HELP! ABS engaging extremely early
Thread Starter
Burning Brakes
Joined: Apr 2011
Posts: 770
Likes: 2
From: Alexandria VA
HELP! ABS engaging extremely early
I have a 2011 Grand Sport, Hawk DTC 70/60 setup on stock calipers. New Nitto NT-01s, 315X18 and 335X18 and knocked out my fog lights and installed ducts to the rotors. ... I do not have any dash lights on and I hold the TC button till both TC and AH is off (I know this doesn't turn off ABS, nor do I want it to). I used to have a 2007 Z51 and with a similar setup, except square 305X18 tire setup, I could brake DEEP into any turn. Now I have to give myself almost twice the distance to stop because ABS goes crazy and car starts darting all over the place when braking in a straight line. Any thoughts??? Is there an aftermarket or rebuilt ABS unit that is built more for the track?
Team Owner
Joined: Dec 2012
Posts: 30,427
Likes: 1,609
From: Outside the Quick Stop N.J.
You did not say what aspect ratio your tires are, but the ABS uses a difference in rotation to activate. My guess is that your fronts are too far from stock sizes for the computers liking.
Race Director
Joined: Mar 2010
Posts: 16,882
Likes: 5,341
From: T-Town WA
2016 C6 of Year Finalist
Slow nite at my house so here ya go...
Assuming the aspect ration on the new tires is the same as original (35 front, 30 rear) your tires look like this:
Stock fronts 275/35/18 = 789 Revs per mile......Stock rears 325/30/19 = 756 Revs per mile;
New fronts 315/35/18 = 753 Revs per mile......New rears 335/30/18 = 779 Revs per mile.
Your ABS on stock size tires says the fronts are running 4% faster than the rears...everything is cool.
Your ABS on the new tires says the fronts are running 3% slower than the rears...WTH!
Or something like that...
Assuming the aspect ration on the new tires is the same as original (35 front, 30 rear) your tires look like this:
Stock fronts 275/35/18 = 789 Revs per mile......Stock rears 325/30/19 = 756 Revs per mile;
New fronts 315/35/18 = 753 Revs per mile......New rears 335/30/18 = 779 Revs per mile.
Your ABS on stock size tires says the fronts are running 4% faster than the rears...everything is cool.
Your ABS on the new tires says the fronts are running 3% slower than the rears...WTH!
Or something like that...
Le Mans Master
Joined: May 2016
Posts: 7,144
Likes: 1,546
From: Florida
Yes on these reasons. ABS is responding to the differences in axle rotations and is programmed to work normally within a window given factory wheel diameters and tire aspect ratios. ABS is reacting BC your axle rotations, front/back, cross through that threshold in some of your driving conditions.
EDIT: Driving conditions, more specifically, defined as the braking times, not the free wheel or acceleration times. The differences are - with TC and AH off, the deltas in axle rotations have no threshold. However, when you apply the brakes, ABS, which cannot be shut off, takes over and does have a threshold. So, when you apply the brakes, ABS sees a wheel spin condition or a skid condition if you're braking in a turn. You have to get "rolling circumferences" front to back, matching factory tolerances if you want this to stop.
EDIT: Driving conditions, more specifically, defined as the braking times, not the free wheel or acceleration times. The differences are - with TC and AH off, the deltas in axle rotations have no threshold. However, when you apply the brakes, ABS, which cannot be shut off, takes over and does have a threshold. So, when you apply the brakes, ABS sees a wheel spin condition or a skid condition if you're braking in a turn. You have to get "rolling circumferences" front to back, matching factory tolerances if you want this to stop.
Last edited by BlindSpot; Oct 16, 2018 at 07:53 AM.
Race Director
Joined: Mar 2010
Posts: 16,882
Likes: 5,341
From: T-Town WA
2016 C6 of Year Finalist
You went from a fronts faster to a fronts slower ratio. ABS thinks your fronts are skidding.
Last edited by ruxvette; Oct 16, 2018 at 11:04 AM.
Le Mans Master
Joined: May 2016
Posts: 7,144
Likes: 1,546
From: Florida
If you want to correct this yourself, you can do it with a tire aspect ratio calculator like this one:
https://tiresize.com/calculator/
I've use this calculator several times on other cars. Use the tire comparison tab. Go the the GM owner's manual and get the exact factory tire dimensions. Put the fronts in TIRE 1 and the rears in TIRE 2, then run the calculator and note the delta in rolling circ. %
Next, put YOUR existing front tire in TIRE 1 and Rear, TIRE 2, run the calculator, compare your rolling circs and the deltas and how far off they are.
To correct the problem, start with only one variable, let's say your existing front tires. Look for a different replacement size. Plug it in the formula leaving your existing TIRE 2 (rears) in the calculator. See if you can find a replacement size that begins to look like the OEM comparison rolling circs. %. If you cannot find anything close, then reverse the search, putting your existing front tire back in the formula looking for different rear sizes that mimick the OEM %.
