Wheel size and ride height

[gost]

I'm about to try to level out my ride height on my 85 before I get a full alignment and am curious if the whell sizing plays a role in the final obtainable height. I have 16" Fuchs 6's and 7's with standard tire sizes and I've scoured all the threads and Bently and 101 and nowhere do they mention wheel sizes and their effect on height. Wouldn't a 16 or 17 inch wheel be closer to the fender lip than a 15" and therefore should not be lowered to a specific listed height for say a 15" whell. Only makes sense to me.
Can anyone elaborate?

Thanks

[SLO-BOB]

What ypu really want to measure is the tire. In your case, you would drop down the sidewall size to make up for the increase in wheel diameter. There are variables but the basic idea is that if a 911 would start with something like a 185/70/15 tire which has a diameter of "x", ideally you would increase the wheel size but reduce the tire size so that it equals "x" also. Generally speaking if you went up in wheel size by "1" you would drop tire sidewall size and get a 60 series tire. This is sometimes referred to as plus 1. Next step would be 17 inch wheels and 50 series tires=plus 2. Unfortunately this theory doesn't always add up as going to a wider tire, such as 185 to 205 somehow increases the diameter. I haven't figured that one out yet.

Anyway, I'm going to 16 inch wheels and I figure I need to have 50 series tires, 205/50/16 in my case, to keep from rubbing. We'll see if that works. They're going to be mounted tomorrow.

[Zeke]

Sebring has got it. Plus 1 is kinda old hat now, but that's the theory. The manuals assume you have stock wheels/tires. Anyway, if your overall tire height is less than stock height, add one half of the difference to the amount of tire showing in the wheel arch, or subtract if your are measuring from the ground, etc. Of course the best way to find a good lowered height is to forget about the tires and look at the suspension. The front a-arms should not be less than level, AFIC unless the spindles are raised on the struts. This is because at level, you only have about 1 1/2 inches of free bump travel to the snubbers. You can cut the sunbbers by one ring, but you are getting into the range where the shock is in danger of bottoming out and that will hurt it if not destroy it.

Adjust the ride height of the rear after you find what you want for the front. Remember that a level 911 will oversteer more than one hiked up 1 degree (all other factors considered neutral for the moment). Then set your camber and wheel alignment close and corner balance before fine tuning the alignment.

[gost]

I'm still stumped. I have new 205/55/16 on front and 225/50/16 rear which are the factory sizes for 16" Fuchs. With a full tank and my girlfriend and 50lb puppy in her lap in the drivers seat ( 170lbs total) I get these figures from ground to fender lip. DF-24 3/4 PF-25 1/4 DR-25 1/4 PR-26 1/8 What to do now? Euro is 25 1/2 front and 25 rear. Dropping the rears I read will raise the fronts a tad. Perhaps that's a good start but again 16" wheels need to be factored in?
Anyone know the answer?

Thanks

[gost]

Milt, So you're saying plus 1 gets added to the final measured lip height?
Now I would be shooting for 26 1/2 in front and 26 on rears? The car has a little over 110K on her so I'm guessing some Tbar sag is going on as well. Is this right?

[Bill Verburg]

You are confusing 2 different issues,

1) the rolling radius of the tire affects the height of the chassis from the ground. Physical sizes for a nominal size vary but a std. 205/55x16 on 6" wheel will have a rolling radius of ~11.5" w a diameter of ~24.9".

Substituting a 225/45x17 w/ a slr of ~11.6" and diameter of ~25" raises the chassis by ~.1" but at the same time puts the tire only ~.05" closer to the fender.

if you had started at a fender height of 25.5" you would have a height of 25.1" after installing the 17s, but the tire would be only marginally neaer to the fender vertically, it would be closer horizontally as well.

2)as long as the tire fits in the fender well it will work fine, the above is an example of 2 different but common front 911 fitments.

