Wheelbase to track

[rwolcott23]

I am working with a group that is designing a DSR racer and I have a question regarding the wheelbase to track ratio. Generally the cars out there are running 1.6:1 (~95" WB/ ~60" track generally). I know this has to do with stability at higher speeds but how much would be lost in going to 1.3:1?

These cars are basically FSAE cars with 1000cc unrestricted engine (4 cyl. max) and 1000lb minimum weight. Top speeds typically reach ~140 mph with lap averages around 60-80mph.

Thanks,
Bob

[rwolcott23]

I am working with a group that is designing a DSR racer and I have a question regarding the wheelbase to track ratio. Generally the cars out there are running 1.6:1 (~95" WB/ ~60" track generally). I know this has to do with stability at higher speeds but how much would be lost in going to 1.3:1?

These cars are basically FSAE cars with 1000cc unrestricted engine (4 cyl. max) and 1000lb minimum weight. Top speeds typically reach ~140 mph with lap averages around 60-80mph.

Thanks,
Bob

[js10coastr]

well...

eq. 5.42 in RCVD gives the "stability factor"(K) for simplified steady state stability and control.

If all of the other terms remain the same, the the eq boils down to K = 1/L; where K = 0 is neutral steer.

The 1.9 ratio gives a simplified K value of 0.526 and a 1.6 ratio gives a 0.625 which is a loss of stablity of 18.75% (assuming all other things being equal).

[rwolcott23]

I'll have a look at that section. How does the loss of stability translate when it comes to lap times? Does it make the car more difficult to handle?

Thanks,
Bob

[Marshall Grice]

you're the one building it...you tell us. There are a lot of xSR's around, you might want to ask them. I'm going to guess there is a reason they are the size that they are. If you want to experiment, have at it.

[mtg]

This is a good time for Claude's explanation of Stability and Response being inversely proportional.

You want some "instability" (aka response), otherwise you wouldn't be able to turn. But if you have no stability, you can't control the car. Somewhere in the middle is what you want. When Justin said the stability decreases when you shrink the wheelbase, another way to think about that is that your response time decreases.

So if you want your car to be more unstable (ie faster response) than your competitors, make a shorter wheelbase. If you want more stability than your competitors, make a longer wheelbase.

[Marshall Grice]

in addition to the response/stability trade off, longitudinal weight distribution (both static and dynamic) should be considered. Also downforce is typically more difficult to generate with a short car, due to less underbody area to work with.

I am curious what your reasons are for wanting to go shorter then the norm.

[rwolcott23]

I guess I am looking to build a better mousetrap. When I was involved in FSAE the cars were about 50 lbs heavier than today and the wheelbases were ~65-70". It seems that recently teams have challenged the status quo and now you see much smaller cars weighing in under 400 lbs and it is paying off big time.

IMHO DSRs are well behind the design curve set by FSAE cars so I am hesitant to settle for the status quo set by existing designs. My goal in shortening the car would be to reduce the weight and add balast in a strategic location.

To me these cars seem huge for only having 200 Hp. It would be fun to design something that could turn the field upside down :-)

[rwolcott23]

What about shifter carts. Those reach speeds of 100+ with a WB/track ratio of ~1:1 right? I'm not trying to challenge anyone here, just looking for some insight. My focus was frame design and analysis so I need some help in the dynamics area.

Thanks,
Bob

[VFR750R]

With shifter carts there is no suspension to speak of. How does that factor in the response? The softer the shocks I could see getting to the point where your response time is equal to the natural frequency of the shocks ability to translate that into motion and you'd have an unstable system. Since a shifter cart has no suspension your inputs are (relatively) instantaneous and therefore a lower W/T ratio can be used without becoming an unstable ie uncontrollable system. A shifter cart also has independent front rear systems. The frame is so soft in comparison to the non-existant suspension that little of what happens at one end gets transfered to the other which also may make it inherently more stable.

