LVDell

Do you have adjustable toe arms with bump steer adjustment in the rear? What about your ride height? Have you had a full chassis set-down and have the shop bump steer the car?

Masa^^

I have standard suspension, so everything is stock. My alignment is -3 camber in front and -1,5 rear. Toe in on front and rear axle. front is lowered, but only marginally. So, I don't have adjustable toe arms, do you see that it would be recommended to have those, would they correct my problems?

LVDell

If you have stock toe arms, then upgrading to adjustable ones would be a good start and go a long way in allowing your shop to properly setup the car to avoid bump steer issues.

schmidtwerk

I changed my entire suspension over to factory 996 GT3rs including RS springs and shocks. Only a slight improvement is bump steer. I would say the spring/strut combo is too soft, I driven other GT3s with Motons and stiff springs with WAY less bumpsteer.

Masa^^

Okay, thanks for info!!
I think that I am going with adjustable toe arms and if the problem still exists, I will wake this thread up again and ask for more help

Atgani

Sorry it's taken me four years to respond

It was a compliment !

As I said in my previous post, the term bumpsteer is used by journalists (and others) when bumpsteer is not the issue.

Bumpsteer can be measured using a specific gauge for the task, or it can be done "manually" thus : (warning it's a long and involved process) !!

Ensure the cars geometry and ride height are set accurately within the manufacturers tolerances. Then jack the vehicle up off the ground and place on level chassis stands.

Remove the road wheels, dampers and springs and then remove the springs (and bumpstops) from the dampers. Refit the dampers to the car without the springs and bumpstops.

Refit the wheels and apply the alignment equipment.

With the suspension at full droop take front and rear toe readings. Raise the suspension (measured at the stub axle centre) equally side to side (by a figure of your choice, say 5 or 10mm) and again take the toe readings.

Continue taking readings at every 10mm of suspension compression until you reach the end of the supension travel (full compression)

Ploting the supension travel against the toe readings taken will give you a bumpsteer curve. The results on a well engineered suspension will actually be much smaller than you'd think.

However if any bushes are worn, a suspension component is bent, OR your static toe settings/ride heights are incorrect, bumpsteer readings may vary from side to side which in turn may cause instability on bumpy roads.

Cars that have been subject to drastic lowering frequently suffer bumpsteer issues as the original rack mounting points were not engineered to run with such heavily reduced ride heights.

The hub and uprights on the 996 RS were specifically re-engineered to eliminate bumpsteer with different pick up points and steering arms.

Theoretically a car such as the 996 GT3/RS with such short travel should not suffer bumpsteer issues. And on that basis I'd say the OPs issue is purely an alignment/ride height one.

The designers of a WRC cars suspension (which can have anything up to 8"/20cms of suspension travel) go to great lengths to eliminate bumpsteer.
The components required can be expensive however . . .


(Click on these thumbnails twice for hi-res images)



Different steering arms are utilised for different surfaces (clearly a tarmac car runs far lower than one being used on a loose surface forest event where ground clearance and suspension travel are paramount)




The steering arm (clevis) can be seen in this image. Differing spacers (arrowed) are also available to fine tune any remaining bumpsteer out.



HTH

LVDell

Theoretically? Not really.

The two big issues at play here are (1) substandard toe arms that DO deflect under travel and (2) ride height.

The easiest was to fix this is to replace the stock toe arms with adjustable toe arms. Not just toe in/toe out adjustment but rather BUMP STEER adjustment on the vertical plane at the mounting point connecting to the wheel carrier.

Attached is a picture of the PMNA units from my GT3.

Atgani

Hi LVDel, I'm not clear as to wether you're concurring with my comments or disagreeing ?

I note your car has Moton dampers fitted, do I take it your car is used mainly for track use ? And that the issues you experienced (and required you fitted the modified components) were experienced during track use ?
You'll note the OPs issue was that his car was proving difficult to drive on bumpy roads.

I never experienced any noticeable bumpsteer whilst driving my bog standard 997 GT3 on the road (and Englands roads aren't the best) !

But if you were experiencing bumpsteer on track ? (I take it that means you have poorly surfaced, undulating/bumpy cornered tracks) ? then yes I've no doubt that uprated front toe arms would be beneficial, remove unwanted compliance and lead to a reduction in less than optimal geometry in extremis. For road use I'd say both the components you replaced would be beneficial but not essential for "bumpsteer free" driving.

In MY experience Porsche have engineered the bumpsteer out of their suspension/steering systems. Having worked with a geometry specialist in the UK it became noticeable that the majority of bumpsteer issues were not inherant design issues, merely set up issues. in the main caused by incorrect ride heights front and rear, and incorrect front camber and toe settings.

However on frequent occasions the front wishbone bushes were found to have delaminated or worn. When these were replaced and the manufactures settings applied accurately to the vehicles in question the previous "bumpsteer" issues were banished.

In extremis (and with proper slicks on track) yes the standard rubber bushings will be deficient and deflect excessively. I had just such a problem with the rear toe links on my 996 GT2. However for road use and with the manufacturers recommended settings (and ride heights applied) the stock components will not induce bumpsteer.

Playing devils advocate, is it possible that the fitting of the components you mention was combined with an accurate geometry set up and possibly THAT (rather than the fitting of the components themselves) cured the issues you were experiencing ?

I'm all to aware of several low mileage (and less than year old) UK 997 GT3s that were found wanting in the handling department. On being checked, their geo and ride heights were found to be a long way from Porsche AGs recommended settings.

LVDell

Both.

I think the problem lies in the fact that we are talking about a 996GT3 driven on the track and you are talking about a 997GT3 on the street. At regular ride height with factory settings driven on the street you're not going to experience the bump steer we are talking about having on the track.

As far as proper geometry, it will only be achieved with the parts I referred to. It's a combination of parts and a setup guy that knows what they are doing.

va122

I agree were talking apples and oranges here. Road and track.

bobsan

hay Masa, just spotted your post, I'm new to this forum and Porsche, but do know fair bit 'bout suspension,
you're in the right area when you notice the ARB (swaybar) makes a difference. have to remember there are two springs working on each corner of the car, the damper spring plus the bar which acts as a spring also.
These two must work in harmony with each other, the bar must complement the damper spring not fight it,
this is where I think bump-steer and also tram-lining which is often misread to be bumpsteer is being induced. Try disconnect one end of the bar (push the toe link out the way) and run with just the dampers, see how that feels, if the bars adjustable bring it up in tension one hole at a time, if you could see the two spring on a graph, you'd want the bar coming up underneath the spring in tension terms and never rivalling the damper spring. Trial and error im afraid, good luck.