Tom: good news/bad news - rears locking up before fronts

[PhoenixGTS]

Now that I have matching compounds on all four corners I can make some better comments regarding my Tom' 40mm rear calipers (I have had them for years). I used to have sticky rear tires and regular fronts so I disabled my proportioning valve by removing the little rubber o-ring (I kept the o-ring soaked in brake fluid) but the front would still go up in smoke with the rear not even thinking about it. Now I have Toyo R888s on all four corners. I had braked hard a few times but never locked up the new tires. Took the girlfiend out Saturday and gave her a show of the now unbelievable braking with the sticky front tires (wear rating 100). Other than her screaming and getting a little car-dizzy, the interesting thing is that the rears locked before the fronts - although I think it was very close. What do you think Tom? Put the rubber o-ring back in the prop valve? Or keep it out and fully utilize your calipers?

 

[99 R/T 10]

Put some EBC greens on the rears, that ought to take the bite out of the rears........................

 

[TexasPettey]

I can't comment on the O-ring, but I do have Tom's rears and Dave's SRT fronts & adjustable prop valve. I also run R888's on the track. I have to have the prop valve closed quite a bit or the rears will lock up. I think I'm between 2 & 3 turns. When I was stock Gen1 fronts & Tom's rear, I was about even on braking between front and rear with the stock prop valve. If I was heading downhill, I'd lock the rears just a bit before the fronts. Otherwise, the fronts would lock first. If you disabled the prop valve, I'd expect you have way too much rear brake.

 

[Chuck 98 RT/10]

Put a less grippy pad on the rear.

 

[Toby]

I would check your tire pressures as well.

 

[Tom F&L GoR]

Easy thing is what Toby said and check tire pressure, especially side to side. Next easy thing is put the o-ring back in.

Last thing is harder - do you have non-standard wheel diameters (different front to back, like 18's and 19's?) What is the front tire size? If the front contact patch is somehow relatively larger than before, then you can get more braking capability from the front, meaning the rears could lock first. And the more you can brake, the more weight shifts to the front, so the lighter the rear gets and the less the rear wheels can brake, so the less pressure you should have. (This is why they have prop valves.)

As Mr. Pettey points out, an all stock system is balanced, so I'm looking for what has changed that.

All-in-all, I'm impressed. But you need to stop doing it.

 

[PhoenixGTS]

do you have non-standard wheel diameters (different front to back, like 18's and 19's?) What is the front tire size?

Stock 99-02 Viper 18" wheels. Stock size 275-35-18 front and 335-30-18 rear Toyo R888s which are 100 wear rating at 30 psi each corner (although I will recheck).

More sepcifically if was the driver's rear that locked before I released pressure with the passenger rear just barely starting. One more data point: like most/all GTS my car sat low on the driver's rear so I have my adjustable-height shock in that position screwed up almost an inch higher so the car is now level. Back when I had the mixed compounds this stopped the passenger front from locking way before the driver's front and caused the front to both lock at the same time.

I really do not want to put the o-ring back in the prop valve if it will be overly aggressive in releasing rear pressure. But if you think I will not be wasting the ability of your calipers I might try it. My goal with the post was to give you somewhat surprising data.

 

[Tom F&L GoR]

The safety issue is that you come across some combination of sticky tires, road surface, panic, etc, etc, and the rears lock causing you to lose directional stability. All the occupant safety systems (seatbelt, airbags, crush structure, steering column compression) are optimized for frontal impact.

If you do put in the o-ring, it means you won't have to let off the brakes when a rear wheel locks up and you might be able to brake that much harder. Don't know for sure. Perhaps the ideal situation is an adjustable valve that lets you find where all four imminently lock simultaneously.

No, I'm grinning like an engine builder watching a customer doing a big burnout... sort of the reverse of that, I guess... But I don't want to see something go wrong, either.

 

[Toby]

Your alignment may be off as well if you have adjusted the ride height.

Too much camber either way will reduce grip under straight line acceleration and braking.

Once your alignment is verified and you still have lock up...reduce air pressure in the rear by one to two pounds and try again.

If that still doesn't work...reconsider your brake set up and have a professional inspect your suspension for further clues to your braking problem.

 

[PhoenixGTS]

Your alignment may be off as well if you have adjusted the ride height.

I had an alignment done after I went to the current height so I think I am good there.

I did some more "testing" at lunch today and the rear is definitely locking before the front - and both rear tires pretty evenly. I have located the proportioning valve rubber o-ring in my spare parts storage and will be re-installing it this weekend. Haven't bled the brakes since I took it out years ago it will be good to do that. Man this car will stop hard with sticky tires on all four corners.

 

[GTS Bruce]

Get a prop valve. Turn in and brake. Push under braking indicates too much front. Loose equals too much rear. Simple and does not take into consideration other factors including alignment. Should be able to tune F/R bias, push/loose fairly easily under turn in braking providing you pres on the brakes and don't just stand on it causing a lot of weight transfer. (Back in the day bias tuning) GTS Bruce

 

[PhoenixGTS]

GAWD replacing the plunger on the proportioning valve SUX! That little spring is so tough it is a night mare for a small hands/light grip guy like myself to get the threads started on the cap. I remembered it being a PITA and I was correct. Tomorrow I bleed then I will know where we are Tom.

BTW, should I bleed the fronts first? Or the rears?

 

[Tom F&L GoR]

In theory you should only need to bleed one rear caliper. I think every time I've done that it's been fine. Looking forward to the results.

 

[PhoenixGTS]

I've said it before and I'll say it again: if I had to work on cars for a living - I would starve (same with home improvement projects). I get it done, just VERY slowly. The brake bleed was complicated by my floor jack giving up the ghost right when I got the fourth jack stand in (my garage ceiling is too low for a lift). And of course the car was in the way of the garage door by about two inches so I could not close the garage door and have lunch. Luckily, my Dad lives 11 miles away and bailed me out with his floor jack. Bled all four corners with vacuum bleeder and I think the pedal feels firmer than before so it looks like it needed a bleed anyway. Of course while I had all four wheels and tires off I had to clean brake dust off the back of the wheels. Tom you will be proud to know there was just as much or more dust on the insides of the rears as the fronts. I have not driven yet, but hopefully the proportioning valve does not take away a ton of brake effect. At least I will be going in a straight line.

 

[PhoenixGTS]

Finally got out for some "testing" of the brakes with sticky tires (Toyo R888s), the proportioning valve intact, and Tom's 40mm rear calipers on a 98 GTS. Ambient temperatures are in the 80s here so the asphalt is hot, grip is good, and testing to see what tire locks up is hard given the sticky tires. When I brake hard from 60 mph or more the breaking force is tremendous but the tires will not lock. The brakes are as perfect for non-ABS as I have ever experienced. I can get lock if I brake super hard from a lower speed like 40 mph in which case the passenger front locks first (even with my driver's side rear coilover adjusted almost an inch higher than the passenger side rear). So other than the braking being 100% better especially due to the huge change in the rear due to Tom's calipers - and of course from the 100 wear rating tires, the balance at the limit under big pedal pressure appears to be like stock.

 

[Tom F&L GoR]

>The brakes are as perfect for non-ABS as I have ever experienced. <

Cool! Time to change the thread title! Any chance someone you know has an accelerometer (sometimes used for side g-forces in autocross) and can measure the deceleration rate?