Front Wheel Drive Set-Up

[Point-5]

Calling on all race engineers out there for some help.

I'm racing a front wheel drive hatchback.... Its about 160bhp, weighs 900kilo's, uses Coilovers with only bump rate (no rebound), corner weights left to right are perfect, front to back is about 65/35, using a controlled 15" slick tyre!

I'm relatively new to racing and I need to know a few things. Any help would be hugely appreciated.

Toe Angle - In, Out or square? I feel out seems slightly better. Would that be right? I also think that if I can get the rear of the car moving around more it could kill some of the under steer? Would toe in or out achieve this?

Camber - Due to the type car there is a relatively large amount of body roll - I started off with 2degrees neg but outside of tyre was rolling. How much is too much? Also how important is rear camber angles?

Castor - I have maximum castor posible within regulations. Running at about 1degree.. Too much?

Helper Sprrings - Any advantage?

Spring Rates - Should front be stiffer than rear or vice
versa?

I've bought and read over most of the preferred books but most relate mostly to RWD Formula Cars.

Thanks in advance
.5

[gttouring]

well i found running a frontie was that i had 'rocks' in the rear.
High aire pressure (like 38 psi) and very stiff rear spring and shock combo. Why? to reduce under steer and squatting on acceleration.
Now i liked toe in and about 2.5 to 3 degrees camber. because when i was braking i had no 'dog sliding on a wet floor' wheel behavior.
and as you accelerate and the nose like to lift the tire move back and out like when you compress it too much (this is my observation)
I think spring rates might be almost equal, a little softer in front to my liking as it allow the weight to squat on the wheel as you transer under braking and actually turn. a 900 kilo car hmm? what is it? and what spring do you have now, rmember set up can have a lot to do with your driving style and preference...
My old sentra racer had 300lb/inch springs, and 325 lb per inch rear. very stiff but i like to toss it about.

[graham bahr]

ALTHOUGH I RUN A RWD CAR I'VE HAD A FAR BIT OF INVOLVEMENT IN MAKING FWD CARS HANDLE SO HERE GOES, YOU DONT SAY WHAT SORT OF CAR AND SUSPENSION SET UP YOU HAVE SO I'M GUESSING A BIT WHEN IT COMES TO REAR SPRING RATES BUT THESE SETTINGS TOOK 7 SECONDS A LAP OF MY MATES FWD ESCORT ON THE SILVERSTONE STOWE CIRCUIT (SUB 50 SECOND LAP)

THE THING TO BARE IN MIND THAT CARS ON SLICK TYRES REQUIRE TOTALLY DIFFERENT SPRING RATES AND GEOMETRY TO CARS THAT USE TREADED ROAD TYRES.

I WOULD START WITH
FRONT SPRINGS 700LB
REARS 500LB
FRONT CAMBER 2 DEGREE NEG
FRONT TOE 2MM IN
REAR CAMBER 3/4 DEGREE
REAR TOE 2MM IN

AS FOR TYRE PRESSURES IT DEPENDS MASSIVELEY UPON RIM AND TYRE SIZES FOR EXAMPLE ON MY CAR WITH A 7 INCH RIM AND A 210 TYRE IT NEEDED 30PSI, THE SAME RIM WITH A 200 TYRE NEEDED 28 TO WORK, WHEN I FITTED THE SAME 200 TYRE ON AN 8 INCH RIM I HAD TO DROP THE PRESSURE TO 24 PSI

TYRE TEMPS ARE THE WAY TO GO HERE TO FIND WHAT WORKS BUT I WOULD SAY 25PSI IS A GOOD A STARTING POINT AS ANY WITHOUT KNOWING WHAT YOUR ACTUALLY RUNNING

FAR TOO MANY PEOPLE USE EXCESSIVE AMOUNTS OF CAMBER TO KEEP THE TYRE IN CONTACT WITH THE ROAD, WHATS NEEDED IS STIFF SPRINGS TO KEEP THE WHOLE THING FLAT IN THE FIRST PLACE, YOU DO NOT SAY WHAT SIZE WHEELS YOUR RUNNING, IF THEY ARE MORE THAN 8 WIDE YOU WILL ALMOST CERTAINLY NEED TO REDUCE THE CAMBER ANGLES ABIT FROM WHAT I'VE QUOTED, ESSENTUALLY THE WAY TO MAKE A FWD CAR GO QUICK IT TO KILL THE UNDERSTEER BY MAKING THE REAR SLIDE (RWD YOU CAN DO IT WITH THE THROTTLE)

