Technical Spec on Wheels

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Jim Grass
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Technical Spec on Wheels

#1 Post by Jim Grass »

I'd like to start a new topic on wheels, one that include some technical data for those looking to buy 'outlaw' wheels.

Here is my quandary; I have an early production 356A cabriolet (61061, very first car I ever purchased back in 1970 for $500!), it came with an updated transaxle type 644 and some interesting mag wheels (Crestline, see photo, oops scratch that, my photo is too big) that measured 6"x15" with ~0 offset (hope you don't mind I listed an approximate number here) with 5 on 205 bolt pattern. Mounted to these wheels was a set of Pirelli Cinturatos 165 SR 15, that just barey fit in the rear. After sitting in my garage for 30 odd years I've got the bug to source some needed parts, and wheels are my focus at the moment.
I now have a type 741 transaxle with disc brakes that is slated to replace the 644, so I need new wheels with a bolt pattern of 5 on 130, but choices are limited, and I want to make sure I order something near perfect for the job. I'm tending for VTO Classic 8 in 6"x15" 5 on 130, but the offset appears to be 24mm. What I don't know, because no one seems to discuss this topic, is what are the required offsets for outlaw wheel sizes? I don't want to use extra long studs with a stack of spacers to correct for too much offset. I'm thinking, based on what I had, 12mm offset should be plenty; but I don't know, so who can help me with offset numbers on the A body?

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Barry Brisco
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Re: Technical Spec on Wheels

#2 Post by Barry Brisco »

Hi Jim,

I'm stunned and impressed that you still own the first car you ever purchased and that it is an A cab!

I can't answer your specific offset questions (I would think the offset would depend on the type of wheel you choose) but I can suggest that you read the article "Wheel Weights and Types" at http://porsche356registry.org/resources ... ypes-.html which lists different wheel types that can be used on a 356, both drum and disc brake cars.

Sorry, I've never heard of "VTO Classic 8' wheels. If they will fit on a 356 I would appreciate your posting photos and what the wheels weigh so I can include them in the list in that article.

If your photos are too large to post on 356Talk, Google "resizing photos" to learn how to reduce them.

Best regards,
Barry Brisco
1959 356A Coupe 105553, Ivory / Brown
2009 987 Cayman, Carrera White / Beige (daily driver)

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Ashley Page
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Re: Technical Spec on Wheels

#3 Post by Ashley Page »

Barry Brisco wrote:Sorry, I've never heard of "VTO Classic 8' wheels.
Googled "VTO Classic 8' wheels" - they are a minilite knock off and they look pretty nice.

Can't help with the offset question as i am not near my car or extra wheels

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Jim Breazeale
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Re: Technical Spec on Wheels

#4 Post by Jim Breazeale »

Jim Grass wrote:I'd like to start a new topic on wheels, one that include some technical data for those looking to buy 'outlaw' wheels.

Here is my quandary; I have an early production 356A cabriolet (61061, very first car I ever purchased back in 1970 for $500!), it came with an updated transaxle type 644 and some interesting mag wheels (Crestline, see photo, oops scratch that, my photo is too big) that measured 6"x15" with ~0 offset (hope you don't mind I listed an approximate number here) with 5 on 205 bolt pattern. Mounted to these wheels was a set of Pirelli Cinturatos 165 SR 15, that just barey fit in the rear. After sitting in my garage for 30 odd years I've got the bug to source some needed parts, and wheels are my focus at the moment.
I now have a type 741 transaxle with disc brakes that is slated to replace the 644, so I need new wheels with a bolt pattern of 5 on 130, but choices are limited, and I want to make sure I order something near perfect for the job. I'm tending for VTO Classic 8 in 6"x15" 5 on 130, but the offset appears to be 24mm. What I don't know, because no one seems to discuss this topic, is what are the required offsets for outlaw wheel sizes? I don't want to use extra long studs with a stack of spacers to correct for too much offset. I'm thinking, based on what I had, 12mm offset should be plenty; but I don't know, so who can help me with offset numbers on the A body?
Jim

There is no such thing as a wheel for a 356 with 0 offset. Your wheels have offset, you are just stating it incorrectly.

Regards
PS: A wheel with 24mm of offset should be just about right for your car.
Jim Breazeale
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Jim Grass
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Re: Technical Spec on Wheels

#5 Post by Jim Grass »

Jim Breazeale wrote:There is no such thing as a wheel for a 356 with 0 offset. Your wheels have offset, you are just stating it incorrectly.
These wheels are oddballs for sure, and maybe I don't get it, but here's what I found: The wheel width from outer edge to outer edge is 6.63", the flange webbing is 5/16" (so the wheel inner width is 6"); then if I put a straight edge across the backside and measure down to the mounting surface its 3.19", and from the frontside down to the same surface its 3.44". From this I calculate a negative offset of 3/8" (I was thinking it should be the otherway around), and my first measurement was a little crude so I thought they were more or less equal amounts (hence ~0 offset). I take offset to mean the distance from the rim center line and the mounting surface; positive offset moves the wheel center line inboard (most common), negative moves it out.
With the wheels and tires I originally had you couldn't fit a pinky finger between the tire and the fender. Seems that if I went to a wheel with 0 offset, or even a small amout of positive offset I would be better off, but a whole inch more of offset seemed too much. I was afraid to order wheels with this much offset because they might cause rubbing against the inner fender wall.

