Ride height data compiled

[logician]

Hi,

I've been researching ride heights in preparation for setting up a relatively stock '73 911S for track use. In the process, I have compiled some data that I think may be useful for others, so I would like to share it.

Ride height is often quoted in terms of fender lip height. People often cite Bruce Anderson (p201 of "Porsche 911 Performance Handbook"): "The correct ride height will result in a measurement of about 25 inches when measured from the ground up to the rear fender lip."

The factory specifies rear ride height as the height of the torsion tube center above the wheel center. Euro height is specified by Porsche as torsion tube 12mm (.5") above wheel center. Anderson writes about this specification also, and recommends a ride height where the torsion tube is 1.25" below the wheel center. He also says that "anything below 1.5 inches is probably too low, and the suspension geometry will not be correct."

So I took some measurements on my 911S, and also a '74 911 that was set to Euro height, and compiled the following table of specifications. The fender heights assume a tire diameter of approximately 24.5", which is the common factory tire size (205/55-16, 225/50-16, etc.). If you use larger or smaller diameter tires, then the fender heights will be raised or lowered relative to the table. Also tire wear affects fender measurment significantly, as tread thickness can be about 1/3".

Rear Torsion
Fender -Wheel Spec
------ ------- ----
27.5" +1.5" USA height
26.5" +.5" Euro height
25.0" -.75" Anderson "correct ride height"
24.5" -1.25" Anderson recommended height
24.25" -1.50" Anderson minimum height
23.75" -1.8" Juan's excessively lowered 911S

Table accuracy is ~ .25".

One note here: when Anderson writes about "correct ride height", he is not clear about what he means. The text would lead you to believe that he means Euro height, but 25" rear fender is 1.25" lower that Euro height as spec'ed by Porsche.

I'm inclined to set my relatively stock suspension to around the Anderson minimum height of 24.25". Currently, the car is lowered to about 23.75, which I believe is too low.

-Juan

[logician]

Hi,

Sorry. the table didn't reproduce very well. Hopefully this will come out better!

-Juan

Rear fender	Torsion-Wheel	Spec
27.25”	+1.5”	USA height
26.25”	+.5”	Euro height
25.0”	-.75”	Anderson “correct ride height”
24.5”	-1.25”	Anderson recommended height
24.25	-1.5”	Anderson minimum height
23.75	-1.8”	Juan’s excessively lowered 911S

Note: fender heights for USA and Euro cases were corrected. Previous revision of table had these at .25" higher.

[masraum]

interesting, good info. When I set mine my thought was to set it to Euro/BA height using the Porsche recommended measurements, but it seems none of those agree. I'm interested to see if anyone else has anything to say on this.

Currently my front fender is about 25.75-26" and it looks a little high to me. Now I don't know where I will end up.

Darn!!

[logician]

Hi Steve,

I'm curious if perhaps there is a difference between SC/Carrera flared rear fender heights and '76 and older rear fender heights. Can you tell me the following measurements on your rear suspension: 1) fender height, 2) torsion tube center height, 3) wheel center height?

Thanks!
-Juan

[RoninLB]

Originally posted by masraum
I'm interested to see if anyone else has anything to say on this.

I've said a few times on these subjects to match fender height to "true" height to get an idea if the tub is twisted.. to find out how correct your fender height is, for whatever reason.

[addictionMS]

So here is another way to look at it, I decided that fender lip height was too easy to get inconsistant measurements from, so I use the botton of the spring plate where the torsion tube is, the cylindrical part that extends into the wheel well, and the head of the the bolt that holds the front cross member to the chassis. I figure these are about as close as you can get to points which are fixed in space relative to the chassis and tightly bound to the actual suspension.

Both my race car and my street car (both pre 1970 chassis with 198X suspension and 3.2/915 drive train) are about 23.5 inches at the rear lip and 23.75 inches at the front lip, which translates to about 9 inches +- .5 inches in the rear at the spring plate and 3 3/8 inches at the bolt head in the front.

I also struggled with ride height verses corner weighting, and have been told it really is a balance between the two to get a car set up right. I have comprimised ride heights to get closer on corner balancing, allowing the car to ahve abot 1/2 difference betweend sides to get a better balance.



The race car is on 23/31 torsion bars and the street car is on stock 85 setup.

Jim

[masraum]

Man, I was not expecting that. My rear is what you have for Euro height, about 26.5" and about .5" difference between the two.

To me it looks like an old '70 Nova. I thought I was going to lower it to Euro height, but I definitely want to lower it at least an inch maybe 1.5.