[gost]

I see. A standard 15" wheel with a 185/70 or 215/60 tire would be a base for the standard euro heights and any wheel sizing changes would have to be factored in to adjust this final height based on their profile and associated wheel dia.
Thanks for straightening it out for me Bill, Sebring and Milt. Simple math!

[Lukesportsman]

Bill may disagree with me. And I'll respectfully accept this since he is head and shoulders beyond me in Pcar knowledge.

Due to variations of rims and wheels and their respective overall diameters, I think from a theory standpoint fender height isn't that useful. Sure we all use it to "compare" and to make it "look right". It seems your question, though adequately answered in that it doesn't matter, is more to core of theory. Does this or that matter? Would increasing things change other variables?

Now, I'm not saying Bill's description is inaccurate at all. I'm just saying that using the TB to axle center takes the wheel diameter out of the equation. This is good and possibly bad if you stray far from OEM fittings. Using the TB/axle gives one a better idea of where their suspension setting are putting them in respect to geometry, bump stops and shocks. (how could it be bad? My car at factory specs would a CG of a bus :O )

If you consider extremes like my 18's. To get to my 25.5 & 25 ride heigth I had to go far below Euro suspension settings. Sure I look much like many "Euro" set cars, but my suspension is lower than that in respect to shocks and bump stops. This therefore is where I stopped in order to keep my geometry within reason....I need to raise the spindles or get some 215/35's.

Just another way of looking at it, that may make it clearer or just mud.

Later,

[SLO-BOB]

Luke-I'de love to see pics of your car.

Gost-That's basically it. I should say that, imo, ideally outside diameter of whatever tires you're running would be as the factory intended. For one it keeps the speedo accurate, but also you hopefully avoid the dreaded rubbing issues. Rubbing is also created by going wider than factory obviously. You state that you have 6s and 7s with standard tire sizes. I don't know what factory tire spec was for your car but if you are running factory spec, your task should be an easy one. If you are running other than factory spec, compare the outside width and diameter of what you have to what you "should" have. Then also factor in Milts excellent observations. A bump-steer kit can be added to compensate for a lower car as well.

And you thought this was going to be easy. Wheel size and lowering still seems like alchemy to me even after 20+ years of toying with this stuff. Factor in the inconsistent build of the 911 chassis and you have no "hard and fast" rules unfortunately. What might work for one may not work for another when it comes to pushing the envelope in tire diameter and width.

Now remember I said "ideally" when comparing factory spec diameter to upgrades. That is merely for illustration. Yes, it would be nice, but the likelyhood of maintaining that diameter is slim. Like I said, in my case I am reducing the diameter from factory spec by about an inch. Why? Because I am increasing width. If I go to the same diameter on my narrow car I'll have rubbing issues. Sounds bad doesn't it? Then I'm adding negative (sounds weird) camber to help so I can lower the car more-within reason. Hopefully Euro spec. In theory this should leave me 1/2 inch of extra room above the tires to reduce the chance of contact under load. The end result "should" be a car that sits at Euro ride height, and has a 1/2 bigger gap in the wheel openings than factory spec. In other words-lower but with more vertical room.

[Paulporsche]

Most of these"Euro Height" discussions are based on Bruce Anderson's book, in which he uses 25.5F and 25R as a guideline. This was based on a wheel diameter of 25" and using an SC rear flare, and took into account looks, fit without rubbing inside the rear wheel wells and road clearance. Since then a lot has happened in tire and wheel technology and design. As others have said, once you get past an 8 x 16" rim and/or a 225 wide tire you are really on your own. For those of us w/ narrow bodies and 6 or 7s and 215 max wide tires, this works great, and depending on location and type of driving, some can even go lower.

From a looks standpoint, I like to see the wheel arch even w/ the top of the tire (hence 25 for a 215/60/15) or maybe even slightly lower if you dare. If you then add 1/2" to the front, you get a slightly nose down attitude, which also looks good and allows for front wheel travel.

