How much can the front and rear tire diameters differ?
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How much can the front and rear tire diameters differ?
Hi folks. Is there any Porsche info on how closely the front and
rear tire diameters have to match in order to prevent damage to
the viscous torque transfer unit? It seems that the published specs
of the officially approved tires are not exactly the same. I want this
info for considering what range to stay within if I were to run
other tires and sizes if I were to change wheels etc.
thanks,
Joe
rear tire diameters have to match in order to prevent damage to
the viscous torque transfer unit? It seems that the published specs
of the officially approved tires are not exactly the same. I want this
info for considering what range to stay within if I were to run
other tires and sizes if I were to change wheels etc.
thanks,
Joe
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Joe,
I have never seen anything in writing. Sometime back I remember Viken on Rennlist had some information. I believe it was to keep the fronts and rears within 3% of each other. You might look him up and ask the question. When you get the answer, please post it. I have been running five tenths difference for sometime now without any troubles. Remember, the rears wear alot faster than the fronts, so over time the ratio will change from the new tire spec.
I have never seen anything in writing. Sometime back I remember Viken on Rennlist had some information. I believe it was to keep the fronts and rears within 3% of each other. You might look him up and ask the question. When you get the answer, please post it. I have been running five tenths difference for sometime now without any troubles. Remember, the rears wear alot faster than the fronts, so over time the ratio will change from the new tire spec.
Last edited by cjv; Feb 4, 2004 at 02:08 AM.
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Joe,
On the Rennlist forum, one reply refered to a PCA tech by the name of Joel Reiser. When I looked into his response on the PCA tech forum, he quotes 4%. There are a few responses to this issue. If I was to install Michelin Pilot Sport Cups sizes 345's and 265's in the 18" inch wheel I would have a one inch difference which would fall within the 4%. I questioned Rob King, the owner of S Car Go regarding the above sizes and he said no way would it work. He said he has experimented with the Porsche awd and encountered PSM and anti lock brake problems when one half inch difference was exceeded. If he is correct, it is not so much a percentage issue as it is a fixed number issue. What are we to believe?
On the Rennlist forum, one reply refered to a PCA tech by the name of Joel Reiser. When I looked into his response on the PCA tech forum, he quotes 4%. There are a few responses to this issue. If I was to install Michelin Pilot Sport Cups sizes 345's and 265's in the 18" inch wheel I would have a one inch difference which would fall within the 4%. I questioned Rob King, the owner of S Car Go regarding the above sizes and he said no way would it work. He said he has experimented with the Porsche awd and encountered PSM and anti lock brake problems when one half inch difference was exceeded. If he is correct, it is not so much a percentage issue as it is a fixed number issue. What are we to believe?
Last edited by cjv; Feb 4, 2004 at 09:50 PM.
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You might want to try this web site and punch in different numbers and see what you might be able to run.
http://www.miata.net/garage/tirecalc.html
http://www.miata.net/garage/tirecalc.html
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Warning: tire size calculators are bunk!
Hi, and thanks for being helpful, but you just tripped one of my
long-set hot buttons for warning others. The tire specs such as
275-40-17 etc are *only approximate*. Calculators are incapable
of what they hope to do, because of how off those numbers
can be from reality. The only accurate dimension data for a
given tire is from the separately published vendor's spec sheets
for the tire. I have seen two 245-45-16 tires *from the same
manufacturer* differ in diameter by 2%! The only difference
was that the two tires were different model types. ( all season
vs. summer etc)
long-set hot buttons for warning others. The tire specs such as
275-40-17 etc are *only approximate*. Calculators are incapable
of what they hope to do, because of how off those numbers
can be from reality. The only accurate dimension data for a
given tire is from the separately published vendor's spec sheets
for the tire. I have seen two 245-45-16 tires *from the same
manufacturer* differ in diameter by 2%! The only difference
was that the two tires were different model types. ( all season
vs. summer etc)
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