924srr27L Lightweight Doors / Suspension / Bump steer etc..

[924Srr27l]

If I get a chance I'll run some weights for you to see what might still be available. I'm away currently so don't have access to a Blue Book. If someone can remind me of the tube specification I'll also run a mock up to demonstrate how heavy they aren't.

It is actually quite nice to have projects where you haven't reached the absolute end as, that way, there is always something more to do or to look forward to.

I would also seriously consider taking the car on some track days. It is so much more fun than driving on the road in this overcrowded, over restricted day and age. Having had a very light car it is rather nice to be able to "cock a snoot" at some of the more exotic machinery. Tight circuits are best though (Brands Indy, Mallory Park etc.) as othwise you get mullered on the straight by brute power.

Ok, thanks

Run some weights up on what ?

I did a PCGB Track day last November @ Oulton Park on Michelin Road tyres and it went well, but not good enough that it makes me

want to do more or Race it, because I'd then want it to be properly prepared and competitive (Caged and Race suspension and trailered to events etc..)

I've just done 300 miles in the 924 over the last 2 days across the country and back and whilst some parts of the 6 hours in total were fun.

generally the rest of the journey was too harsh and uncomfortable. All due to the Bilstein B6 Mono Shocks, stiff springs and the bumpsteer.

I need to decrease the front spring rates, possibly the rear T bars also, and sort the wishbone / track rod angles asap!

Also the R888 / NS2-R track tyres are coming off, and I'm going back to softer walled Pilot Sports'

R

 

[944 man]

I doubt that you have bump steer. Bump steer is where the wheels angle of attack changes as it moves through the suspension range. What you will have is hitting a bump and it bouncing you off line.

 

[924Srr27l]

I doubt that you have bump steer. Bump steer is where the wheels angle of attack changes as it moves through the suspension range. What you will have is hitting a bump and it bouncing you off line.

The car never had it when it was at the Standard ride height, the lowered front ride height reduction changes the wishbone & Track control arm

angles.. and makes the car's Roll centre worst than standard.

Standard ride height

Lowered ride height...

Bump Steer which is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the road interacting with improper length or angle of your suspension and steering linkages.

The solution is to lengthen the lower wishbone ball joint & Track rod points with Steel Stems..





R

 

[944 man]

I KNOW what bump steer is Roger.

Unless you have interfered with the steering rack's mounting or ridiculously lowered the vehicle then it wont be bump steer, it'll be bumps knocking you off course. You should always use the softest springs that will do the job. True bump steer, should you ever encounter it, is rectified using shims, lots and lots of tea, quite a lot of swearing and a hell of a lot of time.

 

[924Srr27l]

I KNOW what bump steer is Roger.

Unless you have interfered with the steering rack's mounting or ridiculously lowered the vehicle then it wont be bump steer, it'll be bumps knocking you off course. You should always use the softest springs that will do the job. True bump steer, should you ever encounter it, is rectified using shims, lots and lots of tea, quite a lot of swearing and a hell of a lot of time.

I'm not convinced you do know only Your description doesn't add up

"Bump steer is where the wheels angle of attack changes as it moves through the suspension range" ?

Your description that the "wheels angle changes" when the car is compressed and rebounds over bumps does not cause any bumpsteer issues on standard ride height cars because their design and geometry is set to minimise & eradicate this.

It is happening on my car when I encountering bumps in the road because the lower wishbone and track rod angles are not parallel to the ground but at an approx. 12% angle. As per the diagrams the Roll Centre position has now changed to an adverse position and is worst than standard.

Some racers (Not all) that have identified this, hence they change and lower the 2 connection points (Track rod & Wishbone)

or the rack could be lowered too as Porsche did on the 924GTP le mans cars.

(but this would also require a longer steering shaft and I'm not going down that route! )

The 2 images (post 83) of the modified lower wishbone and stub axle are on loan to me from a Championship Winning Race team, these items

are from a very Successful Race car which had the Geometry corrected to eradicate the (Same) bumpsteer issues I'm experiencing.

Fitting Shims on any component is not going to set the wishbone and track rods straight, the 2 extension pins on the Race stubs are 50mm.

It's all in hand and will not require any swearing or tea, just some drawings and the services of my machine shop and Fabricator.

R

 

[924Srr27l]

Some More Suspension Tuning Info:

- If you reduce your ride height by an inch, then you will lose 1" of total compression in your suspension travel. To compensate, the spring rate must be raised. Usually an increase of 15% to 20% will do it.

- If you do not compensate for the reduction of shock travel, then you will bottom the shocks before the suspension has hit full bottom.

- Never set the front so low that the lower arms are inclined upward toward the ball joint. You will never get the front aligned properly and the camber gain curve will be very erratic when the suspension compresses with considerable bump-steer.

R

 

[924Srr27l]

The MK1 Golf boys like their ride height slammed and hence with the same strut design (That's where Porsche got it from)

they also have the same Bumpsteer issues...

Here's a pic where the extensions have been fitted and you can see how flat the wishbone and Trackrod is.



Here's an extension pin for the Track control arm..



And another example on a lowered 928..



R

 

[924Srr27l]

I KNOW what bump steer is Roger.

