Porsche 718 Cayman GTS bumpsteer on a factory SPASM setup problem..any idea where to look ? 2

[FreshDriver]

Hello Everyone,

I have a problem with my Porsche Cayman 718 GTS.

There are two types available: a 10mm drop with PASM (Porsche Active Suspension Management) and a 20mm drop with SPASM (Sport Porsche Active Suspension Management).

Initially, my car came from the factory with the 10mm drop (PASM) suspension. However, I spoke with Porsche and they agreed to retrofit the 20mm drop kit (SPASM) for me. As part of the retrofit, they replaced the original Porsche parts associated with the 20mm drop setup, including the shocks, springs, and bars. They also reprogrammed the suspension module and all related components. Consequently, my car now has the same specifications as a factory 20mm drop configuration.

Upon receiving my car after the retrofit, I noticed that it had some bumpsteer. I discussed this issue with the service shop manager, who assured me that everything had been changed according to the manufacturer's guidelines and that the alignment was done correctly. However, Porsche claimed that bumpsteer is a "normal condition" for a 20mm drop car. Interestingly, my friend has the same specifications but their car came directly from the factory, and they don't experience any bumpsteer.

I'm at a loss about what to do next. Is there anything specific that I can check to identify the problem that Porsche might be overlooking? It feels like they are dismissing my concerns with the argument that it's a "normal condition." Any help or suggestions would be greatly appreciated!

Thank you all so much for your support!



Regards



Marcelo

 

[FreshDriver]

Hello All

I had my car aligned using the Hunter alignment values specified for the -20mm Porsche Cayman 718, which are included in the Hunter machine's database. I've attached the graph showing new reading, as well as the charts comparing the alignment before and after the procedure. Any comments or insights would be greatly appreciated.

Thanks for your comments

FD

 

[cibachrome]

Take video of the car being driven over a moderate symmetric bump. There's a good chance one of your electronic controlled dampers is out of action. (But the ride steer is still horrid)...

 

[FreshDriver]

Hello Cibachrome

Will take a video this weekend..

When you say "But the ride steer is still horrid)..."
You refer to the reading shown in the graphic after alignment ?

 

[BrianPetersen]

So your camber was out of whack on one side both front and rear (bad), and is now OK, and you had static toe-out before on both front and rear (bad), and now have slight toe-in both front and rear, which should be good. There's still a lot of bump-steer by the numbers, but it's now very close to the same left to right.

Repeat your drive-over-a-bump test, and report back with findings. I have a funny feeling that the car will behave now.

 

[cibachrome]

It's on the high side of just about every one of the many, many, many sports cars I have K&C data for. If so, 1 or more of your front tires ought to start showing shoulder wear.

Static toe-out is a cheap Ackermann steering patch, also bad for adding understeer, reduces max lat, and makes the steering gain nonlinear thruout the range of 'sporty' driving. Especially on tires as oversized as come with this make/model.

 

[FreshDriver]

We took two cars to the hunter machine: my car, equipped with a retrofitted 20mm drop Porsche suspension, and another identical car with the same suspension, but factory-installed. Essentially, we had two identical cars, both with the same suspension setup; one was installed by a Porsche dealer, while the other came with it as a stock setup. The car was slightly out of alignment, but you can still check the toe movement during compression of the suspension on a factory stock suspension.
How do you see it compared to my car ?
Here is the graph from the other car:

 

[sierra4000]

Are very similar bumpsteer values for both cars,

after alignment your problem still remains?

then it really seems that "only" damping problem
when different damping from left to right creates different instant ride height this mean different toe angle for opposite wheels
or
also different damping will create different wheel grip (during wheel vertical vibrations) over highfrequency road unequalities

what your wheels size? (unsprung weight)

Radek

 

[sierra4000]

Go to damping test ,
of course your problem also can comes from rear axle

 

[FreshDriver]

Are very similar bumpsteer values for both cars,

after alignment your problem still remains?

then it really seems that "only" damping problem
when different damping from left to right creates different instant ride height this mean different toe angle for opposite wheels
or
also different damping will create different wheel grip (during wheel vertical vibrations) over highfrequency road unequalities

what your wheels size? (unsprung weight)

Radek

I have 20" inch wheels
Any way to measure the damping ?
After alignment have not checked carefully, will do it today.

 

[BrianPetersen]

At this point I'd say the bump-steer that you have in the car, is what Porsche designed it to have.

