Cupcar

Anybody know how much difference in front to rear tire size one can run before the ABS starts to wonder what's going on? I want to run 275/40 rear with 235/40 fronts in 17". The difference in revs per mile is on the order of 4.3%, will the ABS sense this and get its braking program screwed up?

marc 1xx1

cupcar:

the owners manual says 15% of original tire size.

Cupcar

Marc- Thanks for the tip on the owners manual. I couldn't find the 15% reference you gave, what page is it on?

However, a statement I found in my owners manual (WKD 964 021 92 5/91 edition) quoted below and found on page 36 may be the answer:

"Note
The control unit for the ABS brake system is set for standard tire sizes. If non-standard size tires are fitted, the control unit may misinterpret the speed of the vehicle, because of the variant data it receives from the sensors on the axles. If the difference in rolling radius exceeds approx. 6%, the control unit deactivates the ABS system and the ABS warning lamp lights up."

It doesn't say whether this is a 6% difference from the original size or a 6% difference between front and rear sizes.

Adrian

Dear Cup Car,
I am afraid the difference is 6%. It is quite easy to check. The original tyres installed had 851 rotations per mile (I am doing this US style because it is the easiest to understand) front and back. So you need to keep within 6% of this original pre-programmed figure. Now it is also compared front to rear. So 6% gives you 50 revolutions per mile. However if you drop the front to say 810 you cannot raise the rear by more than 10. Everything has to stay within 50. I do recommend that you stay within 40 to allow for tyre variations. You can get these figures in the USA from Tire Rack. On the specs page.
The 6% is from original installation. 4.8% would be as far as I would go personally but I think it will be okay.
Ciao,
Adrian
911C4

Jeff Curtis

Cupcar, I run those EXACT tire sizes on the track...without issue.

My ABS works fine, belieeeeeeve me!

Adrian

Dear Jeff,
The tyre manufacturer used is also important. You might have no problem with say Bridgestones of this size but somebody might have a problem if they used Yokohamas. In fact Yokohama is a good example. On their German language web site they actually show the speed differences for the same dimensions tyres but different models. We have proof that tyres of the same "technically correct dimensions" vary by up to 25mm in width. One has to take care and do each 964 on its own calculations based on what is going to be used.
Ciao,
Adrian
911C4

PS: there is a famous 968 story along these lines written in Excellence in the early 90s.

Cupcar

Interesting this topic of tire revolutions per mile. My 4.8% difference calculated in my post above was from the Kuhmo spec sheet. I went to Tirerack and looked up the revs per mile for several representative manufacturers of 255/40-17 tires for an OE size RPM database. I found:

BFG 831 Rev/Mile
Bridgestone 830
Dunlop 772 (must be wrong)
Michelin N0 836
Michelin N1 826
Pirelli 827
Yokohama 833

Interesting Michelin RPM varies with Porsche type number. The average of above is 830, ignoring the Dunlop number. 3% of 830 is 25 like Adrian said so I guess going up and down 25 RPM from 830 gives a range of 805 to 855 RPM. This means that that essentially all 17 inch tires that anyone would normally run work, and Jeff is right his ABS works!!

The only Kuhmo sizes out of this range are a 205/40 X 17 at 877 RPM and 335/35 X 17 at 799 RPM,I don't think anyone would run the 205/40 on the front anyway, but a Turbo bodied car may have ABS problems with the big tire on the rear if it fits. <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

marc 1xx1

Cupcar:

Next time I will check my facts better...good catch Adrian.

The paragraph regarding the differential % is on page 36 of my owners manual. The last paragraph in the ABS Brake System section.

Jeff Curtis

Well, let's see here...I have three sets of wheels for the car.

One set is for the street, with Bridgestone RE-730s on them...they're 17" 996/Boxster "Twist" wheels.

The second set has Kumho ECSTAs on them, they are 17" Cup II 993 wheels.

The third set has Hoosier road racing slicks on them, they are 17" Cup II 993 wheels.

I don't seem to have any issue with the ABS doing anything goofy...and believe me, I test my applications out in their applicable environments.

Adrian

Dear Jeff,
I also think it should be pointed out that you have done some rather major modifications to your brake system on your C2. I personally prefer not to use highly modified 964s such as yours as a guide. The installed modified components will have an effect, maybe positive maybe negative on a vacuum boosted system. I cannot tell and it si theory only. The vast majority of owners who ask these questions have basically standard set up 964s. My help and words are primarily aimed at these people.
The C2 ABS is less sensitive to these changes as compared to a C4 but Porsche do not provide limits just for fun either. I have had to help a number of people with C2s and ABS fail lights and more than once it was the huge investment made on new wheels and looking kool tyres that cause the problem.
This is why I advise every 964 owner to do the calculations themselves for their own 964.I have clear evidence from 964 owners around the world that what works for them did not work for somebody else and vice versa. Before anyone spends huge sums of hard earned cash doing such things please work it out and be sure yourself.
Ciao,
Adrian
911C4

Cupcar

Jeff-I bet if you go to <a href="http://www.Tirerack.com" target="_blank">www.Tirerack.com</a> data base checking the speciications for each tire, you will find that all your tires are in the range of 805 to 855 revs per mile, so you are OK.

For example the Ecsta V700's you said you have in 235/40-17 are at 850 RPM and the 275/40 are at 815 RPM. I checked the 16 inch and 17 inch sizes that were standard for 964 sizes and they fall within 827 to 836 RPM for an average of 830 as baseline.

Adrian, where did you get 851 rotations per mile as the average for the original tires? Is this the actual design number for the system?