I could be WAAAAY of base though.

Does your track have to be that wide, maybe a good compromise is a smaller car overall so your w/t ratio doesn't get out of control? Then slaloms and small radius corners are easier to navigate as well.

[BryanH]

The Porsche shop that I do EFI work for is now home for 2 West DSR's. Very interesting cars, overpriced at A$75000 over here. I agree, first thing I thought was the rear end is too long.(after RMIT R04 everything is fat!) But they are using the "transmission" as a mount point for the rear wing and bodywork, they are on the min weight and there is a direct relationship btwn driver age and ideal wheelbase, they are aiming these cars at the more mature racer who quite possibly can't drive a greasy stick up a dogs arse and need the extra wheelbase versus a 19 y.o. kart racer who could use the response of a 80" wheelbase
Cheers

[Peter7307]

Before you go to far down the design path you may want to consult with your driver and ascertain their preferences if any.

Otherwise you may end up building something no one will want to set foot in at worst or designing a car which is not taken to the optimum level on the track because the driver has no confidence in it at best.

Either way your efforts are not fully utilised.

Pete.

[Peter7307]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BryanH:
2 West DSR's. Very interesting cars, overpriced at A$75000 over here. they are aiming these cars at the more mature racer who quite possibly can't drive a greasy stick up a dogs arse and need the extra wheelbase versus a 19 y.o. kart racer who could use the response of a 80" wheelbase
Cheers </div></BLOCKQUOTE>

Maybe it's only the old far...eerr "more mature" drivers who can afford the prices being asked?

Posted by an old far...I mean more mature person !

Pete.

[Craig Simon]

I am new to engineering as a mature age 1st year student at RMIT but I have been interested as an amateur race car builder for a while. My thoughts are that the current DSR car WB ratio is driven by rules and aero. Firstly the rules require the feet behind the front axle line which effectivly sets the minimum wheel base. I just measured this up yesterday for my own xSR style car and I got 2252.5mm from pedals to center of rear drive sprocket (this was moderately reclined seating position) choosing a WB ratio below 1.5 is going to be a wide car! or you will have to sit quite upright both will effect frontal area and drag. The second since they don't have a massive amount of horsepower they are chassing as much low drag downforce as possible which means underbody tunnels which needs floor plan area, and also the same with reclined seating position and narrow cars.
As a last thought most if not all of the current lemans cars are around a 1.6 WB ratio (Audi R8 1.65) is stability a desirable feature for drivability of high downforce race cars as any yawing will result in a reduction of downforce which would not be fun for the driver on turn in or mid corner?
Also what tracks are the DSR running on? from the videos I have seen they are all "full sized" tracks with large radius high speed corners (relative to autoX fsae etc) what is less directional stability gaining you with respect to the competiton? I don't know that the Stohrs/Wests, Radicals etc are the only solution but they are fast and I would want to be able to quantify the speed advantages gained by reducing the WB before committing to a "new" design.
In support of your idea Ferrari clearly struggled for pace at Monaco with their long wheel base car.
please let me know if I'm way off.

regards
Craig
keen "old" 1st year Engineering student

[Pete Marsh]

Hi Craig,
I agree totally with your reasoning but am also not clasically trained (ie, I'm old to). It seams to me with first years with your understanding RMIT will remain a force in FSAE for some time to come.
Do these wheelbase to track ratio formulas imply a motorcycle is infinately stable at any speed?
Has anyone in FSAE increased the wheelbase of their car for the sole purpose of improving stability? If so I'd love to here the outcome of any testing of a long vs short car. Or alternativley does anyone believe the minimum wheelbase is unstable at 'x' speed? Even for an old far...

Pete.

[VFR750R]

Leaving motorcycles alone which have completely different dynamics, what about a Top Fuel car. It is very stable in a straight line, but couldn't 'turn in a 40 acre field'. Even a car with a very high frame stiffness would have very slow response with a 240" wheelbase.