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930 3.3 on 15' fuchs Vs 16': feedback wanted!

Hi,
I am new to 930 wonderland and I am exploring ways to make it as nimble as a narrow carrera chassis. I have a very nice example and would be keen on keeping it as stock as possible in terms of equipment/interior so stripping it is not an option.

I am already considering stronger torsion bars front and rear (21/29).

I am also consdering putting it on 8&9 15' Fuchs wheels instead of its "native" 7&8 16' Fuchs (I already have a pair of 9*15 so finding a pair of 8*15 should not be too difficult). I know that a lot of people are going the opposite route... However, I think that there are great advantages in going "smaller" (in diameter, not width):

Unsprung weight is much improved which will make the car nimbler
Overall gearing is shorter (with 225/50/15 tyres of sticky toyo or yoko) which is a good thing with 4 "high" gears

I know that 3.0l Turbo as well as early 3.3 turbo were offered with 15' Fuchs but I was wondering if I could get some feedback of 15' Vs 16' Fuchs.

Mathieu
Old 10-27-2008, 08:22 AM
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The overall hieght of the tire (or lack of) will help with your effective gear ratio but begins to look cartoonish when you fall below 24.5" tall. At that hieght (24.5) on a 15" rim you have ~4.75" of sidewall. That is HUGE in terms of sidewall flex potential.

Here is some data on available sizes:
225/50-15 = 8.85"wide, 4.42" sidewall, 23.86" tall
245/50-15 = 9.64"wide, 4.82" sidewall, 24.64" tall
245/45-16 = 9.64"wide, 4.34" sidewall, 24.68" tall (+1 Sizing)
245/40-17 = 9.64"wide, 3.86" sidewall, 24.71" tall (+2 Sizing)

The sidewall rigidity of the 17" will be VERY noticeable.

Wheels and ties are likely the most easily changeable item on a car. Get the set that best serves your needs and store the stock wheels for the next owner.
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Old 10-27-2008, 09:55 AM
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Hold up! Before you make what I would call a backward move by going to inferior tires, I would suggest you simply try things like adding more rake to the car and you can change toe in and camber settings.

What you should be asking, imho, is 'who knows a really good shop in my area that will mess with my car and make it handle the way I want'.

Going back to smaller tires with flexible sidewalls will give you wheelspin and less grip and less braking and lower cornering speeds.

These cars are highly adjustable. Even getting adjustable sway bars allows you to fine tune the car.

I love this website, and the guy that runs it to:

http://www.rennsportsystems.com/2b.html
Old 10-27-2008, 10:27 AM
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Thanks for your answers.

Going with 50serie 15', there are some semi track/street tyres that are excellent (such as Toyo r888 for ex) and which are supposed to have very rigid sidewalls, so I do not worry too much about that, although I would have loved to hear some hands on experience from people who have just done that.
Btw, cars like carrera 3.0l RS had turbo wide flares (and pretty much a 930 chassis) and were fitted with 8&9 15' fuchs, so this should not disrupt Porsche handling set up for the 930.

The more I think of it, the more I am tempted to do it as it can address both the "agility factor" and the high gearing issue of the 930. The 4 speed gearbox is really geared high as compared to the 89" 5 speed (notably the 3rd and 4th gears). Alternatively, is it common to put a shorter final ratio to a 930 4 speed unit?

Regards

Mathieu
Old 10-28-2008, 04:24 AM
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I'm doing the same thing with my widebody 3.2. No mine's not a turbo car but I'm doing it for the same reasons you are. 8&9x15 Fuchs with 205/55 (or 225/50) fronts and 225/50 or 245/50 rear tires.

Here's an example. It appears to be on 7&8x15 Fuchs with 225/50 and 245/50 tires:


IMO opinion anything smaller than 24" tall looks a bit funny...but...it makes a noticable difference in gearing. If you go with 205/50 front tires then consider 225/45 rear tires. These are ~23" tall. But even 24" tall tires make nice gearing changes vs. 25" or taller tires.

I'll probably keep a second set of wheels that are 17's so I can switch back and forth when desired.
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Old 10-28-2008, 05:15 AM
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Hankook RS2, Toyo R888, Toyo RA1, Toyo T1R are all available in the 205/225 sizes. Plus others...

245 is a very limited size.
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Old 10-28-2008, 05:22 AM
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Make sure you got good clearance on the brakes Calipers when using 15" rims.
Old 10-28-2008, 07:09 AM
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Remember that on the 935 they used a taller diameter wheel to increase the contact patch of the tyre. You are going to decrease the contact... The great Norbert Singer worked his magic on this.
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Old 11-08-2008, 02:56 PM
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My understanding is that the increase in contact patch that you get get with greater diameter wheels (for the same width) is only true with respect to tyres that have a strong "dome" profile. In other words, the higher the dome, the bigger the benefit on contact patch.
If you use a R888 tyre with a racing profile, I think fitting 15' wheels as opposed to 16' will have a very limited effect on contact patch.
If the quest for the biggest contact patch possible makes sense on a 935 - where you have huge power and rules restriction on wheel's width (and every little thing is used to make a difference) -, I think it is less relevant for a street/track slighlty modified 930.


Quote:
Originally Posted by NathanUK View Post
Remember that on the 935 they used a taller diameter wheel to increase the contact patch of the tyre. You are going to decrease the contact... The great Norbert Singer worked his magic on this.

Last edited by 1100sport; 11-08-2008 at 04:04 PM..
Old 11-08-2008, 04:01 PM
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If you want the car to really handle you are going in the wrong direction. Go to lightweight 9" and 11" by 17" rims and 255 and 315 tires. With stiffer bars and adj. sways your car will be transformed. Drive a 930 setup like I have described and you will be sold. I speak from experience. I have driven my car w/ the 16" tires on and off the track...the 17" wide shoes blow away the old school setup. The extra rubber allows you to get much more aggressive cornering, braking and with the gas. Your margin for error with those 3 items will also increase...again from experience

Your tire choice will also increase by about 10 fold as well. As time goes by 16" and 15" choices are getting slimmer all the time.
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Last edited by F350Lawman; 11-08-2008 at 11:10 PM..
Old 11-08-2008, 10:57 PM
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Pardon me for intruding, I have *absolutely* no experience, but I think that most replies are missing the point.
He's not looking for better grip or trying to shave a whisker off his lap time. He's looking for better feel and agility, such as Lotus was doing when they went to thinner front wheels when they introduced the Elise Mk2. The loss in absolute grip and precision is (supposedly) compensated with more feel and agility.
Old 02-13-2010, 06:48 AM
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Oops, old thread :| ...
Old 02-13-2010, 06:49 AM
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Quote:
Originally Posted by Bothari View Post
Oops, old thread :| ...
don't worry about it - I'm enjoying the read.
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Old 02-15-2010, 07:22 AM
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