It will probably increase your tyre temperature too, as there will be greater resistance to vertical compression, but this will vary hugely on the individual setup.)

The tyre sizes are usually such that the tyre temperatures from the front tyre compared to the rear tyre on the same side are similar, especially the loaded side. It may seem obvious, but the more work a tyre does, the hotter it will get.

It will quickly become very difficult to be specific regarding actual car setup as there are so many variations of car, especially Formula Ford, so many peculiarities and oddities that what will work for one will almost certainly hinder another, but the trends usually apply. However, in the first picture in this thread, I would be very surprised if that right front had a tyre temperature hotter on the inside than the outside.

Food for thought I am sure,

Regards,

Pete.