okindao

Okay guys here is the story.......last week hadpss9's installed and they also rolled my rear fendrs to be able to accept bigger tires te rear. Was on the highway nd driving over 75 mph and the anti lock brake light comes on the warnng panel.

please note that drove over 100 highway miles after new suspension prior to new tires without the light coming on so it seems to me to be due to the new tires

My suspension guru says that it is because the new tires are wider than he old ones ( 255 40 17's) and that the computer senses that the front and rear wheels are not in snc speed wise. and that this is exagerated as the speed increases, whch would explain why it did not happen immediately after leaving the tire place but 15 minutes later after i got onto the highway and was gong over 75 mph.

I turned off the car and restarted and the light went off, and stayed off for 5 minutes but never got to drive over 40mph. Think i will take it on the highway tomomnorw and get it over 75mph and see if it comes on again.

I am going to take to the dealer ( friend ) who says he can put it on the scope to check the fault) my guy thinks we can solve by one of 3 ways :

1. get bigger front tires ( i run 225 45 17's ) to help equalize
2 get wider rear wheel ,they are only 9 inches wide fiske wheels,
3. remove the sensors but that will disable the anti lock brakes

Appreciate any feedback this esteemd group can provide

Thanks in advance ..cant wait to get to the track but think i dont want to loose the anti lock brake capability of the car

BTW car is

97 c2 Cab
40,000 miles
brand new pss9's and sway bars, no engine modifications

Regards
dennis

allill

Dennis, is your car a c4? I went way wider on the rear (255 to 295) and have no problem with the abs light.

okindao

no it is a c2

allill

What size tires are you running and what wheels?

okindao

275 40 17's on rear with 9 " Fiske wheels

225 45 17's on front with 8.5 fiske wheels

Trevor-uk

That means your rear wheels are rotating around 2.8% less than the fronts (786 revs/mile against 808revs/mile). Your old 255s were closer at 806revs/mile - only 0.2% out. There are sensors that read wheel speed variance and determine when the ABS should kick in but I don't know what level of variance they need to see in a non-braking situation to throw an error. My guess is that it is related to this. Maybe go up to 235/45s on the front (do they exist?)

okindao

im sorryi was incorrect the fronts are 235 40 17's and the rears are 275 40 17's

sdenior moment is that still enough of a difference to cause the sensors to flip out ???

Thanks

Dennis

Falcondrivr

I would think that you want to keep the wheels close to the same outside diameter. The would mean that you should be at a 275/35/17 on the rear...At 275/40, you have almost a 10% bigger diameter on the rears. I bet your speedometer if a little off too.

Garth S

Their diameters are 24.4 and 25.6 " respectively ( 852 vs 814 rev/mile) ..... likely too much for the multi channel ABS brain to process. Porsche specifies oem tires that are a nominal 25" od in the 16 to 18" range - width is not a factor, until the selected tire is too wide for the rim: a 275 is pushing the limits of a 9" wheel width.

okindao

I am confused but others have mentioned that getting a wider wheel for the rear may solve the dilema,right now i have 9 inch rear wheels and the toyo fact sheet indicates 9 inch to 11 inch width is okay for that tire, will adding .5 or 1inch solve the abs issue ??? Also will adding that amount have impact on room for the tire uder the fender?? i had the fenders rolled and they are kay now just wondering if adding bigger wheels will cause another issue

Thanks for the help on this guys,

B-Line

Before you go flipping out, I would just hang back and see if it goes away.
It might just be a electrical glitch on a 10 year old car.

Now, if you're driving and stand on your brakes and the ABS doesn't work, then I would be concerned. But ABS is an easy enough thing to test. Just go drive you're car like you stole it (in a very safe place, parking lot, etc.) and see if you're ABS is having problems.

B

okindao

Two things have convcinced me not an electrical glitch:

1. Happened within 20 minutes of putting wider tires on for the first time
2. The diagonostics at the deaer indicated a wheel gear issue which translates to mean one wheel (the front ) is going muchg faster than the rear wheeel ( due to the size of the tire differential

Now the issue for me becomes what if anything to do about it, options being
1. turning off abs which is not a great option
2. getting wider wheels for the rear ie 9 inch to either 9.5 or 10 inch wheels
3 possibly getting bigger tires in front but do not want to hurt the turn in currently have 235 40 17's in front
4 donig nothing and living with in on the track as street tires obviously d not have the issue

What do you all think ???

Thanks for the input

Dennis

Garth S

.... OK, I agree with the TOYO charts - I was referencing Michelin's. In any event, 1/2" is no big deal , and in no way will influence the ABS issue.

The ABS sensors are a simple fine wire coil on an iron core that sit in each hub next to a notched steel 'gear' on the half shafts .... which act ,in effect, as a mini electrical generator producing an AC current when the toothed wheel turns: the frequency ( in Hz) of this AC signal from each wheel is compared within the ABS module .... and the brain must be at its happiest when all four frequencies are equal and will stay that way within a prescribed +/- window that allows for turning corners, etc; however, if one or more individual signals falls outside that window, the ABS activates when the brakes are applied .... or sometimes gets confused and goes off line
At 60 mph, that average frequency is 13.33 Hz X number of teeth/axle.
With your differing tire diameters ( not widths) , it is possible that the frequency difference generated by the slower turning rear axles is enough to jump outside of the window that keeps the ABS brain happy ... when the magic speed of 75mph arrives: that is assuming that the window is of fixed width .... but that may be a lousy assumption ...

Repeat the experiment!

Pan58

Here try this http://www.powerdog.com/tiresize.cgi I use it to determine Dia of my tires Stk and then try different wheel and tire sizes to keep it with in less than a 1%

okindao

I have no doubt that is the issue, i tun off the car the fault goes away when turned on and then at some point above 40 to 50 mph when the computer does its thing based on youre description the light comes back on which effectvely disables the anti lock brakes....now that we know what the issue defintely is the question still remains what to do, you state the .5 inch increase in rear wheel will not solve it how bout 1 inch?? i suspect that will solve it due to another friend with a 95 coupe running the same tires as myself and not havng the issue, still waiting to hear frm him on size of rear wheel ie 9 or 10 inches

Dennis