[thrown_hammer]

My car sits at 24.5" in the front and 23.75" in the rear. That puts me at the low end of the graph. Seems a little low to me, but the car was set-up by a guy that knows his stuff. It seems to handle nicely.

[jluetjen]

Something that I keep in mind -- no matter what Bruce or anyone else says -- the ultimate determination of the "right" ride-height/alignment specs etc. for competition use is the stop-watch. If you are faster, it must be better, if you are slower it's not. So any ride-height should only be considered a starting point as opposed to the "right" ride height.

For street use -- I've come to the conclusion that there is a fairly wide range of values which will work fine. If your ride height results in the suspension hitting the bump-stops or the front A-arms being flat or "down hill" to inside, you are most likely too low.

It's interesting how there can be so many different "ideal" ride heights -- even from the same source.

[phaques]

check out the following thread on some puzzling measurements regarding FRONT ride height. competition ride height

A couple of us are pretty confused on how to square the porsche method of measuring ride height with the fender height method. any insights on this greatly appreciated......

[KTL]

jluetjen makes a good point (as always). There is no one definite height that is correct for all cars. Weight, tires, alignment, shocks, springs, sway bars, ............ are all going to influence what is a good ride height for your car. Lap time certainly tells what's best. Still, if your car is 90% street driven then you'll have to make some compromises (curbs, driveways) and you're not going to "feel" the ride height issue simply by doing some spirited driving on the street!

I think the fender height method is rather general in nature. Seems like just a starting point to me. I say this because your fender heights are not necessarily going to be exactly the same side-to-side after you fine tune your corner weights. Also, cars with sway bars that do not have adjustable drop-links (factory setup on years 74 and later) will be fighting the corner weights when you reconnect the sway bars following your corner balance adjustments!

[logician]

Phaques:

> A couple of us are pretty confused on how to square the
> porsche method of measuring ride height with the fender
> height method. any insights on this greatly appreciated......

The basic problem is that you don't really know what the fender height should be for Factory Euro setting. People commonly assume Bruce Anderson's 25" rear fender height (from p201 of his book) is the Euro height. If this is your assumption, then there is a 1.25" discrepency between the factory setting and the fender height method, as the factory setting will result in a 26.25" rear fender height.

However, as user Forrest pointed out to me in a separate communication, you might question whether Anderson really meant to say that 25" fender height is Euro spec. Quoting from Anderson:

"What most people consider to be 'Euro Spec' is actually lower than the true European specifications. The correct ride height will result in a measurement of about 25 inches when measured from the ground to the rear fender lip."

So what does Anderson mean when he says "correct ride height ... of 25 inches", the lower spec, or the true Euro Spec? Perhaps his meaning by using the word "correct" is to endorse the lower specification, rather than say that 25" is the result of the true Euro spec.

Here is a second interesting clue. In sorting through all the literature, I found the following footnote on p44-04 of the Carrera workshop manual:

"Height specifications apply to new vehicles. Vehicles which have been operated can be up to 10mm lower, i.e. downward tolerance can be 15mm."

So, it would seem that the factory specifications should be reduced by 10mm ~.4" when used in the real world. This reduces the discrepency between the Factory method and the Bruce Anderson 25" fender height to .85".

Jim:

> So here is another way to look at it, I decided that fender
> lip height was too easy to get inconsistant measurements
> from, so I use the botton of the spring plate where the
> torsion tube is, the cylindrical part that extends into the
> wheel well, and the head of the the bolt that holds the
> front cross member to the chassis.

So actually, it's really easy to measure the torsion tube center, "the cylindrical part that extends into the wheel well", and thus measure ride height exactly as the factory measures it. That way you can exchange ride height specifications with your friends!

I just take a tape measure and hook it over the top of the torsion tube, extend the tape to the ground, and read the length. Don't forget to add in the length of the tape measure body to what you read off of the tape. I then measure the diameter of the torsion tube, and subtract 1/2 of that diameter to get the height of the center of the tube from the ground.

If all you want to do is measure the relative heights of the left and right sides, then you can just compare the tape measure readings, without adjusting for the diameter of the tube or the body of the tape measure.

-Juan

[IROC]

Fender lip ride height measurements are essentially meaningless without also noting the tire sizes on the car. A 205/50-15 on an the rear of an early car is going to result in a fender lip measurement that is drastically different than the 275/40-17s I have on mine, for instance. A 205/50-15 is only 23 inches in diameter where the 275/40-17 is closer to 25.6. That's (in theory - there are always inconsistencies in tire sizes) a difference in ride height of about 1.3 inches - all else being equal.