Read this thread and you see more references to the issues with Bumpsteer with lowered cars, then you'll really know what it is.......

Hopefully.

https://rennlist.com/forums/944-turbo-and-turbo-s-forum/849644-my-front-roll-center-bump-steer-solution.html

R

 

[John Sims]

Hi Roger,

I think you will find Simon knows what he is talking about, but the above is all interesting information.

My understanding is bump steer is a bid of a misnomer as it isn't bumps making the car steer as such, the effect is more prevalent when the suspension compresses on one side due to corning forces. But none the less I agree it would be better to get back to a more neutral geometry. I would be a little wary of the extended ball joints. I did it to mine and they broke.

It could also be that the suspension isn't set up ideally for the current configuration. Setting up suspension is a bit of a black art and just because the readings are within limits for the standard car doesn't necessarily mean it is the best setup for a car with revised suspension. Even manufacturers can get it wrong.

You may find that you have come to a point where the car is too light for the suspension (or the suspension is too stiff for the car). Light cars are a great deal more difficult to get to run smoothly on the road - especially the state of roads in the UK. Sluggy was obviously set up for the track and, on the road, he was like some maniac terrier. Every pebble you ran over was likely to spear you into the nearest ditch.

With regard to running weights - as mentioned above - I can do an estimation on the weight of the underseal etc. I am a bit busy currently so it wont be for a few days.

And a final aside - you shouldn't really use other peoples images from the internet. I appreciate they are in the public domain but this is something of a grey area currently in respect of copyright. Also be aware they are often trying to sell a solution or it is someone trying to justify having bought something. Just because it is on the internet doesn't make it gospel.

 

[924Srr27l]

Hi Roger,

I think you will find Simon knows what he is talking about, but the above is all interesting information.

I know I have excessive bumpsteer issues but (somehow?) he doubts I do ?

He's also suggesting the cure is shims which will not sort the angles at all, So I'm not convinced he does

(know what he's talking about) on this subject.

My understanding is bump steer is a bid of a misnomer as it isn't bumps making the car steer as such, the effect is more prevalent when the suspension compresses on one side due to corning forces.

It's on the straights where it's worst with a bump for both sides, not one side or in cornering.

But none the less I agree it would be better to get back to a more neutral geometry. I would be a little wary of the extended ball joints. I did it to mine and they broke.

What type did you fit ? ..welded on or bolted? what length, where from and how much lower than stock was the ride height?

It could also be that the suspension isn't set up ideally for the current configuration. Setting up suspension is a bit of a black art and just because the readings are within limits for the standard car doesn't necessarily mean it is the best setup for a car with revised suspension. Even manufacturers can get it wrong.

I disagree, it's only a black Art when it's not understood. The stock standard set up on the 7 other transaxles I've owned did not have any excessive Bumpsteer issues at all, because none of them were lowered.

You may find that you have come to a point where the car is too light for the suspension (or the suspension is too stiff for the car). Light cars are a great deal more difficult to get to run smoothly on the road - especially the state of roads in the UK. Sluggy was obviously set up for the track and, on the road, he was like some maniac terrier. Every pebble you ran over was likely to spear you into the nearest ditch.

The spring rates were calculated and selected as a base and this will be the first change, the original Shock absorbers were Spax Gas on car (adustable units),

I've since changed them to Bilstein B6 as I felt they were not performing good enough. However the B6 non adjustable valving is too Stiff and this has made the car more harsh with less travel speed, so I may also consider and try a more basic shock set up (Sachs or KYB) as well as lower spring rates. I'll be heading toward rates similar to standard cars (150Lbs) which is half what some 944 Suspension kits are but they are for 1.5 ton cars of course..

With regard to running weights - as mentioned above - I can do an estimation on the weight of the underseal etc. I am a bit busy currently so it wont be for a few days.

Don't worry thanks, I don't think I will go down this route. Once the Suspension is done, that 'll be it till next year then I may consider a 3.1 Litre Engine N/A 8v on ITB's.

R

 

[John Sims]

Roger,

I have PMed you. Please consider what I have said.

 

[924Srr27l]

Roger,

I have PMed you. Please consider what I have said.

That's Interesting............[]

Thanks

R

 

[John Sims]

Roger,

I have PMed you. Please consider what I have said.

That's Interesting............[]

Thanks

R

 

[John Sims]

With regard to the failed ball joint it was the Rennbay kit. I had been less than sympathetic with the curbs at Bedford when it failed but I'm also not sure the so called race car specialist who did the work for me was quite as good as he thought he was.

 

[924Srr27l]

With regard to the failed ball joint it was the Rennbay kit. I had been less than sympathetic with the curbs at Bedford when it failed but I'm also not sure the so called race car specialist who did the work for me was quite as good as he thought he was.

Ok but this sounds like the replacement Balljoint kits for the original wishbone?

Only I'm not using the OE balljoints, but a High quality 2 or 3 piece Male Rod End, and as illustrated longer Steel stems to move the positions lower as per the pics on post 83 for the wishbone & 87 for the Track rod end.

