Wheelbase To Track Ratio

[BenB]

I saw on an earlier thread that 1.6:1 is the "optimum" ratio for wheelbase to track. This is supposedly because it is a "unstable" as you should make the car for a normal person to drive it. I noticed that F1 cars are in general 2.0:1, but most sports cars conform to the 1.6:1 spec. F1 drivers should be able to handle a more unstable car than most people, but their car designs don't seem to reflect this.

F1 cars obviously have no problems changing direction. Does anybody know why having a long wheelbase is not an issue for F1 cars?

I was speculating that maybe the aero benefit of having a long narrow car is great enough to justify not conforming to the 1.6:1 rule.

[Drew Price]

How much does an F1 car weigh, versus an 'average' sports car?

How much more aerodynamic download?

What portion of the width of the car is tire?

Best,
Drew

[Marshall Grice]

F1 cars have rules that limit maximum track width. Probably means that they are not at the optimum size.

[VFR750R]

Aero, you can't seperate mechanical grip considerations and dyrnamics when looking at racecars. ALL racecars are built with aerodynamics as the most weighted variable in the equation, or there are rules which limit what can be done. You'll almost never see a series where all competitors run the the middle of the range when rules are given. They'll all be at one extreme or another.

[carlosfierros]

hi guys?
Interesting issue,could anyone explain to me where does come from the fantastic 1.6 ratio number?
Thanks...
Carlos

[Tim.Wright]

Another thought,

F1 cars run at such high speeds that yaw damping is very low, making them very responsive(unstable). They probably need a slightly longer wheelbase to make them more stable. Yaw damping is proportional to WB^2 if I remember right.

Higher wheelbase, more yaw damping, more stable in yaw.

Tim Wright
Suspension & Vehicle Dynamics
Curtin University

[BenB]

Another thing that increases the responsiveness is the low polar MOI, which is a typical characteristic of formula cars, however there are many formula cars that have a 1.6:1 ratio. I think the new Formula Mazda is 1.65:1.

Drew, can you explain why more downforce would favor a narrower track?

[Marshall Grice]

Drew, can you explain why more downforce would favor a narrower track?

It doesn't. A longer wheel base favors more aero. As stated before, the rules limit the maximum track width to limit the performance of the cars. If they were wider they would be faster.

[Drew Price]

Originally posted by BenB:
Drew, can you explain why more downforce would favor a narrower track?

I am pretty sure I am the only Drew being referenced, however, that is not what I meant to say, what I meant by more aero load was more downforce compared to the 'normal' sports cars we are referring to as sticking more to 1.6:1 rule.

The yaw damping hypothesis is interesting, but does that have to do with vehicle speed, or just geometry?

Marshall, I think I am with you on this one, a longer track would have (probably) a higher polar MOI than a similarly built car, and would not be as nimble, but add in aero to the long car, and now you have the inherently stable high speed characteristics of the long wheelbase, plus all the extra tractive capacity from the aero, plus the extra space you need to house the aero elements, and keep their flow pretty clean, etc.

Only reason I can think aero would favor a narrower track is possibly decreased frontal area, but that is so application and condition specific that I don't think that assumption can be made. For our rules, the wing element span is dependent on track width, go too narrow, and you throw away some of the advantage in the first place, less beneficial aerodynamic load, for additional weight.

Best,
Drew

[Pete Marsh]

I wonder what is the ratio for a shifter kart? People can be found that drive them, and they go very fast, have aero even.
Do you think you will ever hear "I pushed my front/rear track out another 1/2" and now its just crazy, I can't control it!" ??
Is there anyone on here that has driven or set up a shifter kart and can comment?
I could see too much caster being a problem when widening a kart( or just in general) but not the track ratio.

Pete

[Ecks]

Shifters have wb/track equal to about 1.1 or so, using the rear wheels as the track measurement. In my experience, this has not caused ongoing control problems. Karts in general will do what you tell them to do, for better or worse. While a suspended vehicle will damp out rough steering inputs to an extent, a kart will expose them in a very upfront way. This in my opinion, is what makes karts such good learning tools.

But I digress...

One should keep in mind that small wb/track ratios do not make for an unsteady vehicle, mathematically speaking. It just means that the vehicle is more responsive, which can cause adverse control loop feedback issues, i.e. steering overcorrection.

As for F1 cars, like what was said before, wb/track takes a backseat to aero and rules considerations. Also, F1 yaw rates are typically much lower than what we experience in FSAE, so comparisons are invalid.

On a final note, our FSAE Auto-X tracks are generally tighter and more technical than most kart tracks, yet many of the best teams elect to use a wb well above the 60" minimum (pity that rule exists). Perhaps this is due to weight distribution goals?

[Tim.Wright]

Drew, yaw damping is inversely proportional to velocity. Therefore, to maintain the same stability/response, a faster vehicle needs more yaw damping. This can be achieved with a longer wheelbase (and track to a smaller degree), or tyres with a higher cornering stiffness.

Yaw damping is the main reason why a (non-aero) car feels like it's 'floating' the faster you go.

Since F1 cars are so quick, they need the additional yaw damping to keep them stable enough to control. But I agree the wheelbase (in an F1 car) probably has more to do with other things such as packaging and fuel tank size than handling.

Tim Wright
Suspension & Vehicle Dynamics
Curtin University