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-   -   Anti-dive spacers (http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/529905-anti-dive-spacers.html)

Flieger 03-07-2010 01:59 PM

Anti-dive spacers
 
I am thinking of adding anti-dive spacers to my 911. Can anyone post a photo of the 930 spacers? How much do they lower the front A-arm mounts?

Thanks.:)

http://forums.pelicanparts.com/uploa...1268002790.jpg

equality72521 03-07-2010 02:07 PM

Not sure how much thicker they are. Somewhere around 1/4" I guess.

http://forums.pelicanparts.com/uploa...1268003222.jpg

Flieger 03-07-2010 02:25 PM

Cool, thanks alot. They look pretty simple to make. SmileWavy

equality72521 03-07-2010 03:09 PM

Yeah, it's just another spacer spot welded onto the original spacer.

equality72521 03-07-2010 03:16 PM

Here is the difference.

http://forums.pelicanparts.com/uploa...1235851620.jpg
http://forums.pelicanparts.com/uploa...1235851654.jpg

fredmeister 03-07-2010 03:26 PM

The chassis is also slightly different on the turbo cars where the steering rack mounts. It is higher so that the rear crossmember is higher and thus between the added spacers and the higher rear cross member height you get a significant amount of anti dive.
Adding spacers to the front mount alone should help a little but be sure to account for the loss of thread engagement with longer mounting bolts there.

911st 03-07-2010 05:40 PM

Bruce Anderson in his book gives the exact amount of lift in the back and drop in the front.

I wonder if the steering rack cross member is different or of the chassis sheet metal is lifted in the rear. It would have been easyer to do it at the cross member. Might be able to just mod the stock 911 unit but not sure.

If not, can we purchase that panel.

Anyone have pictures or details on the back attachment.

911st 03-07-2010 05:47 PM

PS, Porsche never really made "rake' a part of a 911's intended set up from what I could ever confirm. It would seem that rake would to a lesser degree 'tilt' the front.

I wonder if rake helps add to the 930/turbo look front's anti dive geometry?



Any thoughts on this?

proffighter 03-08-2010 01:57 AM

I think this is stock for Euro 930's (without the spherical bushes) I've never seen others here:

http://forums.pelicanparts.com/uploads15/0141255125093.jpg

One more thing: It could be possible, that there are different crossmembers, because after lowering the car, I wanted to lift steering rack using shims like recommended sometimes, but there is no space between tunnel and rack/boots to fit. Clearance between boots and chassis is about 5mm on top.

911st 03-08-2010 06:00 AM

Looked it up. The front of the suspension is lowered 7mm and the rear is raised 13mm.

He clearly states the mod to the rear of the front suspension is different sheet metal but I still wonder.

If it is the sheet metal and the cross member is the same it would raise the steering rack. This is why other's have noted that the steering rack spacers will not work on a 930 or turbo look.

Anyone have pictures of the rear front suspension rear mounting points on a Turbo and NA 911?

proffighter 03-08-2010 07:55 AM

Quote:

Originally Posted by 911st (Post 5223719)
Anyone have pictures of the rear front suspension rear mounting points on a Turbo and NA 911?

Like this?:

http://forums.pelicanparts.com/uploa...1268067347.jpg

911st 03-08-2010 08:25 AM

Roland,

Great pic!

Can anyone tell from that pic of the sheet metal is different from a 911 or if it is the cross member?

Pic also shows very well how close the arms are to the body with the 930 geometry.

911st 03-08-2010 08:34 AM

I am wondering if we lower a 930 to where the front A arms are not sloping down and or the rear spring plates are not sloping down, if that would defeat or work against the 930's anti squat geometry.

Lifting the front spindles on the front struts of a lowered 930 is such a nice compliment to the rear suspension's more aggressive camber curve. On top of that it may also be needed to reduce the potential for losing some of the fronts anti squat when lowered.

Just guessing.

Flieger 03-08-2010 02:20 PM

The only way that lowering the car effects anti-dive is in the cosine of the difference in the angle * the A-arm length. The effective lever arm changes by a few millimeters as the A-arm goes through its arc, being greatest at level.

The ant-dive achieved through angling the axis of the A-arm is independent of FVSA angle. Some of the moment of the center of gravity pitching forward under braking is reacted through the suspension arms themselves, bypassing the springs. At a maximum anti-dive, the suspension is as good as locked and there is a pure pitching moment of the center of gravity about the front contact patch.

A-arm FVSA angle effects the roll center, not anti-dive.

