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well, maybe...
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Posted by gorper (more from gorper) on Fri, 5 Dec 2003 15:43:15 Share Post by Email
In Reply to: Rim size on C900. . ., Tyler, Fri, 5 Dec 2003 12:27:14
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Hi Tyler,

I'm reasonably certain it'll work (see http://www.ronalusa.com/cars/saab_4lug.html) -- if Ronal sells a 16x7.5" for c900s, then it stands to reason that yours will fit, right? Provided, of course, Ronal specs a 40mm offset. You could call them and ask very nicely what offset and tire size they use. :-) Or, IIRC, "KB88900T" runs a 7.5" Ronal – ask him for offset (and possible spacer) measurements??

But, may I make a suggestion? Do your homework. Seriously, not trying to be a wise@ss:

Do a search on Saabnet for "7.5" "x7.5" "et40" "et 40" "40 offset" etc. and see what you come up with. Take some notes. :-) Remember, though, that what works (or doesn’t work) for some people may not work (or will work) for you – there are a lot of factors at play here:

. . . tire brand/size/condition
. . . alignment specs
. . . spring setup/condition
. . . trim (e.g., SPG fender plastic)
. . . driving style
. . . vehicle weight (couple 100#s separate the lightest & heaviest c900s)
. . . local road conditions

Now, get some data: roll your car up onto a set of ramps, get beneath it, and take some actual measurements, front and rear. For the fronts, measure with the tire pointing straight ahead and at full lock pointing inward, which pulls the wheel closer to the spring due to caster. Measure off the tire, and also measure the actual section width (not tread width) of the tire.

Note that your numbers are only valid if you plan on keeping the current camber/caster settings. If you plan on adding more negative camber/positive caster, which, in my experience, is a *good thing*, then you’ll have to do this afterward because you’ll lose some clearance.

Now, find the specs on the tire you plan to run on the new rims, go to tirerack.com (Saabnet sponsor, BTW), and learn how to determine actual section width based on rim width. Calculate this distance.

Use the offset calculator below to plug in the *section widths*, not the rim widths; this way, you’re dealing with measurements that are as close to reality as possible. Compare the actual section width of your existing setup with the one you plan on running, and see what you come up with.

Without actually having the rims and tires you want, I think this is about as close as you can get to determining how much space you're working with.

Caveat: I don’t know much about how the rear tire moves with respect to the body during cornering. The inside tire falls away and the outside moves in but, by how much, I don’t know. I’m sure that if there is, in fact, a lot of lateral movement during corning, you could remove the slop by using new bushings in the panhard rod and elsewhere; perhaps buying some poly ones from Brad S. @ KCSaab (also a Saabnet sponsor) would be beneficial.

One last thing: theoretically, a 7.5 rim @ ET40 sticks out a whopping 1mm *less* than an Aero rim @ ET27 (extends 25mm vs. 26mm, respectively). That same 7.5" rim will stick in, again, theoretically, 25mm. So, given the same brand/size tire on both rims, I'd say you could conclude that, for the fender arches anyway, you'll probably get some rubbing in the rear, but it wouldn't be any worse than what many people experience with Aeros.

One more last thing: On a c900 with new-ish SPG springs and 2* negative camber, pointing straight ahead, I measured 30mm from spring to sidewall (205/50-16 Kumho Ecsta). (sorry, don't remember the measurement when turned). 5mm is close but, provided the wheel doesn't rub the spring at, say, 3/4 of full lock, I'd be happy & would run them.

One last last thing: Do your homework.

Tirerack info page:
http://www.tirerack.com/tires/tiretech/general/size.htm



posted by 65.101.15...

http://toy4two.home.mindspring.com/offset.html

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