I expect you'll find that you can no longer run same size wheels and get there. Either the fronts will need to go down 1", or the rears up one, which will put you back into the factory OEM setup.
https://tiresize.com/calculator/
I've use this calculator several times on other cars. Use the tire comparison tab. Go the the GM owner's manual and get the exact factory tire dimensions. Put the fronts in TIRE 1 and the rears in TIRE 2, then run the calculator and note the delta in rolling circ. %
Next, put YOUR existing front tire in TIRE 1 and Rear, TIRE 2, run the calculator, compare your rolling circs and the deltas and how far off they are.
To correct the problem, start with only one variable, let's say your existing front tires. Look for a different replacement size. Plug it in the formula leaving your existing TIRE 2 (rears) in the calculator. See if you can find a replacement size that begins to look like the OEM comparison rolling circs. %. If you cannot find anything close, then reverse the search, putting your existing front tire back in the formula looking for different rear sizes that mimick the OEM %.
I expect you'll find that you can no longer run same size wheels and get there. Either the fronts will need to go down 1", or the rears up one, which will put you back into the factory OEM setup.
Last edited by BlindSpot; Oct 16, 2018 at 11:27 AM.
Thread Starter
Burning Brakes
Joined: Apr 2011
Posts: 770
Likes: 2
From: Alexandria VA
Thanks for the replies. I will try less aggressive pads first at the event in two weeks. I will also look into a new bosch ABS unit.... Its crazy that I used the exact same tires, wheels, pads (4x18" C5z rims, NT01s, 70/60 pads) on my 2007 Z51 and had zero issues. Then new car, and larger calipers, lots of issues. So I got larger tires with larger rims and they helped a little (maybe) but still far worse than the dinky OEM brakes on the Z51. I will keep you all posted.
Racer
Joined: Apr 2014
Posts: 489
Likes: 63
From: Mason OH
Thanks for the replies. I will try less aggressive pads first at the event in two weeks. I will also look into a new bosch ABS unit.... Its crazy that I used the exact same tires, wheels, pads (4x18" C5z rims, NT01s, 70/60 pads) on my 2007 Z51 and had zero issues. Then new car, and larger calipers, lots of issues. So I got larger tires with larger rims and they helped a little (maybe) but still far worse than the dinky OEM brakes on the Z51. I will keep you all posted.
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Thread Starter
Burning Brakes
Joined: Apr 2011
Posts: 770
Likes: 2
From: Alexandria VA
Sorry, I am skimming through a lot of info and putting together a plan of attack... I am going to start with testing less aggressive pads I already have to confirm its not too aggressive of a compound instead of immediately dropping $1200 on a new set of tires (maybe $600 if I only have to by one pair). From there, I will mess around with tires. I also know that there are many racers out there that use C6 Grand Sports and I would assume they are not maintaining the stock rotation ratio OR have replaced the ABS unit.
Race Director
Joined: Mar 2010
Posts: 16,882
Likes: 5,341
From: T-Town WA
2016 C6 of Year Finalist
Hey, OP...what is the aspect ratio of the tires you have on your car? 315/XX/18 and 335/XX/18. I'll do the calculations for ya...
Thread Starter
Burning Brakes
Joined: Apr 2011
Posts: 770
Likes: 2
From: Alexandria VA
FYI, I changed from 4.5% ratio to 1.8% ratio. Not major but could be it. Again, I used to run a square set up so 0% ratio difference and had zero issues on my 2007. I believe the ABS sensor runs on each wheel independently.
Melting Slicks
Joined: Jul 2016
Posts: 2,521
Likes: 219
From: Los Angeles CA
The aggressive pads could possibly build up and foul the abs sensors. Sometimes the pad material is magnetic and can cause this if it builds up on the sensor. I had it happen to my mustang on the track with hawk pads and Hoosiers. It's much easier to clean the Ford sensors however.
Not even sure this is possible with a vette hub though
Not even sure this is possible with a vette hub though
Safety Car
Joined: Feb 2016
Posts: 3,662
Likes: 386
From: Killeen Texas
ignore all the internets peeps talking about tire ratios, there are tens of thousands of us tracking our cars with 315 square and 315/335 combo. the two most popular track sizes out there. thats not your issue, ive run both combos personally.
Thread Starter
Burning Brakes
Joined: Apr 2011
Posts: 770
Likes: 2
From: Alexandria VA
Yeah... thats why I didn't go out and buy new tires. I spoke with Joe at Phoenix Racing and he said it has to do with the POS Bosch ABS unit and to mess around with brake compounds depending on compounds and tire widths.
Last edited by ShaWING66; Oct 16, 2018 at 09:19 PM.
Melting Slicks
Joined: Jul 2016
Posts: 2,521
Likes: 219
From: Los Angeles CA
Did he say if the pad dust could foul the sensors like on my other cars?