R

 

[John Sims]

With regard to the failed ball joint it was the Rennbay kit. I had been less than sympathetic with the curbs at Bedford when it failed but I'm also not sure the so called race car specialist who did the work for me was quite as good as he thought he was.

Ok but this sounds like the replacement Balljoint kits for the original wishbone?

Only I'm not using the OE balljoints, but a High quality 2 or 3 piece Male Rod End, and as illustrated longer Steel stems to move the positions lower as per the pics on post 83 for the wishbone & 87 for the Track rod end.

R

I am sure it will be a much better solution. I was never 100% happy with the extended ball joint although it was the assembly that failed not the pin. Perhaps if the conversion had been better executed (and I hadn't been monstering kerbs) it would have been fine.

With regard to the black art of suspension, every car is different. As I suggested, even mainstream manufactures and F1 teams can get it wrong. It is often not one specific thing as so many possible variations can have an impact on other things. We can all read books on the subject, run simulations, (well perhaps not everyone can run simulations), trawl the perceived knowledge and half truths of the internet but, at the end of the day, you need someone with Jedi like qualities to set it up and get it all to work properly.

 

[Eldavo]

Am I right in my understanding that the suspension arm itself and the length of the ball joint remains unchanged - this kit adds a spacer/longer rod for the track rod end to sit parallel?

If so, I'd be interested in one. I have 968 front arms on a substantially lowered car and don't want to mess about with changing the ball joints but would consider a solution that levelled off the steering arms.

 

[924Srr27l]

Am I right in my understanding that the suspension arm itself and the length of the ball joint remains unchanged - this kit adds a spacer/longer rod for the track rod end to sit parallel?

If so, I'd be interested in one. I have 968 front arms on a substantially lowered car and don't want to mess about with changing the ball joints but would consider a solution that levelled off the steering arms.

Ideally you need to extend both to fully cure the excessive Bumpsteer issues.

The wishbone and the track rod, so their positions are both back where they would be

on a standard ride height car 44 or 68.

But if you only want to change the Steering Bumpsteer issues, (Track rod) something like this Kit will suffice:

http://www.elephantracing.com/suspension/bumpsteer/944bumpsteer.htm

Search for Bumpsteer Kits Porsche 944, most are from the US

If you wanted to extend the wishbone also without changing it, you'd need an extension to be made.

The Golf Mk1 has a 17mm Pin and they use something like these,

Ideally they need welding to the Stub axles...

Which is why I'm doing the lot and making Modified wishbones with a Male Rod end, then a Stem will be welded

to the stubs, and also the track rod will have an extension welded on to lower the Track Rod ends which will also

be fitted with M14 male Rod ends.

The VW Golf track rods originally fit on top of the Stubaxle arm, so they have a "Flip kit" where the stub axle is drilled straight

and not left tappered which allows the arm to be fitted underneath with a Spacer.

R

 

[924Srr27l]

...this is a 1970's designed car and will only need the Alignment checked once the wishbone and steering arm have been extended no need for any specific rare Jedi set up's etc..

A fair point except, having dramatically changed the weight of the car and potentially the centre of gravity as a result, you may benefit from consideration of spring rates, compression and rebound etc. You have already mentioned this above and this could well manifest itself in the symptoms you have already outlined.

My apologies Roger - I edited your post rather than replied to it. It has never happened before. Sorry about that, I can't retrieve the old post content otherwise I would.

John [&o]

 

[924Srr27l]

...this is a 1970's designed car and will only need the Alignment checked once the wishbone and steering arm have been extended no need for any specific rare Jedi set up's etc..

A fair point except, having dramatically changed the weight of the car and potentially the centre of gravity as a result, you may benefit from consideration of spring rates, compression and rebound etc. You have already mentioned this above and this could well manifest itself in the symptoms you have already outlined.

My apologies Roger - I edited your post rather than replied to it. It has never happened before. Sorry about that, I can't retrieve the old post content otherwise I would.

John [&o]

No Probs,

My Car's ride height was set up on corner weight pads / scales and has a 255kg Front / 250kg Rear average on each corner.

The spring rates are 190lbs front / 220Lbs rear, ARB's 26.8mm front / 18mm rear and the balance is exceptional with no understeer or oversteer, but the excessive bumpsteer and roll from the wishbone & Track rod pick up points is affecting the car's Roll and straight line stability.

So once this is sorted it will I'm sure be a dramatic improvement as also will a decrease in front Spring rate down to 163lbs (Hyperco 2.5ID Race coils) and I'll also back off the camber from -2 front / -1.5 rear to -1 all round which will make the car a lot more drivable for the road and also the front will have more bite and handle better with less roll once the bespoke wishbones and stub axles are fitted.

A lot of people idea's on handling can only be what they have experienced and done on the road and circuit (Competition)

Many years ago when I was racing I had an invite to Bruntingthorpe to test a Prototype which went well,

1000cc R1 Engine, Spaceframe lightweight chassis / body (550kg) and a 6 speed sequential box etc..

My friend Jon Lee built and successfully raced this car against radicals etc..

This car later on had a Hayabusa engine fitted.

For those that fancy an original Mini but with 147bhp, he currently sells a Kit to fit an R1 Engine!

www.lynxae.co.uk

R