'75 911: re-index rear torsion bars

[Facey]

Hey guys,


I recently purchased some new wheels and tires and have encountered fitment issues on the rear. Previously rolled on stretched out 265/35/18, trying to switch to square-shouldered 285/40/17. Shocking to me is the overall height of the new wheels.

Old combo:



New Setup



Basically an entire inch taller, roughly the same overall-width however non-tappered edges. Once I set the car down I realized I was in trouble; almost touching just sitting there. I test drove the car 1/4 a block, immediately turned in the next parking lots entrance -> RUBBING!!!

Looking at it carefully, squat on acceleration or lean during cornering is what I need to give more clearance for. I believe my rubbing issue will be resolved by raising the rear between 1/2" & 3/4". Roughly 4 years ago I went through this process, but I barely recall changing & adjusting the torsion bars

Seriously wishing i went with the adjustable plates: Pelican Parts.com - Adjustable Spring Plates

Anyways -> some step by step help would be appreciated. Sequence of tasks -> regarding taking off the cover, marking key angles/splines/bolt locations, loading/unloading the shock, supporting the trailing arm, etc.





Note:
I cannot find my 101 projects books (believe that I have 2), however I am still feeling ambitious, any help would be appreciated. Pretty shocking to me, the change up front fit fine, from 235/35/18 to 255/40/17 (not settled yet):


IIRC I should be able to get some extra clearance by adding some negative camber? What is that process like?

[Flieger]

I would start with 37 degrees spring plate angle and see where that gets you. Maybe Hans put his good eye on it back in 1975?

I am just curious to see if that number on the spring plate means anything.

[kodioneill]

Use the search function you'll find plenty of info there. If you have a Bentley manual there is a step by step process in there also.

[Chuck Moreland]

Some more information would be useful -

Where is the rub - inside or out?
Existing Ride height?
Camber?
Torsion bar size?
Type of flares on the rear?

From the looks of the front it doesn't appear your car is low to start with.
Raising the car is probably not the best path forward.

If you simply raise the car, you will reduce the propensity to rub, but you will not eliminate it. Soon enough you'll hit a bump or corner hard enough to compress the suspension and have rubbing - at least momentarily.

Ditto putting in stiffer torsion bars.

Further, the increased ride height will likely hurt the appearance and handling of the car.

What you need is to have the right backspace, camber, bumpstop and flair combo to ensure the wheel can move through it's full range of motion without rubbing.

I assume the rear flares are turbo. That's plenty of flare and you should be able to fit 285 under turbo flares.

Specific recommendations depend on answers to the questions above. But in general you need to adjust the spacer width to get the wheel better centered in the well, through it's full range of motion. You can also tweak the camber to get clearance, understanding that this will have side effects impacting tire wear and performance.

If you are threading the needle and putting the largest possible tire, your best play is to take the torsion bar out so you can work the suspension through it's entire range of motion - up to the bump stops. Adjust back space and camber to make it perfect.

At full compression, make sure the bump stops are compressed before the tires are touching anything. You can pack the bump stops if needed to make sure this is so.

Yes, the adjustable spring plates make life a lot easier and are the way to go especially for cars (like yours) that don't have factory adjustable spring plates.

I'd recommend the Elephant Racing QuickChange variety for ease of torsion bar indexing, replacement, bolt-together design, lighter weight, and superior build quality. From pelican here:
Pelican Parts.com - QuickChange Spring Plate Kit

[Facey]

Hey guys, thanks for the replies.

- I have the hayes manual but it's too vague -

Car was actually low before, front has been raised to get in and out of drive-way, and is sitting high in that picture (had just mounted the front wheels). Rear was likely due for a raise anyways.

Where is the rub - Outside edge - Fender lip
Existing Ride height? Lowish - rear a little lower due to clearance needed in front for garage - would get a measurement but am currently have it all up in the air
Camber? zero or even slightly positive, has some toe in as well
Torsion bar size? 28mm
Type of flares on the rear? Turbo - narrow body (stock) running gear/trailing arms

What you need is to have the right backspace, camber, bumpstop and flair combo to ensure the wheel can move through it's full range of motion without rubbing.

That's exactly what I attempted to go with, 8.5" & 10.5" (9 & 11lip to lip width) with backspacing to not require any spacers. For previous wheels I've already rolled the rear right fender, so I could do that for the rear left to get a little more clearance (though pretty scary).

Some negative camber and a small raise in height sounds like a place to start, will be checking back here regularly incase more help is offered

[Facey]

Originally Posted by kodioneill

Use the search function you'll find plenty of info there. If you have a Bentley manual there is a step by step process in there also.

I tried a few before posting. Kurt V & ride height, raising rear , setting height, etc.. really not finding much -> I imagine its well covered in 101 projects however I cannot find either copy I own.

[Facey]

Successfully raised the drivers-side one spline (9 degrees of spring plate resting angle), also went from 0 camber to - camber (measured 3 degrees at hub surface I believe), unplanned but it looks like I have added toe-in. Haven't measured while resting, as I switched to working on the other side and everything was going a little too smooth. Strut mounting bolt was put back in cross-threaded... and being a Sunday no one is open aside from Canadian-Tire & Lordco, neither of whom have the required bolt... will be checked early in the am and if not found I'll be ordering from our host...priority shipping :P

[Facey]

Originally Posted by Flieger

I would start with 37 degrees spring plate angle and see where that gets you. Maybe Hans put his good eye on it back in 1975?

I am just curious to see if that number on the spring plate means anything.

It was the resting angle the spring-plate was set at. 37 drivers side rear, 39 passengers side rear.


Note: I understand I'll have to get the car re-aligned asap, have now changed toe, camber and ride height.... getting the tires under the car was priority #1.