One front brake locking

GregS

GregS

ProjectX
I just fitted new brake pads to all 4 wheels, went for a drive to bed the pads in, but when I got to heavy braking from 100kph it keeps locking up the left front momentarily. It doesn't stay locked, just while heavy braking, releases as soon as I take my foot off the pedal. I took both front wheels off and neither side is dragging abnormally, just a slight rub. The disc is not running out, no variation in thickness. I am using stainless braided brake hoses. I bled both front calipers with 3 pedal pumps, no air came out. Pedal feels good and firm. With constant light pedal pressure it takes similar force to rotate each wheel. I even tried swapping pads from left to right it case 1 side was different material, no change, left still locks to get a puff of smoke off the tyre.
I am running out of ideas. I might have to strip the calipers down.
I suppose pushing the pistons in to take the new pads may have dislodged some crud that can jam the right hand piston enough to stop it working to 100%? Can't actually make the left side work more than 100% to cause it to lock.
Any suggestions welcome.
 
It sounds like your calipers are in pretty good shape, most of the time a caliper piston will fail to retract because of corrosion behind the seal causing it to drag. It sounds like yours is retracting.
Unloading the left front could cause the front left brake to lock. Any chance the left front is "getting light" under hard braking? Excessive toe might also contribute to brake lock? Are the steering components in good nick?
 
Have you tried swapping the slide wedges from side to side to see if they are causing the caliper to stick?
 
Clean the wedges and wedge surfaces, smooth them with fine grit sandpaper, lube with silicone caliper lube.... and if all else fails the front calipier seals are cheap and it's an easy job to replace them.
 
Greg, your car is right hand drive correct? X1/9s have always had a tendency to lock the front wheel opposite the driver's position. Right hand drive X1/9s already have a right side weight bias, that varies somewhat with fuel load, so add in the driver and the left front wheel is typically about 100 lbs light. That's close to 25% and the lack of vertical loading on one front tire will make it far more likely to lock under hard braking.

I would be sure that the brake hoses are in good condition, the caliper pistons move freely (in and out) and the calipers themselves move smoothly on the sliders. A caliper binding on the sliders will perform poorly and the opposite wheel will lock well before it.

If its not the first, its likely the 2nd.
 
Tire pressures both 24psi cold.
Yes, right hand drive. But in all the track use over the last few years I have never had any trouble with brake lockup. The wedges and calipers slide nicely, should I lube them at all? I just put a bit of graphite powder on the wedges. I will check the pistons are moving freely when I get a chance. I think the braided hoses are ok, they are not holding pressure back, and the calipers bleed easily with light pedal pressure.
 
GregS said:
Tire pressures both 24psi cold.
Yes, right hand drive. But in all the track use over the last few years I have never had any trouble with brake lockup. The wedges and calipers slide nicely, should I lube them at all? I just put a bit of graphite powder on the wedges. I will check the pistons are moving freely when I get a chance. I think the braided hoses are ok, they are not holding pressure back, and the calipers bleed easily with light pedal pressure.
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Take a close look at the pads. Try to see if they are the same compound/construction. Its possible they got the compound mixed from one set to another. Not likely but it is possible.

Absolutely lube the sliders/wedges. There are a number of lubes that work. I have had good luck on street cars with using anti-seize. It is stable and tends to not boil off. For a heavily used track car there are specific caliper greases but I have had good luck with high temp wheel bearing grease. Whatever you use, go lightly with it.
 
Update, finally got to fit new caliper seals to both fronts, rubber grease on them, caliper grease on the wedges. Brake hoses good, removed stuff from front boot (8kg). Test drive heavy braking from 100ks still locking front left prematurely. Swapped front wheels/tyres L to R. Still tending to lock L front, but maybe not quite as easily, occasionally R front locked at lower speed. I am starting to think the tyres have gone off a bit, and I can't brake with them as hard as I am used to. Toyo R888s, I haven't sprinted on these for a couple of years, but they are stored in plastic garbage bags at a pretty stable temp. I will have to try with my newer Dunlops to see if they are better, and I could adjust the front coilovers to load the left front a bit more to allow for driver weight on the right. I think I have tried everything else I can think of.
 
