Rear ride height adjustment

[NorthernThrux]

My rear Koni shocks came from Paragon last week and eventually when the weather cools off a bit, I'll install them and while I have the car jacked up, I'll attend to the ride height adjustment as well. The car is a bit too low for my liking at the back and the speed bumps that are on my way to work. Not to mention the ramps on my lift.

Currently, the rear axle centre is 301.5 mm from the ground ("A") and the centre of the rear torsion bar cover is 267.5 mm from the ground ("B"). So the torsion bar centre is lower than the wheel centre by 34 mm. The Porsche spec is that the torsion bar should be higher by 12 mm, so in theory I need to adjust its height up by 34+12 mm = 46 mm. Not that I want to go that high. With reference to this figure, I am at -34 mm, not +12.



I see lots of measurements relating to fender heights (mine is 24.5 rear, 25.5 front in inches) but not references to the factory ride height measurement method. So let's say I want to raise my rear ride height to 25" rear (front would stay where it is at 25.5". The half inch higher front seems to be the way most people set these cars up. It means I have to raise my torsion bar centre height by half an inch (12.7 mm which is a lot less than 46 mm). Seems pretty straight forward. I'll just need to calculate what the new spring plate arm angle should be and then adjust away. There used to be a great calculator for this that I read years ago, but that bookmark is dead.

By the ER calculator (2400 lb, stock 23 mm torsion bar), the USA ride height gave a spring plate angle of 39.3 deg and the Euro ride height (whatever heights these were) gave 36.4 deg, which is close to the 36 deg Robert suggest setting it too. What were USA and Euro ride heights? I see confusing differences in various places.

I just wanted an ESR sanity check here because if I were to raise it 46 mm according to what I believe is the factory spec (i.e. a spring plate angle of 39.3 deg theoretically), then the rear fender height will go up 46/25.4 = 1.8" which takes it to 26.3". Which means the front would be something like 26.8" for factory spec. Were these cars really that high as delivered? I think I can believe that based on old magazine articles, but looking for validation here.

Ravi

[John912]

I’ve set up stock 65 and 67 911s at 36° even , which is the factory setting. Although I never took any other measurements, The rear ride height on both cars looked right to me

The factory manual I have shows from 69 on with 23 mm torsion bar diameter, the setting should be between 36 degrees 30’ and 37 degrees. I would think 39+ would bring it too high. The factory manual does call for 39 degrees on 68 models with 22mm torsion bars

[Richy]

Urban myth , there was not a "European ride height" until 1974 with the new bumpers , the impact bumpers had to be a specific height in the US . For your car a 36 degree spring angle would be ok . 1 degree is about 3 mm so change the angle 2 degrees for lower than stock height .

[Arne]

Ravi, I set mine at 36° as well. Here's a quote from my build thread, sounds similar...

I've felt some thumping over some larger bumps in the back, got to wondering if I was bottoming out as it did look rather low in the rear. Did some measuring, determined the rear ride height was way low, almost 2" (50mm) below factory spec. So I put it in the air and pulled it apart to check the spring plate angle. Factory spec for a '72 is 36.5-37° -- my car was at 32° on both sides. I reindexed the torsion bars to set the angle just a touch under stock spec, at 36° even. Much happier now. Car doesn't look like it's squatted in the back, and the thumping noises are gone. Still sits a bit under factory height, by about 10mm.

[NorthernThrux]

Thanks everyone. Will aim for about 36 deg. Richy, the guidance of 3 mm per degree is very useful, so I can go from what it is currently set up to, to the desired height in one trial (lol). I read Bruce Anderson's book last night and he says "Adjusting the torsion bar one inside tooth up and the swingarm one tooth down produces a swingarm change of approximately 50 min and a vehicle height change of approximately 6.5 mm". Hmmm. It's easy math anyways, once I can measure the distances from the torsion bar centre to the perpendicular to the contact patch and the centre of the axle. I'll figure it out.

I found this old road test panel for the 2.2S in R&T, back when it was a real magazine with technical specs and all. Measuring from the scale, the front fender lip was 27" and the rear was 26". Hub centre is at a hair under 12" which is consistent with the 301.5 mm I measured, so it's all self-consistent. These cars were setup pretty high from the factory, at least in the US. That 27" front is entirely consistent with my estimation of the front from the rear 39 deg spring plate angle that ER says is stock US ride height. I like it when the numbers come together.

Ravi

[gled49]

I lowered a lot of cars in the SC days, and drivers side was always set higher, so when I lowered them I kept the same difference. With stock torsion bars drivers side will drop 1/2” when you sit in the car.

[Arne]

I found this old road test panel for the 2.2S in R&T, back when it was a real magazine with technical specs and all. Measuring from the scale, the front fender lip was 27" and the rear was 26". Hub centre is at a hair under 12" which is consistent with the 301.5 mm I measured, so it's all self-consistent. These cars were setup pretty high from the factory, at least in the US. That 27" front is entirely consistent with my estimation of the front from the rear 39 deg spring plate angle that ER says is stock US ride height. I like it when the numbers come together.

Actually, the US didn't get a special ride height until the impact bumpers in MY 74. For cars prior to impact bumpers, all had the same spec world-wide.

But yeah, they were rather tall from the factory. Check out some of these pics from the Car and Driver '72 test:

[terry911t]

I find this thread to be helpful...
http://forums.pelicanparts.com/porsc...sion-bars.html

[911MRP]

Even with this young lady sprawled on the front wing ( fender) the ride height of a brand new car exhibited at the British motorshow October 1971 (model year 1972) looks high by today’s approach....



The under wheel arch finish and tyre brand with external rim balance weight used in that era for this RHD example also reasonably clear in the shot.

[NorthernThrux]

I lowered a lot of cars in the SC days, and drivers side was always set higher, so when I lowered them I kept the same difference. With stock torsion bars drivers side will drop 1/2” when you sit in the car.

That is the way my car is setup right now. It kinda drives me nuts even though it's correct in terms of nominal corner balancing because I can just visually detect the lean from left to right when it's parked in the garage and you have a nice square reference frame surrounding it.



It's clear to me that the high delivered height of these cars was achieved by a 39 deg spring plate angle. And that 36 will bring it to something more aesthetically pleasing and better handling. I suspect I'm sitting at 33 right now. Robert set the car at 36 deg he thinks, but it had been in the air with the wheels hanging for 27 months at the time and that might have changed the effective spring constant of the torsion bar until it had a chance to settle again.