Eliminating Or Reducing Understeer

[Guest V8ish]

I have a 205 track car running Billys and a strut brace, rear end lowered to match. It is handles well normally and these are the only mods I have in this area.

 

I am running 135bhp, 125llbs and struggle to get the power down out of corners and find the car understeers if I give it the slightest oppurtunity in or out of bends. The last trackday it had spanking new Eagle F1's on so tyres can be discounted (unless the size was too big 205/50/15?)

 

Any thoughst on reducing understeer guys?

 

Many Thanks

[tagy 1]

I would look into adding some negative camber on the front wheels (309 wishbones or adjustable topmounts) & an LSD to help get the power down.

[Guest V8ish]

Negative camber.... I never thought of that. My car is about 50mm lower than standard and I assume this has never been done.

 

I asume it cannot be done on a standard set up?

Edited February 13, 2009 by V8ish

[projectpug 0]

The use fo an lsd will help this and the use of 309 wishbones/shafts for a little negative camber:)

[Garry 1]

Unfortunetly the standard set up is not adjustable for camber.

 

A budget option is to run the 309 setup as suggested. The 309 stuff is a little but longer (15mm or so) and it pushes the middle of the wheel outwards whilst the top stays in place creating the camber. Another way is to get adjustable top mounts. Or both like me!

 

New Eagle F1's won't give that much grip as the new, deep tread blocks will flex. To get mega grip you will need some track tyres like Toyo R888's.

A ATB diff is another option to get both the front wheels pulling you around the corner. But beware, you are at the top of a slippery slope that will end up costing you thousands to drop a couple of seconds from your lap times......

 

Another thing to look at is your driving style, are you just flooring the throttle and applying more lock whilst cornering? Try unwinding the steering as you exit the corner whilst applying throttle gradually until you have the wheels straight.

[James_R 3]

24mm Anti roll bar will do it, and if that's still not working plate diff.

[Henry 1.9GTi 36]

was gonna say as above, big fat rear anti roll bar. Helps no end.

[Nathan 0]

Also solid mount the rear beam, my old car had billys, strut braces, 309 bones and an adjustable rear bar and I found that I could induce LOOS which was very easy to control and this meant no understeer. Technically speaking it might not of been the fast way round the corners, but it was dam fun to drive! This method / set up can easyily bite you in the ass if its greasy / wet or your not ready for it.

[zilog 0]

I dont know much in this area, but I suspect a few things to look over are:

1. more toe-out in front

2. a bit more ride height rear, to reduce rear grip.

[cir287 0]

best thing are tyres, if it track only why not go for slicks ?

if you keep the rear end a bit lower than the front you create a little more castor on the front end, this helps. we used to pack down the rear bumpstops by about 10 - 15mm and effectively run the rear arms sitting on them, in the wet you will need to jack up the rear end and pull out the bumpstops though.

a limo should make a big difference but it will cost you

negative camber is always good, havnt tried the 309 bones but sounds good.

stiffer anti roll bars are going to help too.

 

just be aware the more understeer you cut out will make the car much harder to drive in the wet, it will just love to snap on you, mind you if you can catch it just as the back becomes light and accelerate again you can get some super conering speeds but you are on a knife edge (wet or dry)

[MerlinGTI 3]

If your car has been lowered 50mm, this would have already increased the amount of negative camber (on the front) the car has, by a 'guess-timated' (educated mind, im looking a a geo of mine when it was running -40mm) half a degree (if not a tad more) over a standard setup (assuming nothing is worn / bent).

 

I would say 1.5 deg of negative camber (over standard) is the absolute maxium you should be considering, after running this myself and finding it causing a servere lack of stright line traction, it was laughable in the wet.

[Cameron 16]

A couple of complaints about the advice given so far.. Increasing negative camber will make your power understeer problem evan worse, since you will have reduced the contact patch on the inside wheel and with am open diff it will just light up the inside tyre. Secondly, fitting a thicker rear anti roll bar will just reduce rear end grip. It works by making the understeer less noticeable but that's because you're going slower. Fit a thicker bar and watch you lap time increase.

