Anti-dive spacers

[Flieger]

Moving the rear of the axis inwards would couple the longitudinal brake thrust to a vertical displacement of the wheel, in my quick mental picture.

[WinRice]

Max,

Here's a few pics that might show the relationship better.

Notice how close the top of the crossmember is to the body seam.

[Flieger]

Interesting. It looks like the bearing mounts further outboard than on my 1971 911S. My A-arm can bottom out on that chassis seam but that does not look possible in your photos, since the pivot point is right under it.

Thanks a lot for the photos.

Edit: It looks like it is the same width when measuring the position of the torsion bar hole.

I find it interesting that there is a portion of the seam between the inner fender and suspension pan missing just behind the crossmember. If that was right above the A-arm, it would help the bottoming out. Is that just a part of restoration or is that the final configuration?

I hope what little anti-dive there is would help some, although I have only bottomed out a couple of times over bumps, when anti-dive does not do anything.

[911st]

That last pic shows pretty well the indent or relief in the sheet metal that raises the rear attachment point.

Again, on a street touring car with a fair amount of articulation, having some anti dive might be helpful. However, I wonder if on car tuned for the track where dive might will be more controlled by the spring rates it is worth the effort.

[WinRice]

Originally Posted by Flieger

I find it interesting that there is a portion of the seam between the inner fender and suspension pan missing just behind the crossmember. If that was right above the A-arm, it would help the bottoming out. Is that just a part of restoration or is that the final configuration?

That's factory 930. It's detailed in the repair manual.

[Flieger]

Thanks.

I wish I had a 930 chassis.

[Flieger]

Is the 930 crossmember the same as the 911 crossmember, with both height and angularity taken care of with the sheet metal? I saw a page back that the A-arm has a different part number but appears to be the same.

[WinRice]

A friend of mine had a 85 Carrera crossmember, and when compared side-by-side they looked the same.

Makes you wonder why the factory moved the mounts up 21mm, then used spacers to move them down to fit the standard crossmember?

Wonder what their original plan was? Special crossmember to fit the higher mounts, but never got manufactured? Found out they didn't need to go any higher, so the standard crossmember worked?

I bet the 911 and 930 a-arms are identical, but since they came with the front bushing and mount installed, the part number was changed because the welded on front 930 spacers.

[Flieger]

Cool, thanks.

Porsche had a history of putting lots of anti-dive in the first version of a racecar then taking most of it out durin development to make it faster. This happened with the 917, although I think they found that with the 917.30 they needed to add a little back in. Maybe something similar happened when developing the 930? They wanted a nice range of adjustment.

[Walt Fricke]

I don't see why a special reinforcing crossmember would be needed to accomodate the raised chassis mount on the 930. But wouldn't a check of some parts catalogs tell for this discrete part?

I do know that the alu Xmember from my '77 2.7 fit fine on the mounts I raised on my "68 tubbed track car.

[Flieger]

Because the angle of the A-arm axis means the rear bushings need to be angled, not just raised, as do the fronts. Unless you just let the rubber bushings take care of the angularity somehow. That would bind a lot though.

[Walt Fricke]

I see your point, but wouldn't that apply to the fronts as well as the rears? While held in with but two bolts, everything is still set up to have the axis at the same angle, just with less surrounding metal to resist the steel mounts from bending the chassis/bolts?

In my case, that wouldn't apply since I have spherical bearings with coilovers, so I hadn't thought of this. But isn't the angle slight enough that the compliance of the rubber could easily accomodate it?

[Flieger]

It seems a bit crude for Porsche to do that but your evidence is convincing. On the racecars that used this anti-dive they had the Heim joint suspension, and the RSRs that used bushings somewhat like Rebel Racings in that they do not allow angular misalignment did not use the front anti-dive. I wonder how much anti-dive effect actually works on street 930s if the A-arm can move that much anyway, and what about camber changes?

Clint's bushings do work on 930s, but I think everyone must have bee using the spherical washers and extra spacers to make up for ditching the stock front A-arm mounts.

[blue72s]

Reviving this thread..

I don't have a 930 Factory workshop manual but is there an official explanation printed in the manual on why the factory raised the mounts up by 21mm for the 930 model?

Is anti-dive the only reason?

