ride height calculations/questions

[sopor]

Under spirited driving on B roads I'm getting some tyre rubbing on the left rear and quite possibly in the front somewhere as well. So decided to better understand ride heights. Some data:

front wheels and tyre - 16x7 rims, 205/55 tyres

rear wheels and tyres - 16x9, 245/45 tyres. approximately 5mm spacers fitted

narrowbody car

I took some measurements on the left side:

The front wheel bearing cap centre is 307mm (A) off the ground. The centre of the rear torsion bar cap (where the ride height adjuster is) is 133mm (B) off the ground. This gives a difference of 174mm. (A-B). Factory spec is 108 +/- 5 so the car is lowered by 66mm in the front.

The rear wheel bearing cap centre is 303mm (A) off the ground. The centre of the rear torsion bar is 265mm off the ground. This gives a difference of -38mm (B-A). (At factory ride height the centre of the rear torsion bar is above the centre of the bearing cap.)  Factory spec is 16 +/- 5 so the car is lowered by 54mm in the back.

I'm guessing it's v unlikely that there is 20-30mm of adjustment in the front and rear adjusters and I'm into re-indexing torsion bars?

The rake looks about right - 132mm difference between front and rear torsion bar centres.

[Chris T]

I'd say there is at least 20 or 30mm at either end, more in fact. But, you may already be at the wrong 'end' of what is available. Front is dead easy to do, but at the rear (unless you have aftermarket adjustable spring plates) adjusting ride height which is easy, buggers up camber/caster not so easy. However, will simply removing the rear spacers not fix the problem?

[Leicestershire]

You also need to measure toe, camber and caster.

Its very unlikely that the ride height is causing the tyres to rub.

[sopor]

Isn’t it the case that any ride height adjustment will affect the camber and toe, front and rear?
 

if I take the spacers out the tyres will rub on the inside. 🙁 They really are shoehorned in there.

I could fit 225/50s but as the wheels are so wide I’m thinking they will still be a snug fit.

[Chris T]

Rear certainly, front camber to a small degree if you go wild with it. Toe is more complex depending on rack height but again only minor I'd say and besides, after doing the rear you'll need an alignment session right....

[SilverWT]

9 minutes ago, sopor said:

If I take the spacers out the tyres will rub on the inside. 🙁 They really are shoehorned in there

What year is your car?

There should be plenty of room for 245/45 tyres on 9" on anything other than a early IB with no arch flares. A lot of us run that combination, with and without thin spacers.

[sopor]

It's an 88 - @SP72 's old ride. The rear toe adjustment bolt is a fingertip's thickness away from the inner sidewall.

 

 

[longman]

I ran 245 on 9’s on the rear with a similar setup (CG) but with no shims  as under full compression 1 tyre would rub on the lip of the arch .. on the front I ran 3 mm shims no issue..

edit no I didn’t .. ran 225 on 9 and 245 on an 8 back in the day ..

Got that exact same rub line ...

Edited July 30, 2020 by longman

[SP72]

The 5mm shims were purely for aesthetics when I had 225/50 on the 9s.

I didn’t think the 245s would rub - might be an opportunity to switch to 225/50 as it felt better with them on.

 

[WP0ZZZ]

By the numbers it sounds pretty low but it doesn't look so low in the detail pictures. Do you have sideview picture of the car?

[angusb]

 I ended up copying this adjustable stand design, for each wheel to sit level on. Not familiar with rake over torsion bars, but when right the rear wheel arch should be appox 19mm lower than front.

 

[longman]

I’m pretty sure it was set up at Center Gravity with a little lower please by Shirish so should be  pretty much right on , get rid of the rear shims and bolt on some 225’s , as SP says they drive much better that way 

[Chris T]

What he said,. Shirish knew a 911 and his was developed beyond most folk's budget. Don't change working car!