Anti roll bars should ONLY be used to balance a chassis when you have exhausted all other opportunities to do so. I would strongly recommend fitting a plate type LSD.

[Henry 1.9GTi 36]

with a stiffer front as in OP a rear anti roll bar is needed to balance the chassis again Cam. (torsion bars would be good but expensive)

 

benifits of correct camber on the outside wheel contact patch far out weigh any losses from the inside wheel in mid corner speed.

Bigger rear antiroll bar increases weight transfer to the rear outside wheel giving more vertical load and therefore more grip it will just reach its limit first . i.e before the front making it oversteer. Increasing rear roll stiffness will reduce weight transfer at the front effectivly increasing vertical load on the inside, maybe spinning, wheel therefore increasing exit speed. Get the the right size arb and balance the chassis and it will be neutral. Id say aim for a little oversteer so you can be on the power more of a lap. LSD is a great option but one of the most expensive compared to a ~£145 rear arb which also makes the car very fun to drive which it whats it all about!

 

cheapest is to decrease front roll stiffness by disconecting the front arb, but u then induce more roll which gives u alot of positive camber which will reduce maxium lateral G generated. McPheason struts gain -ve camber in bump so as above lowering 40mm will give a little camber. Id also say with a torsion bar suspension setup (and mcpherson strut) any roll in the body will be transfered to the tyre. seeing as 205s have 0 -ve camber at the rear limiting rear roll angle is diserable.

 

good luck, vechile setup is a minefield but its not difficult to make a fun car.

[Cameron 16]

That isn't entirely true, a thicker arb doesn't increase loading on the outside wheel. You still have the same weight transfer and body roll but the inside wheel is lifted so you get reduced grip. That's why a thicker arb makes that car more tail happy, but it doesn't increase grip or make it any quicker. You don't want to be lifting any wheels, the whole point of suspension is to keep the wheels in contact with the ground.

 

Depends on your budget and motivation I guess. Yes a LSD is expensive but it's widely regarded to be one of the most effective mods you can make. If you just want cheap fun then I'll agree that a thicker arb is the way to go but it won't be reducing your lap times.

[niklas 1]

I would put the LSD/ATB at the top of the list. It transforms the car really!

After that it's up to you how much more you want and are willing to pay for... Torsion/antiroll bars are expensive but the only real solution in the end!

[niklas 1]

That isn't entirely true, a thicker arb doesn't increase loading on the outside wheel. You still have the same weight transfer and body roll but the inside wheel is lifted so you get reduced grip. That's why a thicker arb makes that car more tail happy, but it doesn't increase grip or make it any quicker. You don't want to be lifting any wheels, the whole point of suspension is to keep the wheels in contact with the ground.

 

Depends on your budget and motivation I guess. Yes a LSD is expensive but it's widely regarded to be one of the most effective mods you can make. If you just want cheap fun then I'll agree that a thicker arb is the way to go but it won't be reducing your lap times.

 

When cornering you cannot use the suspension to alter the weight transfer left-to-right. When braking you cannot use the suspension to alter the weight transfer front-to-back... Only actual physical weight and placement of weight can do that. (Wheelbase and track are obviously also part of the equation!)

 

But you can and will alter the weight transfer front-to-back when cornering and left-to-right when braking, by adjusting spring rates and antiroll bar rates.

But you are right that lifting the inside wheel is often (mis)used to improve the balance. A front-heavy FWD needs a lot of work to get the right balance so removing rear grip is often used. But with the right spring/antiroll bar rates it's not necessary.

The problem with most pugs are that there are not stiff enough bars available! And there are obviously physical limitations to what can be fitted as well.

[Henry 1.9GTi 36]

as above and cont.. cannot add to or take away from the total weight transfer a car has, it only weights so much I went from no arn to 25mm and the difference in getting the power down with an open diff was very very noticeable.