Anti-squat is achieved by lowering the rear, bringing the line of action of the tractive force (tire contact patch height along ground) closer to the side view swing arm (SVSA) instant center. Hypothetically, if the rear trailing arm is the IC, then when the trailing arm is level, there will be no squat reaction in the springs. There will still be a pitching moment of the center of gravity rearward about the rear contact patch.

Having a high SVSA instant center (IC) will contribute more anti-dive forces under braking.

Because the pitching moments are always present, the anti-dive and anti-squat geometries are mainly to be able to control longitudinal load transfer rates since the total longitudinal weight transfer is constant.

This is the same concept as in lateral load transfer with roll center heights, CG heights, and sway bar/main spring reactive forces.

What we all want is a lower CG, of course.

911st 03-08-2010 04:34 PM

Flieger,

I think I get some of what you said. Wish could I could get it better.


I do not know where the COG is but my guess is it is probably at about where my pockets or seat is when I sit in the car.

Is it the angle of the COG relative to the front wheel contact patch that makes most the dive or squat force?

If so, until we get the front suspension angled equal to the angle of the COG to the front wheels there is going to be dive of some level. Seems we would have to make said angle of the front A arm attachment points greater than the COG/contact patch before there is really any anti dive?

I am guessing adding rake to the car adds about 1 deg angle to the front. Spacing the front of the front suspension down would add maybe about one more deg and spacing the rear of the front suspension up would add almost 2 deg more for an approximate total of about 3-4 deg.

If the COG to front tire patch is say 15 to 20 deg that might have some effect but not get the car anywhere near stopping flat.

However, seems lowering the COG and or adding stiffer springs might have much have more effect on dive or squat than playing with the front orientation. Dose this sound right?

And why do we want to reduce dive?

Is it to reduce the suspensions effects on camber and toe under braking to keep the wheel contact patches at a better orientation under braking. Thus letting us stop faster? Or is there some other reason?

I have been very interested in the idea of working the front to be more like a 930. However, I am wondering if it is worth it in the name of anti dive?

Back when Porsche was monkeying with this the spring rates and tire elasticity was much lower or softer than most of us run now and it was possibly more relevant back then than now. Just a thought.

Playing with the camber curves seems much more relevant I am wondering.

911st 03-08-2010 04:54 PM

My very rough numbers and gut tell me the 930 suspension might reduce the anti dive forces something like 10-20%.

Going from stock to 22/28mm torsion bars reduces it over 50%.

Sway bars having no effect.

911st 03-08-2010 05:01 PM

This might be an argument for larger springs relative to sways bars over the stock apx 50/50 set up?

Flieger 03-08-2010 07:38 PM

#1 Yes, The angle of the tire contact patch to the side view swing arm (SVSA) instant center (IC) is the line of action for the braking force. If this lines up with the center of gravity, then there will be no moment about the center of gravity caused by the braking force. Therefore, no force will need to be reacted by the springs. There will be no pitching moment of the chassis relative to the wheels.

The SVIC is determined by the intersection of a line 90* to the top of the strut and the line through the A-arm axis. The point of interection of these two is the IC. From the IC to the contact patch is the braking force vector.

#2 "Anti-dive" is relative to a horizontal A-arm axis where all of the pitching moment is reacted by the spring deflection.

#3 Anti-dive and anti-squat are useful tools to determine the rate of longitudinal weight transfer. Since we have no sway bars for this direction, we can control how fast the rear tires loose load under braking and how quickly the chassis will react to braking inputs (rotating the car around an autocross cone by sliding the tail around). Remember that the total load transfer is a function of wheelbase, center of gravity location, vehicle mass, braking force, traction, and is not changed by the anti-effects.

#4 Yes, less suspension deflection will also stabilize alignment.

#5 Yes, stiffer main springs are usually more desireable than very stiff sway bars because the stiffer main springs help control the car in both axis- pitch and roll.

jimmcc 03-09-2010 05:37 AM

The crossmember is the same part number for both the SC and the Turbo.

The parts reference shows a two spacers for the Turbo that fit between the crossmember and chassis which moves the crossmember down. The part number is 930 341 135 00.

At the moment my jack is kaput so I can't measure the offset.

Jim
1980 930
1994 993

911st 03-09-2010 06:11 AM

Jim, Good work, thx.

Not only dose this mod reduce dive potental some but is also rases up the end of the A arms up about 1/2" that is closest to the strut. This should also put the front into a better part of the camber curve compared to a 911 set at the same height.

Often we raise the spindle on the strut 15 to 32mm to make improvement in this area. Combining these two could make for a nice improvement in the fronts camber curve I would think.

One of the best parts of the rear suspension on a 930 is it's more aggressive camber curve.


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