rwd and rear antirollbars

[Kev_205]

Do rwd race cars need a rear antirollbar?
My brother has an ex rallycross 309 which is being converted to run honda s2000 engine and drivetrain. The rear antirollbar was huge so it was ditched.

Should we be looking for a smaller one or are we fine without?

[Al Weyman]

Depends on the car of course but IMHO yes definitely unless you like a lot of understeer. I have just upped the rate on my Historic Camaro from 5/8" to 1" and it feels a lot better.

[Notso Swift]

I tend to think of it this way, (although it was 10 years ago when may last track car was RWD) you want the springs to be soft enough so you get good traction, but you want the car to roll around like it is at sea, solution anti-roll bar!
You may want it smaller than the OEM (especially for dirt), but I would still want it

[greenamex2]

If it has a live rear axle, probably not.

If it has an independant rear end then it's a tuning option.

On my Corolla (live rear axle) it oversteer if you fit a rear anti roll bar, no matter what spring and damper rates you use.

[GordonG]

Live axle will have a relatively high roll centre so the geometry is giving some roll resistance by itself, hence you probably dont need an ARB. Double wish bone will have the roll centre lower (or wherever the designer decided he wanted it) so may or may not belefit from a roll bar of some form.

I view it that to get a balanced car with traction you need a certain level of ride stiffness (keep the wheels on the track, off the bump stops/skid plates) and a certain level of roll stiffness (versus the front). If your required ride stiffness (springs) does not give you enough roll stiffness, you need a bar. You may be able to get balance with acceptable roll by tunung front roll stiffness (bar, as it's adjustable) and avoid the weight and complexity of the rear bar, or you may not, depending on your car and a host of other factors.

G

[zefarelly]

its interesting, at least to me, looking at LC's at CC over easter, the wuickest car/driver set up where FiA spec leaf/radius arm set ups, there where several Cortinas with alternative modified set ups, rear ARB, Panhard rod, additional radius arms etc, none of them where that quick.

I have nothing other than single leaf springs on my car and I beat several others

[Kev_205]

well the Honda s2000 uses a double wishbone setup rather than a live axle

[GordonG]

The S2000 is a low sports car and (at least in std trim) the CoG of the 306 will be higher. You are presumably going to jack the suspension up for ground clearance in the rallycrosser, which will raise the roll centre too. Overall the effects could go either way. You either need to do some high power analysis or suck it and see with a range of bars available...

G

[Al Weyman]

As I said it is all down to the car, the reason for one on the Chevs is because thay are nose heavy and tend to understeer so the roll bar neutrulises that to a certain extent, the best solution would be to cure the understeer in other ways not by getting the rear to step out but if by the design of the car you have no choice then roll bars can be very effective, ask Dick Guilstrad the Camaro tuning experts and they will swear by them where as Herb Adams a rival does not. Its all horses for courses and what suits you and your style of driving as well I quess.

[Notso Swift]

Originally Posted by Denis Bassom

If it has a live rear axle, probably not.

If it has an independant rear end then it's a tuning option.

On my Corolla (live rear axle) it oversteer if you fit a rear anti roll bar, no matter what spring and damper rates you use.

That would be because the live axle is working as a anti roll bar, the spring on the opposite side is having an effect on the other wheel. More travel changes that, not just spring rates, (mind you, sometimes you cannot phyically get enought travel).

[coln72]

I did the same as Dennis - removed my rear bar. Sick of snap oversteer on dirt (in third gear!!!!). No rear bar improved it so I softened the rear springs as well.

Depends though on the course tight and twisty dont use one, fast and flowing then go for it.

[Al Weyman]

I think it may also depend on how tight your diff is locked up as well, your spring rates come into it as does the size of your front bar, all I can say is fitting a large bar on my Historic car it feels much more stable and drivable than the understeering pig it was.

[greenamex2]

Originally Posted by Notso Swift

That would be because the live axle is working as a anti roll bar, the spring on the opposite side is having an effect on the other wheel. More travel changes that, not just spring rates, (mind you, sometimes you cannot phyically get enought travel).

I don't have any suspension travel.

If I did then I would just get several degrees of bump steer on my rear axle and would then also be fighting geometry problems.

Must send Toyota a link to one of those freeware suspension design programs some time!

[phoenix]

Originally Posted by Kev_205

Do rwd race cars need a rear antirollbar?
My brother has an ex rallycross 309 which is being converted to run honda s2000 engine and drivetrain. The rear antirollbar was huge so it was ditched.

Should we be looking for a smaller one or are we fine without?

The 309 was front wheel drive with a beam axle at the rear, give a roll centre at road level. In order to give good traction (at the front) and to control roll the rear bar is made very stiff and the front one quite light.