 

Also controlling the roll center through a corner is desireable as with a roll centre moving around alot the car becomes unblances and unpredictable. Lifting a rear inside wheel is good in some circumstances as the roll center moves to the contact patch of the outside wheel and stays there; controlled.

 

It doesnt reduce your grip as its all now being generated by the outside wheel instead of spread across both.

 

all in all for the OP. You have billy's on the front with rear lowered to match. I'm assuming the billy's use stiffer springs so you need to stiffen up the rear to balance the car again. A cheapo rear arb will do this.

 

Henry.

 

ignore above about reducing weight transfer at the front by increasing rear... donno what I was typing and don't fully understand anyway

Edited February 16, 2009 by Henry 1.9GTi

[taylorspug 7]

24mm Anti roll bar will do it, and if that's still not working plate diff.

 

+1.

 

24mm rear bar and plate diff are the two biggest handling changes my car has had. Both me and James are speaking from experience of actually trying these parts, rather than just talking 'in theory' or gleaning information off of the forum.

 

Its funny the amount of people who have been in my car with stiffer bars, not one of them has commented on it having 'less grip' than the standard setup...

[Jon_Bmw 13]

People always seem to forget with a LSD that it will retain most of its value! Use it for a couple of years and sell it on for 75% of what it cost you. That is cheap depriciation in motorsport!

[niklas 1]

Its funny the amount of people who have been in my car with stiffer bars, not one of them has commented on it having 'less grip' than the standard setup...

 

I guess it's just what people call it...

Maybe not everyone knows the actual fact behind the saying, but what it really means is that using a thicker rear ARB increases the weight-transfer to the back, thus the rear wheels take more of the load. So you take advantage of the rear grip to a higher degree.

It's per definition not the same as having less grip but I guess we can say that at a certain lateral force (say 1g cornering) there are less grip available at the rear wheels _not being used_...

[saveloy 8]

As has been suggested, try altering your driving style. Less is more and all that.

Once on the throttle and understeer sets in, stay on the throttle, but dab the brakes with your left foot. Works for me.

[kyepan 291]

I would look into adding some negative camber on the front wheels (309 wishbones or adjustable topmounts) & an LSD to help get the power down.

Sorry to say I tried all of this and it didn't change the inherent balance of the car and still felt nose heavy and like a plough, not exactly fun to drive. the only way to get grip with that setup was to wind up the dampers to silly levels. granted it did turn in a bit more but it was still unbalanced and didn't feel right.

 

so when you load it up and it just slips at the front, or get on the power coming out of bends, there is an answer however

 

The first thing that made a real difference was a front solid strut brace, which i see you already have, as it kept the contact patch stable when loaded up in slower tighter corners, keeping it tracking where it was supposed to go.

 

 

was gonna say as above, big fat rear anti roll bar. Helps no end.

It did in my case, as stated help get the power down out of the corners but only because it marginally increased the rear weight transfer character the balance still felt wrong.

 

The thing that transformed the handling was matching the front and rear spring rates (22mm torsion bars) with the front 200lbs springs, added neggy camber arms to the rear too. You felt the front load up and then the rear after, rather than the front and then understeer. currently using a standard ARB too..

 

Now the dampers are wound right off to soft , the ride is excellent and it still has more grip than before because it's working effectivly, not overloading the front.

 

It's now pretty stunning to drive, in all types of corners from sweepers to little left right jobbies.

 

Also if you're getting any kind of axle tramp when the wheels are spinning you might look at reducing your unsprung weight, as this will help reduce the frequency of the tramp. Moving from 15"speedlines to 4.6kgs C5 steelies has eliminated axle tramp when locking up under braking and when lighting up with a heavy right foot. Now you get a clean squeel from the tyres not a chub chub chub sound.

 

Don't waste your money, match the spring rates first to get your spring rates balanced, then worry about the rest.

Granted torsion bars are expensive but cheaper than a 309 setup, diff etc, stiff rear ARB.

Edited February 17, 2009 by kyepan

[bales 1]

I think that with these sort of topics it is very easy to go off into theory about what should work and how certain things change the balance i.e roll centres, roll axis, weight transfer, camber, castor etc...

