Tire Aspect ration and ABS warning light....
12-14-2003, 03:39 PM
#1
Rennlist Member
Thread Starter
Tire Aspect ration and ABS warning light....
I have had a "sticky" problem wkith my ABS light coming on in my 97 Carrera. It was an occasional event up readily cleared up by turning the ignition off then back on. I did replace the ABS relay in the trunk. All was well until I installed some new race rubber. Almost immediately the light came on and the ABS was disabled. I tried the old turn of the ignition trick...this worked for about 5 minutes, and then bingo, the light came back on again.
This was not good as I was on my way for 2 days at Sears Point (Infineron) with TracQuest to do a little instructing.
At Infineron, the light stayed off for at least one full session and I could drive depending on the ABS to prevent wheel lock up. The next session, the light would come on, ABS would go down, and wheel lock up would commence, not a good thing when one wants to stop from high speed.
The last session of the day, I went over to street rubber in a stock Porsche tire size configuration. Not only did the ABS light not go on, but it stayed off the entire 500 mile (ouch!) drive home. I did another 160 mile drive yesterday...no ABS light.
I just happend to meet Steve Alarcon from Johnson's Alignment/Suspension Specialties in Torrance, CA, and told him of my woes...his instantaneous response, you probably have a problem with tire aspect ratio. You may have on a tire size sufficiently different from factory specs that the ABS Brain is "confused" and cycles off.
I checked the factory spec for 17" tires. 205/50/17 front and 205/40/17 rear, with a 50 aspect ration on the front and a 40 aspect ration on the rear. I then checked my race rubber: 204/40/17 and 255/40/17. Whoops, the aspect ration on the front race rubber is out of spec with the factory specs.
This may well be the problem as I am not seeing the ABS light come on with Porsche spec tires. I guess I could put the 17" race rubber back on tosee if ABS shuts down again, I may do that. In the interim, I will drive with the 18" factory rubber and be happy to have my ABS back.
Once ABS kicks in on the track, or the street for thaqt matter, you will love it. I seem to feel it more on the track, perhaps duwe to the stick tire compounds used.
Please note, I took the car to Porsche dealer...they had no idea of what the problem could be...they did sell me the ABS relay however, and charged me $100 to look at the car. Thanks Steve, as always, you are da man!
This was not good as I was on my way for 2 days at Sears Point (Infineron) with TracQuest to do a little instructing.
At Infineron, the light stayed off for at least one full session and I could drive depending on the ABS to prevent wheel lock up. The next session, the light would come on, ABS would go down, and wheel lock up would commence, not a good thing when one wants to stop from high speed.
The last session of the day, I went over to street rubber in a stock Porsche tire size configuration. Not only did the ABS light not go on, but it stayed off the entire 500 mile (ouch!) drive home. I did another 160 mile drive yesterday...no ABS light.
I just happend to meet Steve Alarcon from Johnson's Alignment/Suspension Specialties in Torrance, CA, and told him of my woes...his instantaneous response, you probably have a problem with tire aspect ratio. You may have on a tire size sufficiently different from factory specs that the ABS Brain is "confused" and cycles off.
I checked the factory spec for 17" tires. 205/50/17 front and 205/40/17 rear, with a 50 aspect ration on the front and a 40 aspect ration on the rear. I then checked my race rubber: 204/40/17 and 255/40/17. Whoops, the aspect ration on the front race rubber is out of spec with the factory specs.
This may well be the problem as I am not seeing the ABS light come on with Porsche spec tires. I guess I could put the 17" race rubber back on tosee if ABS shuts down again, I may do that. In the interim, I will drive with the 18" factory rubber and be happy to have my ABS back.
Once ABS kicks in on the track, or the street for thaqt matter, you will love it. I seem to feel it more on the track, perhaps duwe to the stick tire compounds used.
Please note, I took the car to Porsche dealer...they had no idea of what the problem could be...they did sell me the ABS relay however, and charged me $100 to look at the car. Thanks Steve, as always, you are da man!
12-14-2003, 03:52 PM
#2
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Martin,
I think what he is trying to tell you is that the overall tire diameters (one set of tires appears to be traveling further than the others). is what is causing the problem. So it actually has little to do with aspect ratio. If you had a 40 on the rear with 205s, and went with 315/40s you would still potentially have a problem given the same factory size fronts. On the rears, you increased your overall tire diameter, In the fronts you decreased this diameter. So to your computer, your fronts are spinning faster than the original program, and your rears are spinning slower. This must be outside of its' control parameters and hence your problem.
I think what he is trying to tell you is that the overall tire diameters (one set of tires appears to be traveling further than the others). is what is causing the problem. So it actually has little to do with aspect ratio. If you had a 40 on the rear with 205s, and went with 315/40s you would still potentially have a problem given the same factory size fronts. On the rears, you increased your overall tire diameter, In the fronts you decreased this diameter. So to your computer, your fronts are spinning faster than the original program, and your rears are spinning slower. This must be outside of its' control parameters and hence your problem.
12-14-2003, 04:06 PM
#4
Rennlist Member
Marty-
I can only speak for the 964's ABS which is less sophisticated than your superior 993 car. In my 964 owners manual it states that if the rolling radius of the tires exceeds a 6% variation the ABS light will come on. You can go to www.Tirerack.com and look up the rolling radius or revs per mile specs for your tires and check if they exceed 6%.
You may want to check your 993 owners manual's ABS brake section to see if their is a similar warning for the 993.
BTW where have you been lately anyway?
I can only speak for the 964's ABS which is less sophisticated than your superior 993 car. In my 964 owners manual it states that if the rolling radius of the tires exceeds a 6% variation the ABS light will come on. You can go to www.Tirerack.com and look up the rolling radius or revs per mile specs for your tires and check if they exceed 6%.
You may want to check your 993 owners manual's ABS brake section to see if their is a similar warning for the 993.
BTW where have you been lately anyway?
12-14-2003, 04:21 PM
#5
Rennlist Member
Thread Starter
Cupcar,
I had brain fade when I bought the tires...I wanted to try the Toyo RA-1 tires. They make great practice tires with 8/32 of tread and they are cheap, $500 in 17" and $700 in 18".
The down side, their 17" fronts are either 205 or 235. The 235 is too big for a 7" X 17" wheel....notwithstanding their rollong radius delta problem between fron and rear.
I have been up at the vineyard watching the grapes shed their leaves...nice work if you can get it!
I'll give to a ring...........
I had brain fade when I bought the tires...I wanted to try the Toyo RA-1 tires. They make great practice tires with 8/32 of tread and they are cheap, $500 in 17" and $700 in 18".
The down side, their 17" fronts are either 205 or 235. The 235 is too big for a 7" X 17" wheel....notwithstanding their rollong radius delta problem between fron and rear.
I have been up at the vineyard watching the grapes shed their leaves...nice work if you can get it!
I'll give to a ring...........
