11SECSS

iTrader: (199)

My car is a 3 ch rear and every now a then I get the low trac light along with ABS light on when I run skinnys on the front and bias ply 26x10.50x15 on the back . I would say its like 25% of the time it happens. But when I run my 275/50/15 MT DR with the same rims I never get any lights (tire must not grow like the bias ply does).

SJMMFGINC

Based on your numbers you provided on your first post, you've got a difference of ~1 inch. This is too great of a variance. Someone mentioned tuning. You cannot tune your ABS system. ABS/TCS is per-programmed to work with the stock setup. It senses pulse signals through what is called a hall sensor. Each wheel sensor (your particular setup is three channel so the rear with each front wheel) signal is received by the ABS controller in electrical pulses. These pulses correlate to wheel rotation. The ABS than compares signals and attempts to keep pulses similar (via activating ABS and or TCS).

Here's the problem, your so far out of "normal" range, ABS will NOT function correctly. Here's the analogy I give hopefully this helps...lets say you want to put another cam in your car that is "slightly" different. The engine may still run but it will not operate at optimum. What do you do...well you tune the engines computer to compensate. In the case of ABS, you cannot do this...your modifying the system enough to be out of whack so you essentially have a non-optimum braking system and when you think its going to help, it may hurt as its now operating on false stimuli and adjusting according to what the program is tuned for.

What is the end result...ABS activating when it shouldn't as well as possibly creating worse braking conditions increasing stopping distances as well as creating a more uncontrolled environment. Essentially garbage in...garbage out.

We have an ABS delete kit that will solve your problems so that you don't have to worry about ABS anymore. This way you can run what you want to regarding tires and not have to worry about ABS ever again. If this is not an option for you...you'll have to stay within OEM specs if you expect the ABS system to operate as it was designed. To the folks who think their ABS system may be operating correctly with such a large variance...don't rely on it. It may not be throwing a code or light, but it will not work as it was designed. Remember it's only responding to the stimuli it receives which is not accurate...once again, garbage in, garbage out.

hope this helps.

Steve
Mechanical Engineer
SJM Manufacturing Inc.

justinmc978

Thanks Steve, lots of great info, you're definitely an engineer. I rely on my ABS too much to get rid of them, as my car is my daily driver, so I'll probably just stay stock.

BOBS99SS

iTrader: (2)

Question with my 275 /60/15on the rear i get this issue will going with a 17 in front fix this? I would think it would make up the difference for the rear would it not. If not i will have to lose the abs

SJMMFGINC

Justin, you're welcome. Just keep the tire diameters equal without delineation. If you're relying on them as much as you say, if you don't stay with similar diameters, you're not going to see the results you'd expect.

This is why you'll see tire shops (especially chain stores) want to change all four tires...and keep them the same. Even slight variations will throw off optimum function of ABS/TCS.

Bob, its not rim diameters which affect ABS/TCS concerns, its the overall tire diameter that matters. Calculate the numbers and compare tire sizes. For example, take your tire specs... x/y/z i.e. x=275, y=60 and z=15 as you've indicated, to calculate overall diameter, use this formula: (x/25.4)*(y/100)*2+z=overall diameter. Using those numbers, I'll break it down for you: 10.82*.6*2+15=tire diameter. This means the tire diameter of your 275/60/15 equals approximately 27.98 or 28". If you want to have ABS/TCS function correctly and predictable as its was designed, you need to purchase a front tire and have a resultant of 28".

BOBS99SS

iTrader: (2)

thanks alot sjm, i have not got around to messing with this issue, but i see what you are saying ,

MikeLSS1

Maybe ya'll can use this:

I bought a set of ccw sp16As off a vette. I put 285/35/18 up front and 295/35/19 in the back. I chose the rear because I loved the kdw2 and that was the only size at the time, and the front to get as close as possible to the rear tire size.
This yielded an overall tire diameter difference of .99in. With this setup I had some abs issues. I'd have to say i wasn't ever too easy about it, but i never had a close call.

I then had a kdw explode and had to find a used tire, and it was 285/35/19. This tire combined with the 285/35/18 yielded a 0.71in. difference in overall tire diameter. I have virtually no abs issues except one or two brake pedal pulses once in a blue moon. It also looked weak as hell.

But I would say i was just outside the threshold of abs issues using the 0.71 tire diamter difference.

for example, with the 285/35/18 front and 295/35/19 setup, it'd look like
[19+(2*(295*.35)/25.4]-[18+(2*(285*.35)/25.4)]

with the 2 for accounting for the two sidewalls and the 25.4 to convert to inches.

justinmc978

wow, great info, was wondering what the threshold was, thanks

sscamaro01

I run 275/40/r20 on the rear and 265/35/r18 on the front. Abs, trac control, and brake light are all on. Sometimes reads low trac when braking.