IF YOU WANT TO LET ME HAVE A FEW MORE DETAILS ABOUT YOUR CAR (AND HOW IT HANDLES) I'LL SEE IF CAN THINK UP SOMETHING A BIT CLOSER TO WHAT YOU NEED

[Point-5]

Its a Fiat Punto HGT

Tyres are 175 R15's on a 7inch rim

Suspension uses standard pick up points on front and rear but all bushings have been replaced with spherical bearings. Rear suspension is a trailing rear arm (think thats what its called) The spring is compressed between the axle beam and the under side of the car.

At the moment the springs in the back are about 60% of the fronts(1100lbs/inch) - initially I had it the other way around with the back very stiff but this way is better by almost a second on the stopwatch.

Will toeing the back IN not create extra grip? Rather than letting it loosen up a bit.

I need to find about 8/10ths to be on the pace......... Maybe its just fine tuning that will get this last bit rather than going back to the drawing board?

As for how the car handles - through fast corners its a match or quicker than everyone but in the more techinal corners I seem to be later on the power!

[graham bahr]

175's are going to be fairly well stretched on a 7 inch rim so i guess tyre pressures no higher than 25 psi, have you ever checked tyre temps? aim for an even spread across the tyre, hotter int the middle only suggests your geometry is ok but pressure too high much hotter on the inside is too much negative camber or way too much toe out etc etc its a fairly logical thing.

i do not like putting toe out on fwd cars as it usualy makes them twitchy in the dry and a real handfull in the wet.

1100 springs in the front sounds very stiff to me does the car understeer? my guess is that i is ok but wont want to change direction more than once quicky ie a chicane,

as the punto has the spring half way along the trailing arm that will lesson the spring rate so if you front springs are 1100 and the rears 60% of that then the effective spring rate (at the wheel which is the important bit) will be in the region of 400.

i would say its better to modify an existing set up than to start again from scratch.

when you say you get on the power later than the others why is that? does the car just wheel spin? go wide? with the back very stiff did it spin or are you now faster because its more stable so you have greater confidance to push, maybe the rear was just way too hard before, do you know what geometry settings you are running now? does the back end step out if you lift and can you recover it by planting you foot again? sorry its lots of questions but i need the answers to try and help, the real secret in getting a fwd car to be really competitive is to kill the front end understeer by having the rear on the very limit of adhesion.
tell me a bit more and i'll think some more.

p.s. who's brand of slicks are you using and do they come in other sizes/

[gttouring]

1100 lbs/inch spring. Good if my budget allowed it I would have gone very stiff. but driveing style and car weight very crucial to consider. Tire type too.

[alfasud]

Does the car have limited slip diff? In tight corners there will be weight transfer away from the inside wheels to the outside (both front and rear). The end of the car (front or rear) that is more stiff, will tend to transfer more weight to the outside wheel. If the front is quite stiff and you don't have limited slip diff, then the weight transfers away from the inside front wheel and then it just spins when you try to accelerate out of the corner.

Is this what happens to you?

Cars with a softer front but stiffer rear probably get better drive out of the tight corners. If your class allows LSD you might consider getting one and/or experiment with softer front.

[Point-5]

The reason I'm later on the power is for 2 reasons (1) Mid corner its hard to keep on apex, I'm using the brake a bit to hold it into the corner and (2) its washing out slightly on exit(I could live with that though)

I know the logic behind getting the rear to move around but I can't seem to achieve it on this car - when I stiffen up the rear it gets too hoppy into the corners and the back literally starts to leave the ground on turn in...meaning realy late to get on power. Its also a second a lap slower. The back end doesn't really step out that much even on lift off which is a indication to me anyway that the rear has too much grip. Whn it does go though which is only really in the wet its easily caught/held with power delivery

I think there might be something in the toe angle - I'm testing next weekend so I'll try playing around with that.

I have a tyre pyro meter yeah and I reckon I have the pressure's sorted. They are pretty even but slightly hotter on inside. Only about 10degres Celsius from inisde to outside.

Re: The rear settings - I have it toe'd square with about 1 degree of neg camber. This is something I'm gonna play around with in testing too.

Tyres are Dunlops (compound 770) and its a set tyre for the class. So can't change it.