That was the beauty of the factory system - it took tire size out of the equation. Bruce's method is a good guideline, but it's only that....

Mike

[Wil Ferch]

IROC..
Yes, but....

most OEM Porsche tire sizes have centered around 24.8-25" diameter since the 356 days...pretty consistent...

--Wil Ferch

[phaques]

Will/Iroc,
Good points.
Anderson's book is a little unclear on what is the "correct" Euro spec fender height.
Torsion bar center height is pretty easy to measure - in fact easier to get consistently right, since you can get pretty big differences with the fender method if you don't mark and measure to the same spot on the fender each time.
I make a comparison measurement between the torsion bar and fender, and then measure the fender (which I have marked) to check changes only.
Iroc - that sounds like a pretty fat tire. Most of the wheel/tire combos that will fit under the unmodified body of my 79 sc conform to Wil's statement about consistent overall diameter.
All that said, without some pretty weird tire/wheel combos, the factory spec measurements for ride height would make my car look like I had it set up for some serious offroading, sitting up really high on its suspension. Particularly true of the US specs. My ride height is about 6 inches in front (floor to wheel center about 12 inches, floor to torsion bar cap center about 6 inches), compared with the factory manual euro specification of 4.25 inches, and my car isn't even that low.
And now, a question: What does everyone think the "ideal" relationship is between front and rear ride height? Bruce Anderson suggests a back to front slope (measured where, I'm not exactly sure) of about 1 degree.

[Wil Ferch]

Anderson says that to get the one degreee slope....the fender measurement is 25.5" front / 25" rear. I would think sonething similar can be measured on the door sill if the car is placed on an absolutely flat surface....like a collsion shop's Cellete bench.
---Wil Ferch

[RoninLB]

Originally posted by Wil Ferch
can be measured on the door sill if the car is placed on an absolutely flat surface....like a collsion shop's Cellete bench.
---Wil Ferch

yep.. I'm w/Wil on this one.. I used one of those $25 angle measuring gizmos that can be had at Sears or a hardware store.. I used the door sill for a flat surface.. w/3/4 tank of fuel and drivers weight in the seat.. a prefectly level floor and correct air psi, etc

[logician]

Hi Will,

> Anderson says that to get the one degreee slope....the fender
> measurement is 25.5" front / 25" rear. I would think sonething
> similar can be measured on the door sill if the car is placed on
> an absolutely flat surface....like a collsion shop's Cellete bench.

A digital level works really well for measuring rake angle on a door sill.

If you don't have a perfectly level surface, for example a garage or parking lot with a bit of a slope, you can still get a reasonable result without a Cellete bench. Just take the average of two measurements, but one with the car facing down hill, and one with it facing up hill. Make sure the car is in the same spot for the measurements, but just facing opposite directions.

-Juan

[RallyJon]

With any strut-type suspension, there is a very real minimum ride height--assuming you want the suspension to actually work. On a track-only car, some people make the decision that they'll lower a car to the point where the suspension doesn't work very well, then they use extremely stiff springs and dampers so it doesn't matter and they still turn fast times.

Just because a car is faster around a smooth racetrack definitely does NOT mean the suspension is working better.

When lowering your car, don't fall into the "riceboy paradox" where you look at track cars with un-streetable suspensions and (often) relocated pickup points and copy that look for a street car with softer springs and dampers.

When I got my 911, the front fender was 24" high with 15" wheels, so equivalent to 24.5" on a 911 with standard wheel diameter. At that height, the front A-arm was sloping upward from inside to outside. It doesn't take an engineer to figure out what that does to the roll center and the camber curve.

[Wil Ferch]

RallyJon...excellent points
We don't want to get into " awkward camber curves" and such suspension esoterica...but..
A good way to tell if you've gone too low is when either of these two things happened:
- the ball joints end up higher than the inboard pivot points of the front suspension. They should be equal-to... or lower.
- in similar fashion, said another way..at static ride height....the front lower control arm should be angled-down toward the ball-joints...or , at most, "flat"...not angled down toward the inboard pivot points.
I think I've once calculated the geometry to say that this allows no more than 1-1.5" lowering from the 25.5" front / 25" rear fender measurement basis ( SC/Carrera body)..before such bad things happen. This is independent of the bump steer question, but it's related in that this too changes as you lower the car. For bump steer, note that you can't insert a spacer of much more than 1/2" thickness...so if you lower 1/2", you can correct for this. Lowering more only means a bump steer kit gets you "kinda" back where you were. Then...it's either heat-n'-bend the steering arm in a very precise way, and/or use the ERP bump steer kit... both of which are a story for another time..
---Wil Ferch