I went through an exercise like this on a 124 spider. First off, I thought the right front was locking up. No, it was the left front not reacting fast enough unless I really jammed on the pedal. Then, it seemed like a slight pull in the opposite direction when I released the pedal.

Problem solved when I took both calipers back off and again cleaned the grove that flat sided o-ring goes in. Surprising how a tiny bit of crud caused me so much aggravation.
 
I may have found what is causing one front brake to lock up momentarily. After fitting some used Yellowstuff pads and finding it's still braking unevenly, but a bit different to before, I noticed many fine hairline cracks in the disc surface of both front discs. On one surface only, the inner surface looks normal. They are Brembo discs, still about 10.5mm thick. The largest cracks you can feel with your finger nail. I wonder how many track laps I have done with them like that. Now to order some new discs and try again.
108967234_819195755283392_3961317784550078325_o.jpg
 
That took some serious heat. You might consider finding some Uno Turbo brakes which have vented discs, you apparently need them unlike the rest of us posers...

Good sleuthing.
 
If only the Uno Turbo brakes were readily available in aus, or anywhere. I have ordered new slotted discs, will give them a try first. With the pads I use it actually brakes pretty well. I have been mulling over the different brake upgrade options for years, but I just haven't been convinced that the slight braking improvement would outweigh the losses due to larger diameter heavier vented rotors (like flywheels) that have to be slowed down and sped up for each corner. If I was convinced it would improve lap times I would have done it. I currently have the larger 38mm calipers on the rear, but I would have to make changes at the rear as well to maintain balance.
 
I believe the Uno Turbo calipers were the same piston diameter and had only a nominal difference in rotor diameter. Shouldn’t cause a change in braking, just in the amount of heat you can put into the brakes.

It is clear you are putting too much heat into the brakes (yes they are working well) but they don’t have enough mass or means to get rid of the heat you are generating. The Vicks front brakes which use LADA parts allows you to use 13” rims so they would be an option with brand new replacement parts readily available for cheap and more modern sliding pin calipers to boot.

Just a thought.
 
That is caused by heat cycling and hard on the brakes. Toss BOTH rotors, replace both rotors and pads. Then braking test after the new pads have been properly bedded in.

Brakes feel different on track? Track duty can be really hard on brakes in ways that will not happen repeatedly in performance street driving. If the power train is up rated, there is greatly increased demands on braking performance and thermal capacity.
IMO, for a up rated power train, the stock brakes are not good enough for track days or similar.

Yes, Fiat Uno Turbo front calipers are same as stock, 48mm, rotors are slightly larger diameter and vented. The increase in brake thermal capacity is significant. Brakes are another reasons to move away from 13" wheels if the exxe is performance up rated.

Bernice

GregS said:
I may have found what is causing one front brake to lock up momentarily. After fitting some used Yellowstuff pads and finding it's still braking unevenly, but a bit different to before, I noticed many fine hairline cracks in the disc surface of both front discs. On one surface only, the inner surface looks normal. They are Brembo discs, still about 10.5mm thick. The largest cracks you can feel with your finger nail. I wonder how many track laps I have done with them like that. Now to order some new discs and try again.
Click to expand...
 
Thanks for all the inputs. I have already ordered new slotted rotors, so I will give them a try first, the next 2 track days (1st for this year) are on fast flowing circuits, not too hard on brakes. The pads were brand new, just have to bed in on the new rotors. In the meantime I will have another look at the options for vented rotors, and better calipers, I want to stick with 13 inch wheels. Wilwoods maybe? Whatever I change to will have to have very good pads available for them, preferably in Australia for cost and time, so that's another consideration.
 
If I can chime in on the wheels, I like the look of the 13's on the X as well. I have done the larger rim swaps on other cars but with the feel of the X I really think the 13's look right. 14's don't look bad but I think when you get up to 15's I think the car looses something.
 
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