It seems to me the main reason was to lower the front body relative to the front cross member for lower center of gravity.

If the cross member was spaced down by 6mm from the mounts raised by 21mm, then the difference is 15mm, meaning the 930 front body sits 15mm lower than the standard (Euro) 911 while the height of the cross member above the ground remains the same.

[WinRice]

Originally Posted by blue72s

Reviving this thread..

I don't have a 930 Factory workshop manual but is there an official explanation printed in the manual on why the factory raised the mounts up by 21mm for the 930 model?

If the cross member was spaced down by 6mm from the mounts raised by 21mm, then the difference is 15mm, meaning the 930 front body sits 15mm lower than the standard (Euro) 911 while the height of the cross member above the ground remains the same.

The body section of the 930 manual only states the carrier pin is 21mm higher, it doesn't state why.

The manual shows front height adjustment:

Wheel center over center of torsion bar
94mm plus or minus 5mm ROW

85mm plus or minus 5mm USA, Canada, Japan up to end of Model 82 only
The specifications are the same as those for the ROW vehicles from model year 83 onward.

I don't have a Carrera manual handy to compare the specs, but I'm sure somebody does. It would be interesting to see if there is a 15mm difference.

[blue72s]

For the non-turbo 911 (ROW), the height between wheel center and the torsion bar center is 108mm -/+ 5mm.

So, the difference between the two ROW models is 108mm - 94mm = 14mm.

Earlier, I was not aware that the turbo had that height of 94mm, so the 930 front body actually sits at the same height as the 911 (if the wheel diameter is the same) whereas the front cross member has been lifted up by 14mm.

The raising of the cross member gave more ground clearance, didn't it?
Wasn't the SC-RS rally car 930 bodied?

[Flieger]

Originally Posted by blue72s

For the non-turbo 911 (ROW), the height between wheel center and the torsion bar center is 108mm -/+ 5mm.

So, the difference between the two ROW models is 108mm - 94mm = 14mm.

Earlier, I was not aware that the turbo had that height of 94mm, so the 930 front body actually sits at the same height as the 911 (if the wheel diameter is the same) whereas the front cross member has been lifted up by 14mm.

The raising of the cross member gave more ground clearance, didn't it?
Wasn't the SC-RS rally car 930 bodied?

I think the lowest points are still the parting line on the engine case, exhaust, front A-arm bushing mounts, harness bolts through the floor pan, etc. I don't think there is any gain in ground clearance.

[blue72s]

Originally Posted by Flieger

I think the lowest points are still the parting line on the engine case, exhaust, front A-arm bushing mounts, harness bolts through the floor pan, etc. I don't think there is any gain in ground clearance.

Maybe I should have said front ground clearance


As an example, the following is a ground clearance rule from the FIA appendix K (for 901 historic racing):

10.2 Ground clearance. All sprung parts of the car must have a minimum ground clearance of 100mm, such that a block of 800mm x 800mm x 100mm may be passed underneath the car from any side, at any time of the Competition.
Ground clearance may be measured at any time during a Competition, on a surface specified by the eligibility delegate, and in conformity with the Homologation Manual published by the FIA in 1993.

The A-arm you mentioned is unsprung weight so it doesn't count. Correct if I'm wrong but I believe the lowest sprung part of the 911/930 at the front end is the cross member.

In the 930, if you increase the standard height of 94mm between the wheel center and torsion tube center, the front body will sit lower than the 911. (aka 934/935)

[Flieger]

Originally Posted by blue72s

Maybe I should have said front ground clearance


As an example, the following is a ground clearance rule from the FIA appendix K (for 901 historic racing):

The A-arm you mentioned is unsprung weight so it doesn't count. Correct if I'm wrong but I believe the lowest sprung part of the 911/930 at the front end is the cross member.

In the 930, if you increase the standard height of 94mm between the wheel center and torsion tube center, the front body will sit lower than the 911. (aka 934/935)

The front bushing mount is sprung, but I agree that the location of the ground clearance makes a difference as to whether you can get over a speed bump, etc.

[blue72s]

Thanks for the picture. Are you saying that the front bushing mount is positioned lower than the 911 cross member?

How much anti-dive does the 930 have? (percent)