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Login to post Forums » Grassroots Motorsports » Front Brake Lockup

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April 19, 2009 8:36 p.m. pinchvalve UltraDork

Another fun day at the autocross today, but I was having issues with the fronts locking up WAY before the rears. This is common and understandable on a first-gen MR2 with so little weight over the front. With stiffer sway bars and upgraded coils, I get less weight transfer to the front which makes the situation even worse. I am researching ways to reduce this and stay STS legal.

One option is the relocate the battery to the frunk to put some weight up front. This has the added benefit of re-distributing weight and lowering the CG slightly. I don't know how much this would help, but it has other benefits and cant hurt as far as I can see.

The next thing to do is to upgrade the brake lines to braided SS lines. If it does nothing else, it replaces lines that are pushing 25 years old! I also like the feel of ss lines and the added security. Will it help braking performance? Not much, but it will help the overall system so its a no-brainer.

The next thing on my list is replacing the Hawk HP+ front pads with basic OEM pads. This should give less bit in the front for better balance, and I found the tip by searching GRM.com so it must work!

Finally, I have read about removing the guts from the stock proportioning valve to remove the front-lock bias. 1) does this actually work? 2) is this 100% legal for STS? There is no mention of changing the stock components or adding a proportioning valve, but usually if it doesnt say you CAN it means you CANT. The car sees little road use and I can certainly handle the change in braking dynamics. I'm not a bend the rules kinda guy.

Any other thoughts?
April 19, 2009 9:53 p.m. NONACK New Reader

On my ES mk2, I run worse than OEM (cheapest NAPA option) pads on the front and EBC's on the back, and the fronts STILL lock up a little bit before the rears, but at least it's better than with good pads all around.. I'd change front pads before you do anything else, then maybe relocate the battery. Also, as far as I know, changing the proportioning valve is not legal.
April 19, 2009 10:02 p.m. AngryCorvair Dork

Nowhere in the weight transfer equation will you find spring rate or bar rate. only CG height and wheelbase. Did you change either of these?

Or did you change your alignment settings, especially did you increase your negative camber?
April 19, 2009 10:23 p.m. Keith UberDork

The equation involves CG height, wheelbase - and rate of deceleration. If you're in a low-traction lot or it's cold, you'll have more problems with front lockup. The more traction you have, the more weight you'll transfer.

I'd put in an adjustable valve to replace the factory one, if legal.
April 19, 2009 10:59 p.m. moxnix New Reader

proportioning valve change is not legal in STS.

My STS miata is running stock fronts and better rears (forget what is on there right now) to help with brake lockup.
April 19, 2009 11:25 p.m. Strizzo Dork

not sure how the e-brake setup is in the mr2, but the way it is in my mazdaspeed3, if you pull the e-brake up a couple clicks but still before it grabs, it will change your brake bias. i was playing with it in the rain the other day, 3 clicks and the rears lock before the fronts, leave it down and the fronts lock first.
April 20, 2009 12:46 a.m. NONACK New Reader

Strizzo, if that works on MR2s you are my hero.
April 20, 2009 6:43 a.m. blaze86vic Reader

Spring rates do affect weight transfer. If you have stiffer springs in the front and rear it will reduce the amount of dive and squat a car has. This reduces the amount of weight transfer.
April 20, 2009 10:33 a.m. fiat22turbo SuperDork

No, stiffer springs reduce the speed at which the weight transfer changes the ride height. Shocks have a more direct effect on the speed of weight transfer. Neither has any effect on the total weight being transferred.

Do stiffer springs make your car somehow lighter? How do you get less weight transfer with the same forces acting on the vehicle?

Stiffer springs reduce the effects to the center of gravity, the bump-steer and camber changes inherent on macpherson strut suspended cars. All of which tend to be favorable, but it can cause the driver to have to change their driving style as the car may not be as communicative as it was before since the tires reach their adhesion limits much faster and without as much "drama" or physical movement.

Sounds to me like running the front slightly more soft might help the drive modulate the brakes better. The softer settings should allow the driver to drive the tires closer to their limit under braking at the expense of grip in the corners. Ultimately an adjustable bias valve in the brake system will be the proper fix.
April 20, 2009 10:48 a.m. Keith UberDork

Dive is a visible sign of weight transfer, not a cause. That's a common misconception.
April 20, 2009 11:06 a.m. GameboyRMH SuperDork

Strizzo wrote:

not sure how the e-brake setup is in the mr2, but the way it is in my mazdaspeed3, if you pull the e-brake up a couple clicks but still before it grabs, it will change your brake bias. i was playing with it in the rain the other day, 3 clicks and the rears lock before the fronts, leave it down and the fronts lock first.

That gives the same effect as adjusting the shoes closer to the drums on drum brakes - it could cause the brakes to drag a little. Worth a try though. I once did that and the brakes were dragging a bit - they got pretty hot after a long highway drive, but the braking power was awesome.