In a rear drive situation you more than likely want the reverse, with the front having more roll resistance than the rear. This may mean no rear bar at all, but certainly a lighter one than the original setup. This will depend a lot on the current rear suspension configuration and the roll centre height. I should try it without to begin with and see how it feels and handles. The double wishbone setup is LIKELY to have a roll centre above ground level, but lower than a live axle so I think an adjustable, light rear bar may be a good idea for tuning.

[dtype38]

Originally Posted by Kev_205

Do rwd race cars need a rear antirollbar?
My brother has an ex rallycross 309 which is being converted to run honda s2000 engine and drivetrain. The rear antirollbar was huge so it was ditched.
Should we be looking for a smaller one or are we fine without?

Correct me if I'm wrong someone, but I thought the antirollbars were there primarily prevent the tyres rubbing against the wheel arches, and as a secondary effect, to prevent the suspension geometry from changing too far and causing problems. Surely if neither of these are a problem then there's no need for the bar. If the car has understeer problems then wouldn't it be better to soften the front antirollbar than add a rear one?

[Al Weyman]

Never ever heard of that being the reason for anti-roll bars dtype to be honest. The Americans call them anti-sway bars so I wonder if that is more descript.

[Goran Malmberg]

A car is longer Wb than Tw. This means that if subject to 1 g of force from braking or acceleration it will deflect less than from 1 g of force sideways. So, we need so much bar as the car has less Tw than Wb as a rule of thumb. Bars do also serve as an easy adjustment for over-understeer.
The trick is to come up with a spring stiffness that in an effective way make use of the effective wheel travel in question, in all directions. This means that ride height, suspension geometry, tire grip etc must be taken under concideration. It is many times very important to keep roll-camber under control, so swaybars is good in moost cases.
Goran Malmberg

[dtype38]

Originally Posted by Al Weyman

Never ever heard of that being the reason for anti-roll bars dtype to be honest. The Americans call them anti-sway bars so I wonder if that is more descript.

Well I don't use 'em, and my car definitely doesn't "sway" But I have just remembered another use for them... they can be disconnected when it rains

Goran, surely most cars will pull much more "g" under braking than in a corner, so you would need higher effective stiffness longitudinally than laterally.

[Al Weyman]

Yes but look at your car Dtype, all fibreglass body, no roof therefore exteremelly low centre of gravity and an IRS set up so I am not surprised it does not roll much.

[Notso Swift]

An anti-roll bar is effectively a torsion bar that links the right and left. at normal ride hight (or indeed when the spring compression/extension is even) there should be no tension on it. Lateral control of the axle is certainly outside the scope of an anti-roll bar. That is the job of a Watts Link or Panhard Rod in a beam axle. A de Dion has elbows that do that, for an IRS. Wishbone suspension will be 100% controlled by the… wishbones. The anti roll bar is not a suspension link, but a spring link.
Do you need the bar? Well, no, but you a may want it - less spring force when the wheels are level, more when they are not.

[johnny yuma]

most front engine/rwd modified road cars on the track run as big a front bar as they can and little or no rear bar.This is useful in getting the power down without the back stepping suddenly way out-especially on flat chat 3rd gear sweepers!!! If you don't happen to have a good limited slip or locked diff,a back bar is a disaster as it just picks up the inside rear wheel and you get more smoke than Thomas the Tank Engine exiting tight corners.laugh:

[coln72]

Been there done that.......

[Al Weyman]

I don't agree, the larger bar 1" rear bar I have fitted on my car is a definite improvement, I do have a 13/8" bar on the front though. I just could not get the rear to kick out and was getting terminal understeer, it seems far more stable now. I will see how it goes Thruxton may 6th as my session with this car was cut short at Snetterton but for the 6 laps I did it definitely felt good.

Also and unfortunately I do not know the guys name but he is or was a Mallory Park based suspension tester and Guru, anyhow he gave a mates Camaro the once over and his first recommendation was a 1" bar on the back which immediately paid big dividens (I could not catch him for a start!) so I don't think there is any hard and fast rule, it all depends very much on the individual car.

[Goran Malmberg]

Originally Posted by dtype38

Goran, surely most cars will pull much more "g" under braking than in a corner, so you would need higher effective stiffness longitudinally than laterally.

Right, and it will not accelerate by the same power as cornering, most likley. We have a theoretically spring to bar relation, but in the end specific factors for the car in question will govern the final roll stiffness and couple. The tire setup should balance the weight distribution, but if the car dont have acceleration grip enough, maybe some more rear rubber is a solution.

Goran Malmberg

[Al Weyman]

maybe some more rear rubber is a solution.

Which would be nice Goran but when you run in a championship (Post Historic Pre 74) that have a control tyre (225 x 55 x 15 Dunlop) and a maximum 8 x 15 diam wheel then you have to resort to other means.