 

But I think in reality you really need to go off people who have raced them or have a lot of experience in setting them up directly.

 

For example when I did Formula Student at uni we spent ages learning all about the geometry and how to this effected the cars handling and we went into considerable depth in vehicle dynamics with lots of formula's and complex equations for weight transfers etc...so much that even now I can quote exactly what the theoretical effects of more rear roll stiffness is or how to keep the roll centres relatively stable during cornering or stuff that makes you sound like you know lots about designing suspension.

 

In reality though we built what should have been a perfect set up for handling, mid-engined, double wishbone suspension all round, nigh on 50/50 weight distribution with a driver, adjustable everything....did it handle well though....did it bollocks

 

Spent ages and ages trying to set it up and tweak it to get it right and thats with a supposedly perfect base, I guess my point is that with a 205 you arent starting with a perfect base and more than likely it will be skewed to handle one way more than another so just applying theoretical rules rarely works!

 

So erm....(ive gone a bit off tangent now) basically the people who have raced them and set them up and had a lot of experience usually know the little tweaks that work...and these don't always agree with theory so I guess my point is that just sayin add camber, make the back really stiff, raise the rear ride height doesn't always have the desired effect, and its easy to quote what you have read or what other people say...

 

This isn't meant to slag anyone off who said just make the rear stiffer or things like that, or equally to praise anyone who has ever raced a 205, but just that from direct experience suspension geometry and handling are very complex things and not as straight forward as I suspect people think.

[kyepan 291]

hehe, yes good point, i'm far from an expert, and have gone through quite an expensive trial and error routine to get it to handle "right", My "right" may not be someone elses cup of tea. let me reiterate that it was 90% error and only came together very recently.

 

Generally I go by what my bum says to me when driving and how the car behaves in different stages of different corners... I would imagine this is how it was done before the theory existed. So you could say it's about the experience for me.

 

Your also right, each change made a difference to one aspect of the car at one stage of one type of corner.. and these in combination affect differently to each in separation, so you're also right that just doing one thing will not necessarily "solve" the problem. Each change was made in (mostly) isolation, so first it was 250lbs front springs and adjustable dampers, then 200lbs springs, then 309 wishbones, then diff, then stiff rear arb, then strut brace, then standard rear arb and 22mm torsion bars + neggy camber rear arms.

 

However, i'll say it again, matching the spring rates changed the way it felt, the rear now feels more involved and actually like it's taking some load, for me this was the most important change. I don't know what this did to the level of grip, this may not have increased grip one jot, but it certainly changed the feeling from front heavy, to middle balanced, this meant you could carry more speed and were driving faster through the corner, not gunning it out of the exit because you have no grip half way through and had to slow down. won't pretend i understand why, but for me of each of the incremental changes it made the most difference to the driving experience.

 

however it sounds like this is a more complex system than one item in isolation....

Edited February 17, 2009 by kyepan

[James_R 3]

Jsut to counter what's been said, lap times went down on a std 205 with a 24mm aRB in at cadwell, mainly as the car had better track at the fornt (the driving wheels), it did require a different driving style, but it was quicker, mainly hanging the tail out. but then it's more about the grip balance than the out and out grip you have, no point having 3x the grip at the back it will jsut ploug on all the time.

 

I was answering the question of reducing jsut the understeer, not the finer balance of the car.

 

If you want a sweet handling neutral 205 then it's bars, altered geometry (which I've found 309 front end to not be enough) coilovers and matched springs, then there's the mine field of wat rates to go with aswell, depends what it's for, a full on track setup is much easier to spec than a road car as getting the comparmise is no easy thing, my car drives round by moving he back end around and useing the diff and power to control the direction mid turn more than the steering

 

Trying to remove mid corner tail wag is provign to be the biggest issue I've had when you start going stiffer and stiffer with the over all sus package.

 

I'll also add 309 beams are a cheap way to reduce the above, bit kill turn in compared with a similar 205 width beam.

Edited February 17, 2009 by James_R