Damping is not easy to measure on the car. Still ... Do your drive-over-an-uneven-bump test (ideally with both cars), and get back to us.

 

[cibachrome]

You might want to compare your car's data with values from a similar model. Even their best is not exactly symmetrical, eh ?

https://i.imgur.com/IY4I6x3.jpg

 

[sierra4000]

According by wikipedia, Taycan have different weight distribution, can be reason? Interesting asymmetric values,but is really under 2Dg/m difference too much?
I love real cars K&C data !!!

it would also be interesting to know the compliance data of both cars

Some workshops measure damping on the car,i think in this case can be sufficient for dampers comparison.

https://www.youtube.com/watch?v=mGxNRF7W9jA

Radek

 

[cibachrome]

Your damping is speed dependent and most owners have the wheel rolling when they drive over bumps. Somebody would have to trick the car into setting all the speed dependent settings. Usually done with a transmission hack.
As for the compliances, I see no reason that they would be any different essentially for the same basic architecture. Manufacturing issues and net build processes are the reasons for most differences.

BTW: you can easily figure out what the ride steer would be with a 20 mm change in static ride height. It's probably quite a lot. BTW2: the car I showed you data for has ROLL STEER values of 8% exactly on both front wheels. So, same symmetric roll steer but asymmetric ride steer tells me exactly how carefully your OEM build was done. Guess the cause !!

 

[BrianPetersen]

Yes, Cayman (original post) is not Taycan (that link) but same manufacturer so maybe similar design philosophy? Internet says Cayman has 55% rear weight distribution, Taycan is very close to even split. That kinda suggests the Cayman needing more understeer from its geometry. The way I'm reading it is that Cayman (original post) has something like double the bump-steer of Taycan, but what's also striking is the substantial difference left-to-right in the Taycan.

A 20mm one-wheel compression bump in the original-post Cayman makes a 0.18 degree toe-out steer angle change on that wheel. If it's a true one-wheel bump then the other wheel stays put and the steering mechanism splits the difference roughly speaking, so 0.09 degrees of steer-angle change. If the steering ratio is 15:1 then the steering wheel would, ideal-scenario, jiggle by 1.4 degrees. That's not much. Reality is probably less than that because presumably the front instant-center is probably above ground level which means that wheel would simultaneously have its contact patch move outward from the vehicle a smidge which kinda-sorta agrees with that wheel toeing out a smidge ... keep in mind also that the rear is probably toeing in a smidge when it goes over that same bump one wheelbase-length later ... still, all these effects are looking pretty small.

We still haven't heard from the original poster whether getting a proper alignment sorted this out.

 

[cibachrome]

I tend to think hes getting a wheel kick from the power assisted steering. Either electric or hydraulic reaction. Can't clamp it down because of evasive maneuver requirements. Flunko de Mooso Testo.

 

[sierra4000]

I usually find an asymmetric bumpsteer on steering rack inclination, different KPI or with different caster.

Radek

 

[FreshDriver]

Here are two videos.

First one is using normal suspension setting, second one is using sport suspension(stiffer) setting.

First Video

https://www.youtube.com/watch?v=37jCYJl2WpY

Here, the vehicle is passing over a diagonal highway joint, and you may notice steering shake, indicating that the joint is not parallel to the wheel links. When passing over a joint that is parallel to the wheel links, there is no steering shake.

https://youtube.com/clip/Ugkx9fSjZ4eFsg5CYtQAgAu9hl6pRlefXaTC?si=vN347E2cHlsC6zVa

Here is speeding up to 110MPH

https://youtube.com/clip/UgkxZD4Xa75Fqh8bTn2wvvC1uY6cZvtWcAqy?si=iXGDeHETagVMHPsb

Second Video using stiffer suspension setting.

https://www.youtube.com/watch?v=cKVlCk-lgqE

Here is a jumpy road surface

https://youtube.com/clip/UgkxB_2ZHgwbrhk-1hV4L9OUqJydI3iI_zU2?si=DQKDFQEsz-3lACcx

Your feedback is very much appreciated.

 

[SwinnyGG]

Looks pretty normal to me.

 

[sierra4000]

I noticed something similar with a series of offroad vehicles with oversized wheels (low ET big width,big diameter,big unsprung)

Radek

 

[BrianPetersen]

I see nothing out of the ordinary.