Power Understeer

[MM89]

No difficulties turning in, but upon mid corner acceleration, car just understeers.

Koni 8041 Race - Front Rebound 0 degrees, Rear Rebound 180 degrees
Ground Control 10k/9k
25mm/23mm jdm itr ARB
Buddy Club polyurethane rear LCA

Static ride height is setup with 20mm rake nose down and plenty of rear mass removed.

Wheel alignment has not been done since arb were installed and ride height altered. Driveshafts sit flat and original arb sizes were 18mm front, no arb rear.

Without doing an alignment first, I'd like to explore what can be adjusted to ease the power understeer. I gather that the extra roll stiffness in the front from the bigger front arb will cause more push but also thought that rear bar would counter that.

Ideas?

[WMD]

The issue, from what I can read here is balance. Initial turn in is fine, therefore the front may not be too stiff for lateral weight transfer, however when fore-aft weight transfer takes place when you pick the throttle back up the rear is not stiff enough relative to the front to keep the weight over the front tyres. As soon as that weight transfer takes place then understeer occurs.

You appear to have already done some adjustment of the damping settings to stiffen the rear relative to the front, however it may not be enough (the range of adjustment is often not hugely noticeable). Also try tyre pressures - rear higher than the front for example - can help a bit.

Initially, the cheapest aside from these would be to go for a wheel alignment, increasing caster (and camber while at it) at the front to try and allow more grip on the outside front with more of a jacking effect. This may help. May not however cure the weight transfer issue under power.

the next thing i would think might help is an Anti Lift Kit on the front, which in effect should help stop the excessive weight transfer off the front tyres under acceleration.

After that, increasing spring rate on the rear to stiffen the rear overall and allow less fore-aft weight transfer would be something to look at.

These are the 'major' items I guess, then start looking at the more fine tuning options - ie sway bars.

Reducing the size of the Front ARB can be an option, allowing the car to sit more on the outside front under acceleration, which would work well with the above alignment.

Increasing the size of the Rear ARB is another option to stiffen the rear relative to the front. Sway bars however are fine tuning devices largely - the spring rates and damper settings will control the most part of it, assisted in this case potentially by an ALK.

[MM89]

I too have considered installing the smaller front arb but questioned whether it would decrease front roll resistance - more roll and hence unload the inside front wheel throughout the corner?

Regarding tyre pressures, I have been lead to believe that higher front pressures work better. In your opinion, what am I achieving by increasing rear tyre pressures?

Swapping the front and rear springs is definitely on the list to help solve the problem and while I agree with more rebound, I don't think that will eliminate the problem.

Adjustable castor and camber would be lovely, when I actually get access to it..

[WMD]

You will find a level of decreased roll resistance in the front through a smaller front RSB, however in my experience this roll can actually be beneficial. If you think of the purpose of the ARB, it is essentially a levelling device. So as the front 'rolls' onto the outside tyre during heavy cornering, squashing the outside front into the road, the ARB is attempting to level the car off, in effect pulling upwards on the outside front, opposing the lateral force. When you apply power to this situation it is very often enough to induce understeer. Of course if the car is too soft at the front it will roll too much, overcoming the outer edge of the contact patch and losing grip that way instead. But that doesnt appear the case here.

By keeping the rear stiffer it is more likely to rotate around the outside front which is being dug into the tarmac. This stiffness can be the result of the increased spring rates / damper settings (primarily) or larger ARB on the rear.

Tyre pressures are a matter of degree again. Too far either way will be counterproductive, however again (in my experience), the higher pressures at the rear/lower at the front (again, not talking 12psi, just a few psi difference front to back) will enable the car to sit on the outside front more effectively, whilst the higher pressures at the rear will increase the stiffness and help the rear rotate around the front more effectively.

For example, when running semi slicks designed to operate in the hot range of 37 - 42 psi (ish), could aim for 37-38 hot front psi, 40-42 hot rear psi.

[string]

Open diff?

[MM89]

Open diff.

[MM89]

LSD is CURRENTLY not an option too. I will install one when i open the box in a few months. Need money for oil cooler and brakes at the moment.

Btw, string, what static ride height does the spss3 like? I remember someone saying that they're not designed to sit much higher than a gapless fender. I worry that I'll get loads of bump steer and the rear lcas are close enough to flat anyway.

[MM89]

WMD I'll swap the springs over tonight first. See how that goes.

[WMD]

excellent- post back as to how it feels.

[MM89]

It's better but I'm still getting a but of understeer while I'm straightening up for corner exits.
It's understeering later though. And less.
Now that's out of the way I can move onto tyre pressures, damper adjustment, alignment etc.

[MM89]

After loosening the front arb, car feels WAY better. I'll be putting in the original 18mm front arb to see how that goes. steering although still light, feels much better. When I move the steering wheel, the car actually turns a whole lot more. Before it felt like a very long ratio steering rack. Car is even better now under power, but seems like it's very easy to induce oversteer on lift off, not that I mind.
Will report back after some more changes.