Anyhow you should definitely start with a battery relocation.
April 20, 2009 5:08 p.m. AngryCorvair Dork

blaze86vic wrote:

Spring rates do affect weight transfer. If you have stiffer springs in the front and rear it will reduce the amount of dive and squat a car has. This reduces the amount of weight transfer.

false. but the other guys already stated it correctly so i won't pile on.
April 20, 2009 5:20 p.m. 96DXCivic Reader

Look at Stoptech's technical white papers on their website for ideas. They have a lot of great information.
April 21, 2009 8:17 a.m. pinchvalve UltraDork
96DXCivic wrote:

Look at Stoptech's technical white papers on their website for ideas. They have a lot of great information.

Excellent advice. So check this out:

Factors that will increase front bias

* Increased front rotor diameter * Increased front brake pad coefficient of friction * Increased front caliper piston diameter(s) * Decreased rear rotor diameter * Decreased rear brake pad coefficient of friction * Decreased rear caliper piston diameter(s) * Lower center of gravity (i.e. lowered vehicle) * More weight on rear axle (i.e. loaded) * Less weight on front axle * Less sticky tires (lower deceleration limit)

The car has a naturally light front end, and under race conditions there is no spare, no tools, and no jack in the frunk so it is even lighter. Adding high-friction pads made the situation worse, as did lowering the CG by an inch. So the best solution seems to be getting some weight over the front wheels and loosing some friction in the pads.

The front sway bar is adjustable, and I have yet to play with it and tune it, but I understand that in a straight line with wheels straight ahead the sway bar should not affect the braking performance correct?
April 21, 2009 9:27 a.m. AngryCorvair Dork

pinchvalve wrote:

The front sway bar is adjustable, and I have yet to play with it and tune it, but I understand that in a straight line with wheels straight ahead the sway bar should not affect the braking performance correct?

That is correct.
April 21, 2009 10:13 a.m. wherethefmi HalfDork

why not throw the spare in the front next time, then you'll know if moving the battery will help. except when you move the battery later the rear will be lighter.
Dec. 18, 2009 2:36 p.m. Capt Slow HalfDork

sorry to dredge up an old thread but I was wonder what if pinchvalve had any sucsess in dealing with his braking issue.

With the recent cold temps (50 brrrr) I have been fighting a similar issue in a 1st gen MR2 in STS.
Dec. 18, 2009 3:23 p.m. Tyler H Dork

MR2s are awful for this. Moreover, the front passenger wheel usually locks up before the driver side.

You can try to tune it out with any number of suggestions offered, but it is inherent to the car. It is more a factor of excessive front bias from the factory than weight on the front wheels.

Brake less or brake sooner. Try to trim time off in other areas.

The best upgrade part for this issue is more tire.

Are you sure that you can't go flat through those turns?
Dec. 18, 2009 4:09 p.m. Rustfinger New Reader

Tyler H wrote:

MR2s are awful for this. Moreover, the front passenger wheel usually locks up before the driver side.

You can try to tune it out with any number of suggestions offered, but it is inherent to the car. It is more a factor of excessive front bias from the factory than weight on the front wheels.

Brake less or brake sooner. Try to trim time off in other areas.

The best upgrade part for this issue is more tire.

Are you sure that you can't go flat through those turns?

Troof!

Playing around with different pads did help a little for me, but it still locks up if you try to stomp the pedal too late into a turn. I found braking a touch earlier/less helps alot, and as a bonus, got me faster around the course.
Dec. 18, 2009 6:32 p.m. Capt Slow HalfDork

Tyler H said:
Are you sure that you can't go flat through those turns?

The car constantly surpises me. Its a cool feeling when you realize that "scary" feature can be taken flat...

What about machining the pad to reduce braking area? Has anyone played with this? Is it effective?
Dec. 18, 2009 10:21 p.m. AngryCorvair SuperDork

pad area really only affects how much temperature rise the pad sees in a braking event, which affects how long the pad will last.

unless you're talking about machining off the material that is radially the farthest from the spindle, to reduce the effective radius of the brake. not a good idea because it will unevenly load the pistons in the caliper and potentially lead to pistons getting stuck in the bores or premature failure of the piston seals. and the change you'd be able to realize would probably be in the ballpark of 5%, hardly worth the effort or the risks.
Dec. 18, 2009 11:16 p.m. GUNDY Reader

We had the same problem with the front right wheel locking after lightening the car. Some things to try: If the car is an 87 or newer try rotors and caliper brackets from an 85/86 they are smaller. Moving the battery helps but not much. Use a proportioning valve from a 85/86 preferably with out A/C Disasemble the proportioning valve and carefully grind a 1/16 of an inch off the spring to make it shorter repeat if it is not enough
Dec. 19, 2009 12:53 a.m. Capt Slow HalfDork

The car is an 88. The battery is already up front. I will need to go back and read the rules but I dont think I am allowed to mix and match parts from different years

Thanks for the input angry, you pretty much confirmed my all of the concerns I had with that plan...
Dec. 19, 2009 2:02 a.m. Keith PowerDork

I was about to post something about rear bias and cold weather, then recommend an adjustable proportioning valve - and discovered I was basically repeating what I'd posted exactly 8 months ago.

At least I'm consistent
Dec. 21, 2009 3:44 p.m. Capt Slow HalfDork

one idea we came up with is using full race pads up front.

The thought is that the full race pads would not come up to temperature during an autoX and therfore not grip as well...

How do you find "low friction" brake pads, its kind of an oxymoron.

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