(mine's STILL not) 

 

[SP72]

Glossy/Shiny pic - SVP did the original RH/Corner balance Back in 2011 - i loved the stance afterwards, and it still rode really well (205/55, 225/50)

Then lost a fair chunk of weight after visiting Sportwagen in 2013 (60-70kg?) - no further suspension work after the weight loss, so it was due (hence purchasing new spring plates last year).

Side to side pics taken in 2015 on the same size tyres (205/55, 225/50) - 5mm shims added to the rear at this point.  Looks like NSR is slightly lower than OSR?

 

[Beaky]

13 hours ago, longman said:

I ran 245 on 9’s on the rear with a similar setup (CG) but with no shims  as under full compression 1 tyre would rub on the lip of the arch .. on the front I ran 3 mm shims no issue..

edit no I didn’t .. ran 225 on 9 and 245 on an 8 back in the day ..

Got that exact same rub line ...

I have 245 on 9” and have no rubbing. I also had a 5mm spacer But CG removed them.

[sopor]

7 minutes ago, Beaky said:

I have 245 on 9” and have no rubbing. I also had a 5mm spacer But CG removed them.

Cheers @Beaky when you get a moment would be grateful to know your ride height numbers from the CG paperwork. Have booked the car in but next available appointment is 29 Sept.

[Busybee]

Why not try by whipping your 5mm spacers off Felix? 

[sopor]

Taking the spacers off is first port of call and seeing what the clearance is on the inside. As I posted earlier the toe adjuster is about 10-12 mm from the inner sidewall - without the spacer it'll be 5-7mm so a tad close.

Narrower tyres is another option. But also want to raise the ride height; I'm not a huge fan of low ride heights for street use.

[Busybee]

9 minutes ago, sopor said:

Taking the spacers off is first port of call and seeing what the clearance is on the inside. As I posted earlier the toe adjuster is about 10-12 mm from the inner sidewall - without the spacer it'll be 5-7mm so a tad close.

Narrower tyres is another option. But also want to raise the ride height; I'm not a huge fan of low ride heights for street use.

I have a new pair of 225 Conti's if you fancy Felix. Bought for the front of daisy but too wide. Was keeping for Scarlett but they'll likely crack by the time I get to needing to replace them. Suppose I could use on rear of daisy but I like the 245's 

[sopor]

Hmm, might take you up on that. Daisy is WB though so 245s fit with ease and 225s might look too narrow.

[Busybee]

She's so low. Think RB ran 225's on the 9" rims on the back of daisy. I'll Double check them later on. Literally on Daisy for a mile

[ras62]

You really need to start at corner weight before anything else. Ride height adjustments can dramatically affect CW in extreme circumstances. So long as everything is free and serviceable a good independent will have it sorted in a few hours.  

[SilverWT]

Sorry guys, I have a different take on this.

Low ride night does not cause tyre rub.

Low ride height can highlight tyre rub which would otherwise only be apparent on suspension compression caused by more extreme use ( e.g. full load, high speed, big dips, etc.).

Raising ride height will only disguise a rubbing problem.

Camber can have an effect on where the tyre rubs, i.e. the more negative camber at the rear the higher up the arch it will rub.

I run 9" rears, 245/45 tyres and 6 mm shims. I have plenty of room at the wing lip through the whole suspension travel with -2 deg camber. If I bottom the suspension at high speed in a dip (bottom of Paddock Hill at Brands is the only place it has happened) I get rubbing about 65 mm up the rear wing. More negative camber will hopefully eliminate this.

There are hundreds of IBs running the 9" 245/45 combination without issues. A lot also have shims, but shims seem to be a bit too much on some cars.

Take the shims off as a first step.

Mark

[Beaky]

On 31/07/2020 at 10:09, sopor said:

Cheers @Beaky when you get a moment would be grateful to know your ride height numbers from the CG paperwork. Have booked the car in but next available appointment is 29 Sept.

Ok i'll do this tomorrow

[Beaky]