@AlfaSud - No LSD I'm afraid - its a standard box. I am getting wheel spin but not in excess. I'd imagine without a LSD you are never gonna eliminate wheelspin completely?

[Morris 1100]

Toe is something that is worth playing with but be carefull!
Toe-in gives stability, so the car wants to go straight, if you go for toe-out on the front the car will turn in better but it may not go straight down the straights and can get twitchy when you hit bumps.
Toe-out on the rear can give oversteer (big oversteer)
So if you play with toe just go a little at a time and ONLY do one end of the car at a time.

[graham bahr]

i regree with morris 1100 be very carefull about towing the rear out its a recipy for oversteer city, try raisng the rear tyre pressures which will reduce the contact patch on the rear and get it moving without resorting to stiff springs.

does the punto have anti roll bars? i have had great results in the past by thinning the front one down with an angle grinder, if it has the origonal fiat roll bar that bar is probably too stiff (car manufacturers use thick roll bars to stop roll because they cannot do so with the sprngs alone or the customers would be complaining about an excessivly harsh ride)

fwd secorts i have thinned front bars down by 50% and gained much better traction out of the bends, i have also fitted rear bars to liven the the rear up with out having to resort to excessivly stiff rear springs

[Morris 1100]

Sometimes you have to take a long hard look at the specs of a standard car and ask yourself what the manufacturer was trying to achieve....

The factory wheel specs for my car.
Front
Camber ¾°pos (+/- 1°)
Castor 5½° pos (+/- ½°)
Toe out 1.6mm
KPI 10°

Rear
Camber 1° pos
Toe out 3.18mm

Note this car has toe out at both ends and has positive camber both ends, about the only thing it has going for it is the castor!
The car is a Morris 1100!

[alfasud]

graham bahr makes some good points about anti-roll bars. A popular setup I've seen with many alfasud racers is to reduce or totally remove the front anti-roll bar. In the rear, an anti-roll bar is added.

Now in the case of the alfasud, the rear anti-roll bar works despite the rear being a beam axle, because the anti-roll bar is mounted on the trailing arms (strictly speaking I think they are watts linkages) and effectively ties those arms together. Some tuning can be made by adjusting the point where the anti-roll bar clamps on those arms/linkages. Can this idea be applied to the Punto? - I'm not familiar with the rear suspension on those.

I seem to remember seeing some web pages about the Punto Super1600 rally car. Are any of the ideas from the Super1600 tarmac spec rally car applicable to your race car?

Or maybe if it's a restricted formula like Fiat Punto Abarth Cup - http://www.fiatcup.com/technical.html - maybe you aren't allowed to change too much, just settings

[gttouring]

what size tyres do you run?
My friend races his integra at club events and I asked him about this situation, he runs 225/50/15 tyres in front and runs 205/50/15 in the rears, his inside rear will tends to lift in a corner on turn in, and with the thinner rear tyres he over ame the big rear end grip you described, and he kept the tight springs (which i forgot about...)

[alfasud]

I think he's restricted to the control dunlop tires (175-555-15?), so he can't change sizes. One more thing to consider is to reducing weight in the front and moving it toward the rear... things like the battery for example, to give the rear end some more work to do. I'm not sure if the rules let you do that either.

[BSTR]

This may come little bit too late but...

I have a relatively long wheel based FWD touring car (Alfa 156) that has almost 70% of the weight in the front. This makes it difficult to turn in the tight corners.

Earlier I was very unhappy with toe-out on rears (also in front) because it made the car too loose on the back... Lots of lift-off oversteer and could not trail brake into corners.

I tried to zero the toe in the back and I found the car very easy to drive in the fast corners, however in the tight technical corners it was understeering. I got 1,5 seconds off the lap time.

Now I have returned to toe-out in the rear (about 3 mm measured from rim) and altered my driving style. The car handles great in tight corners but must be driven with finesse in fast corners. The key is to drive more smoothly. Extra 1,0 seconds off the laptime - 2,5 when compared to beginning.

I must have learned to trust the car when driving couple of times with zero toe and then extended that trust to more difficult handling setup also. It sometimes may be a good idea to try something very different and what response that gives.

But on the other hand, I earlier had Uno Turbo racer and that one seemed to like toe-in in the back. That one might be more comparable to your Punto. Shorter wheel base and smaller wheels/tires compared to beam suspension in the back may have been the reason. I had some 3 mm of toe-out in the front but 1,5 mm of toe-in in the back.