Dynamic Camber ? 3

[TMcRally]

Hi all

I’m new to the site, thanks for all the threads, a good read.

Something that has bothered me for a while now and I hope someone can make some sense of it for me. All the books I've read (I've been an expert now for a couple of months) the writers state the objective is to have the tyre stand upright 90 degrees to the road in turns.

I'm from Australia and our premier motor sport is V8 Supercars (don't blame me they didn't ask me when they came up with that name). One of the onboard camera shots is from under the car and shows one of the front tyres in action.
The tyre doesn't ever stand up close to 90 degrees, it is always in negative. They have about 16 degrees caster and about -7 camber static.

They have limited tyres for qualifying (similar format to F1) and 3 x 40-50 lap races.

Is there an exception to the rule ?

Thanks
Dave

 

[cibachrome]

A V8_Supercar tire, like an Nascar tire has optimum FY at a camber angle in the 4 to 7 degree range. The camber at peak depends on vertical load for a fixed slip angle: The higher the load, the higher the camber at peak force. On Nascar ovals, the high grip tire is actually the inside tire so load management (aero + static + load transfer) on it is Very important. As a point of interest, when testing such tires, test machine procedures sweep slip angle at fixed loads and camber angles, but given a choice, I recommend sweeping camber at fixed slip. This is because slip angle is pretty much fixed by track geometry and camber is set by load, roll, and upper wishbone selection. The camber sweep is a direct answer to a direct question. I don't feel caster (rate of change of camber with steer) is much of a player here because steer angles are not (or should not) be that large. Your high caster is a vertical tire load (dynamic wedge)adjuster. It obviously affects the tierod loads for steering torque but caster offset does too (Don't worry, its a visual inspection). High kingpin is also a way to do this (maybe better).

BTW: oval track tires (yes even your down-under Dunlops) are built with significant plysteer and plyrat properties appropriate for your turn direction. This further compounds the max force camber calculation. And, FX force is a grip amplifier as well. This means that the tire pumps out more force under tractive power.

If you have video equipment on board for your limited allowed tire tests, pay attention to the sidewall position relative to the plane of the rim. Optimum force occurs when the sidewall plane is parallel with the wheel plane. This is where the overturning moment is appoximately zero. You've probabaly heard of or actually felt a car which was "rolling over on a tire". Under such conditions, the tire carcass has lost lateral structural stability. Good force output but not great force output.

So, my reply to you is: NO the max force is not at 90 degrees to road plane. The sidewall will be up to 2 inches out of the plane of the wheel and the lateral grip will be way low and the tire will chatter. Camber the tire back until the sidewall deflection is small.

 

[GregLocock]

Great answer. I'd add that the V8s are a rather artifical race series, and the tires are grossly overloaded (which is what makes for an interesting race). If the tires actually had to last for an appreciable distance they'd use less negative camber.

" On Nascar ovals, the high grip tire is actually the inside tire "

I've got splinters in my fingers.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cibachrome]

I'm actually not that familiar with the Supercar series, but had some mates over there who needed some tire info.

No comprende the "splinters" comment, but Nascar Speedway inside tires are typically twice the mu of the right side. (2.5 vs. 1.25). Hence the heavy aero consideration on the left (inside) front of the cars and trucks.

Have some great video of this but no way to post. If a driver ever saw what a chattering tire looked like (a tire engineer's nightmare: cords in compression) in high speed video, they would consider a career change.

I forgot to mention the rim width and pressure effects on camber setting but the varibles are the same as is the goal.

Nascar fans should also recognize the other result of tire optimization that occurred 2?? years back: wheel failures caused by the increased tire output as engineering focused on the "harder" tire for more grip. Harder tire = less rollover. BTW: This wasn't necessarily a harder compound but a harder (stiffer) sidewall in the construction. REALLY observent fans would have noticed that the brand of wheel changed in the really strong teams to temporarily solve the wheel fatigue issue, as indicated by the cutout count in the spider!

 

[GregLocock]

"I've got splinters in my fingers."="I've been scratching my head"

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TMcRally]

Thanks everyone,

I’ve got a bit to go before I’m up with this, I’m a bit lost, so - just thinking out loud and please correct me where I go wrong.

All the negative camber is needed on the inside wheel because the control tyres used are not optimum for the loads placed on them. The effect of this is would be if the tyre was at right angles to the road the “too soft for the use sidewall” would flex past the point of maximum support which leads to reduced grip. (is this the tyre pulling the back of the contact patch away from the road before it’s time?).

The tyre in the video was the inside tyre, they don’t run on the ovals but only one track is figure “8” so a strong bias to either left or rights. The particular track had fast entry and exit to the main straight and only one RH turn.

Does this amount of -camber work on the outside tyre as well, or is this principal only for the inside tyre. If you have too much negative on the outside tyre and the inside of that tyre was in contact and the outside edge was off the ground then the only sidewall in contact with the road has already passed the point where it is in alignment with the rim and can only get worse from that point.

The tyre test machine sounds interesting. Do you have a link to an image of one.

FY = Force/yield ?
FX = ?

Thanks
Dave

 

[cibachrome]

The camber (dynamic + static) for each wheel would be set to produced the optimim grip for the load, steer and pressure being used. Some of this actually depends on how the rear is doing because tightening up the front of the car (increasing front grip) makes the car more loose (removes understeer). "tight" or "loose" descriptors are poor information to a crew chief because it does not tell which end of the car is having a problem.

BTW: I've heard the term "happy" for tires that are at optimum pressure, load and steer. A "happy" tire makes a special sound on the track when its cornering. A good test team knows how to discriminate this sound from all the other noise out there when they practise/test.

 

[TMcRally]

So the theory of equalising tyre temperature across the tyre is not really appropriate.

 

[Yawing]

optimum camber in this case seems to be ruled by the very soft lateral sidewall stiffness meaning a higher static camber angle is required to give optimum grip on the outside.

If the car is very much biased for mid corner understeer to get the best traction, the inside front is less important as it is quite often unloaded. But this means braking grip is not the best with lots of static camber. but since these tyres are also soft in vertical spring rate, the tyre can deflect enough to negate a little of this.

cibachrome, in regards to your comment on kingpin generating a change in tyre loads with steering, surely it doesnt? if both the outside and inside wheels rise and fall the same as it does with kingpin angle there shouldnt be a change in "wedge"

 

[cibachrome]

There is a change in wedge. Its desired to settle the rear for more 'forward bite'. Keep in mind the cross symmetry in the front. For oval track, the left and right front are considerably different. Tractive force is biased toward the left rear (that's where the 2.5 mu is).