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Vic Skirmants
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Re: Technical Spec on Wheels

#6 Post by Vic Skirmants »

The trailing arm is where you'll rub first.

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Re: Technical Spec on Wheels

#7 Post by Barry Brisco »

Jim Grass wrote:I take offset to mean the distance from the rim center line and the mounting surface; positive offset moves the wheel center line inboard (most common), negative moves it out.
Jim, I think that's correct. Sometimes I get confused about that, so I looked it up:
http://www.1010tires.com/tech.asp?type=wheels wrote:The offset of a wheel is the distance from the mounting surface of the wheel to the true centerline of the rim. A positive offset means the mounting surface of the wheel is positioned in front of the true centerline of the rim / tire assembly. This in effect brings the tire in to the fender well more. Conversely, a negative offset means the mounting surface of the wheel is behind the true centerline of the rim / tire assembly. This will cause the tire to stick out away from the vehicle.
This diagram shows a wheel with negative offset, right?
Image
Barry Brisco
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2009 987 Cayman, Carrera White / Beige (daily driver)

Curt Crowell
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Re: Technical Spec on Wheels

#8 Post by Curt Crowell »

Jim-Here's some rim offset data:
Stock 4.5" "A": Positive 24mm
Stock 4.5" "C": Positive 42-43.5mm
Stock 911 5.5": Positive 38mm
I have used 6" rims with both 33.4mm and 39 mm positive offset front and rear. I like the 5.5" or 6.0" rims on the rear, but I wouldn't use more than a 5.5" on the front, more restricts the turning radius too much with normal offsets. Curt Crowell
P.S. Barry's picture shows a rim with Positive offset.

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Barry Brisco
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Re: Technical Spec on Wheels

#9 Post by Barry Brisco »

Curt, thanks, I reread the text I quoted and now I see you are right: that diagram shows positive offset. I always get confused with this terminology!
Barry Brisco
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2009 987 Cayman, Carrera White / Beige (daily driver)

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Vic Skirmants
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Re: Technical Spec on Wheels

#10 Post by Vic Skirmants »

Affect, effect; I can never figure out the positive/negative offset stuff. :? :? :?

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Re: Technical Spec on Wheels

#11 Post by Jim Breazeale »

Curt Crowell wrote:Jim-Here's some rim offset data:
Stock 4.5" "A": Positive 24mm
Stock 4.5" "C": Positive 42-43.5mm
Stock 911 5.5": Positive 38mm
I have used 6" rims with both 33.4mm and 39 mm positive offset front and rear. I like the 5.5" or 6.0" rims on the rear, but I wouldn't use more than a 5.5" on the front, more restricts the turning radius too much with normal offsets. Curt Crowell
P.S. Barry's picture shows a rim with Positive offset.

Curt

I don't think your figures for a Stock 4.5" "C" wheel are correct. No, I haven't done any measurements nor do I want to. Those numbers just don't jive, IMHO.

Regards
Jim Breazeale
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Re: Technical Spec on Wheels

#12 Post by Curt Crowell »

Jim-I re-checked my "C" rim offset number, this time I came up with 41.5mm offset. So far I've measured 41.5, 42.0 & 43.5, but I think the 41.5mm is the closest. This isn't like measuring crankshafts for ovality, and I didn't bother to check whether these are 3.7 or 3.9mm rims, but that's probably only be the web thickness, I don't think it's the flange thickness. I guessed the flange is 3.0 mm thick, like the early 5.5" 911's. I'm doing this on a rim with a tire mounted on it, doubt if anyone needs hairline accuracy here. Take a gander at the inside of a "C" rim; the web is pretty far toward the outside. It may be hot in Arizona, but around Chicago we have 90* and lots of humidity. Curt Crowell

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Re: Technical Spec on Wheels

#13 Post by Mike Klapac »

This would be an informative thread...with pictures. How about some shots of a tape measure up to a straight edge on the inside of the wheels? We could have our own spec page on wheel fitment.

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Re: Technical Spec on Wheels

#14 Post by Jim Breazeale »

Curt

Well, I went out and measured some wheels. Using Barry's example I measured a Drum brake wheel, A C disc brake wheel and a later 911 5 1/2 X 15 steel wheel. My measurements were taken from the outer edge of the rim . I divided that measurement in half (to get the center line of the wheel) and subtracted that figure from the back spacing figures to get the "offset"

Standard 4 1/2 X 15 drum brake, steel wheel (dated 12/62) had an offest of 28.5 mm
Standard 4 1/2 X 15 356C disc brake wheel (dated 7/63) had an offset of 43.5 mm
911/912 5 1/2 X 15 disc brake wheel (dated 6/77 but who cares?) had an offset of 45mm. All of my measurements are not exact but as close as can be using Junk Yard measuring equipment.

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Re: Technical Spec on Wheels

#15 Post by Jim Grass »

Using junk yard measurements myself, I see that the width of the drum brake is about 2.1" compared to the disc which is about 2.8" (both measurements were from the wheel mounting surface to the backing plate and axle mating surface), which I believe explains the difference in wheel offsets between the two systems. With an offset of 43.5mm on the 4.5" rims that puts the backspace at about 4", so with a 6" wide rim an offset of 1" (25mm) gives about the same backspace. This is the kind of data I've been trying to get comfortable with before ordering a set of wheels.
Thanks everyone!

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