[gost]

Good info fellas. I have a clearer picture of what's going on. Odd how none of the literature out there (and I have most of it) addresses height and tire sizing. That's why there's this forum. I cracked loose the lock and concentric nut's last night. Needed a 24" breaker bar to do them. I've decided to raise the rear corner that was low instead of lowering the high rear corner. The initial numbers on the fronts indicate that they should even out as well. I'm trying the tripod method and have the rear up on one jack and measured the front heights and found them to be exactly the same 26 1/4" which is 1 to 1 1/2" higher than at level and instead of the 1/2" spread in height on the fronts when level now they are even. Hmmm. Anyone know why that is? It's not stopping my work , I'm just curious.
Hey...Thanks for all the help yet again. The board comes through yet again.

Safe motoring!

[SLO-BOB]

Merely conjecture on my part, but if build tolerance is as they say it is, I would not use sheetmetal as my gauge for the car's stance. Maybe corner balancing instead? I'm sure others can give better info than I on this topic.

[gost]

The view from the rear is noticeable that one side is lower so that low side is coming up. We'll see from there.

[Bill Verburg]

Originally posted by Sebring77
Merely conjecture on my part, but if build tolerance is as they say it is, I would not use sheetmetal as my gauge for the car's stance. Maybe corner balancing instead? I'm sure others can give better info than I on this topic.

Actually the fender lips are a reasonable if not 100% accurate way to assess ride height. After all on a lowered car one of the 2 critical issues is the fender clearance. The other being suspension travel, and tertiarily bump steer.

[SLO-BOB]

Again, I defer to those who have more experience within this realm. However, I do still have a question. If, as is often said, build tolerances on 911s are iffy, is it not possible that a car that is sitting perfectly flat, with weight distributed as it should between the two sides of the car, could have a fender higher on one side or the other? If so, then I can only assume that if one were to "correct" the situation by either raising or lowering the offending corner, one would shift weight throwing off the balance of the car. Am I wrong?

[Paulporsche]

No.

[Bill Verburg]

Build quality is not really iffy, there are some consistent asymmetric anomalies that usually maybe even always show up.

[gost]

OK. I raised the low rear corner which was 25 1/4" before I started and the opp high rear was 26 3/16. I raised it almost as much as the concentric allowed and buttoned up and now the raised corner is 25 5/8 and the opp is 26 3/8. So the corner I raised went up 3/8" and the opp rose 3/16". The opposite corner went up half the distance than the corner I raised. On the front one corner is up 1/4" and the other stayed the same. I need to go up about 1/4" on one front and 1/2" on the other to hit the 25 1/2" mark. I will try to raise the fronts tomorrow and see what happens to the rears then. I tend to feel that the rears will drop which will be good to help hit the 25" mark. I'm going to balance and align on a $20K machine anyway but wanted to experience the struggle myself.

[gost]

So I cranked here and lowered there and what I found is that when I had the fronts at 25 1/2 it looked way too high and the rears were at the same or higher at their lowest settings so I went to plan B. I cranked the one rear I previously raised back to it's minimum setting. What I found was after I scribed the spring plate position before starting any of this, after lowering it the second time the spring plate was actually a tad lower than before. So I loosened the other rear and found that it too dropped a miniscule amount which was welcomed because the rears were high to start. I don't believe the car height was ever adjusted since new. Did the bouncing and the run around the block and everything I read was true. These cars are possessed. Different measurements at different times without doing any adjustments, unreal!. Also figured out to not have the parking brake on, that screws the readings up big time. So I used balast of 170lbs in the drivers seat and I have a full tank of petrol.
This is what I ended up with.
Both fronts are at 25 1/4 DR- 25-1/16 PR-25 1/2. It will have to do until I replace torsion bars next year.
Thanks for all your guidance fellas. The pics are without weight in the seat.

[rlh930]

Unless you have replaced the stock suspension bushings with Elephant bronze bearings, roller bearings, or the like, you will have significant "stiction" in the suspension that will make it difficult to achieve repeatable ride height measurements.