Power, Tire width, weight distribution 2

[LostBoy75]

Hi There,

I am building my car for time attack and I wonder how to calculate proper tire width.

Let's take stock toyota supra mkiv 320 bhp, 1560 kg, 51 to 49 weight distribution, front tires 235/45/17 rear 255/40/17, spring 7,5kg for front and 3,8kg for rear.

Now we striped car and we have 520 bhp, 1280 kg, 58 to 42 weight distribution. We are using spring 18kg for front and 12kg for rear.

I have no idea how to approach to that problem. Any formula idea etc ?

 

[imcjoek]

I think the way this is "calculated" for a race car is as follows:

1) Are your tires narrower than that allowed under the regulations? If so. They are too small.

2) Are your tires perilously close to rubbing bodywork? If not, they are too small.

 

[GregLocock]

Agreed, I don't think you ever see a closed wheel car that uses anything smaller than the maximum allowed/practical, and width is far more beneficial than diameter if you are ekeing out the last bit of clearance.


Cheers

Greg Locock

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

 

[LostBoy75]

OK guys, but at some point friction should be taken under consideration. 80% of racing is acceleration with to wide tires I will lose on acceleration.

OK, I can use 310 wheel at front and rear but I need to put wider rim for front wheels +6kg of rotating mass. I lose on acceleration...

 

[GregLocock]

True. Can you get hold of any performance data for your tires?

Cheers

Greg Locock

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

 

[imcjoek]

With 560 bhp in a 1280kg car I don't see rolling resistance (is this your "friction"?) or wheel inertia as being your limiting factor.

I'd rather have more traction out of the corner, and higher cornering speeds. Especially with the "spins tires in anything under 4th gear" P/W ratio you have.

 

[LostBoy75]

GregLocock
I will check it and post it.

imcjoek
Studying power is pointless. let say we have 200 hp instead of 560, wideness of the wheel will make huge difference.

 

[Junior37c]

gotta concur with imcjoek.

however you can easily enough calculate the differences in power loss to spin up 2 wheel/tire sets of known inertias and decide for yourself if the inertia gain is worth the added traction.

 

[LostBoy75]

I can't find good formula to calculate power lose, Junior37c could you share.

I found some ideas here:

http://www.physicsforums.com/archive/index.php/t-321017.html

GregLocock
Curious but Dunlop here don't know much about them, except price of course. I asked guy how they recognize compound and he said he had to write in the email no idea how they mark it on wheels.

Greglock
I tried to approach to your advice, even it's heavier, to install wider wheel I need to use wheels with different offset. Then I will change tire track ratio, front will be wider then rear.

 

[NormPeterson]

F = Ma
T = I[alpha]

Make sure that you keep a and alpha consistent with each other.


On the driven wheels, rotational inertia shouldn't matter as long as you've got enough torque back there to make them spin. Up front is a slightly different story, but is clouded by things like better lateral grip and predictability (don't shortchange that last item).


Norm

 

[imcjoek]

I have a feeling you are over-thinking this.

There is a thing called benefit-cost ratio. You are spending much of your resources (time!) thinking about something that is not going to have a significant effect on your lap-time (benefit).

Fit the biggest tires you reasonable can. Then spend the time you save running laps and dialing it in. You will come out ahead of the guys that have optimized their lug nuts to three decimal places.

 

[LostBoy75]

NormPeterson
Thanks for basic formulas , I am a little more advance.

"...it shows drawbar pull/1000 pounds to be....(depending on surface)

Asphalt....... 12 to 22 lbs.

The formula for air resistance is.....
F=FA x .0025 x MPH^2

F is additional drawbar pull needed to overcome air resistance
FA is frontal area in square feet
MPH is vehicle speed, miles per hour

imcjoek
Perhaps you are right but I read a few books about chassis engineering and I've always read assumption like those ... let say we have equal weight distribution wheels should be same f/r. Should be wider on more loaded side but with fr cars should be different ... but nobody explain how

 

[NormPeterson]

Lostboy - it was not my intent to supply a more complete derivation. Just point you in the right direction since that was the need expressed in the original post. I did give you one direct hint that you'll need for estimating the effect of heavier, wider wheels that may not show up in the physics link.

More hints - there is more than one force, more than one 'I', more than one [alpha], etc. What you're looking for with respect to wheel weights and acceleration potential is entirely possible to determine using a spreadsheet solution.


Norm

 

[GregLocock]

An alternative is to download Bosch Lapsim and play with that to your heart's content. It will at least give you some idea of the trade-off between inertia, drag, and grip, for a circuit.

I would not (do not) use it INSTEAD of understanding the first principles and hand calcs, but it does give you a better insight.

Bet you end up with the fat tires.



Cheers

Greg Locock

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

 

[Tmoose]

"58 to 42 weight distribution."

58% of the weight is now on the front wheels?

 

[GregLocock]

Ow, good catch. Well your next winter project is to move the engine back by a couple of feet or more.

Cheers

Greg Locock

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

 

[LostBoy75]

yeap, We've already started working on that. I will post pictures

 

[evelrod]

All opinions and posts seem to have merit. Fine. However, I've already been there and done that and I can tell you for sure that I like MY approach to the question better...

Warning...If you do not have a lot of time, a race car with several "spare everything" and a large sum of $$$, this may NOT be appropriate for you!!!

In the '80's I just penned a deal to test for one of the major racing tire mfgrs. After the better part of the summer, I knew EXACTLY which tire worked best! ;o)

I've always been BIG on t and e. (Sound familiar, Greg?)

Rod

 

[GregLocock]

Some curious bacteria has invaded my desk and eaten EVERY book on tires and handling.

Hmm, well I found one.

It says, ultimate grip is proportional to tire width ^0.15, which sounds a bit low to me. So going from a 200 to a 300 mm width tire gives you about 6% more latacc. But you can soften the compound a bit and get some more.

The cornering stiffness also increases. This is a good thing for a race car as it means you'll get less cornering drag. CS is proportional to tire width^.3, that is you'll get 12% more lateral force for a given slip angle, going from a 200 to a 300. Again compound has an effect.

Cheers

Greg Locock

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