One wheel always locking before the other

Shocks, Springs, Brakes, Frame, Body Work, etc

Moderator: Team

AC sports
Pro
Pro
Posts: 360
Joined: Mon Jun 03, 2013 6:25 am
Location:

One wheel always locking before the other

Post by AC sports »

I pulled the race car out last weekend to rebuild the brakes all round.
It's a Fiat 124. Oem discs all round ( class rules), sliding wedge type, single piston.
The car always had an issue locking the passenger side front. ( yes, it also has a passenger seat).
That side rotor was always running 20-30% hotter than the drivers side.
I replaced the calipers with another rebuilt pair, new hoses, new pads, discs were good and no runout.
Took it for a test run and I get a slight pull towards the passenger side whilst braking and that disc still runs hotter and locks up when pushed.
Wheel bearings are tight. Wheels spin free by hand when jacked up and seem to come on equally when the pedal is actuated by someone.
I'm now leaning towards maybe a suspension issue, but if it were suspension related why would the disc run hotter too?
Any suggestions? Camber caster are even. Toe is zero .
The only thing I can think of is that the passenger side wheelbase is 5mm longer. This doesnt seem to affect handling or stability...but braking? It could well be something else.
I need to check suspension bushes ..but still wont explain why one disc is hotter.
ijames
Expert
Expert
Posts: 849
Joined: Fri Jul 04, 2008 3:44 pm
Location: Laurel, MD

Re: One wheel always locking before the other

Post by ijames »

Wild guess: you say new hoses, but how about a partial obstruction in the hard line going to the driver's side caliper? Internal rust, kink, slight crush, bad flare at a fitting ... Anything to unbalance the fluid flow to that side so it always applies just a bit later.
Carl Ijames, chemist not engine builder
carl ddott ijames aatt verizon ddott net
AC sports
Pro
Pro
Posts: 360
Joined: Mon Jun 03, 2013 6:25 am
Location:

Re: One wheel always locking before the other

Post by AC sports »

Thanks for the reply. I actually ran new hard lines to that side when I reconfigured it without a servo. It was doing it before and still now. Chassis isnt perfectly square from an old off....its within 5mm all round though. Not sure that would be enough to affect anything.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: One wheel always locking before the other

Post by David Redszus »

AC sports wrote: Sat Apr 03, 2021 10:55 am Thanks for the reply. I actually ran new hard lines to that side when I reconfigured it without a servo. It was doing it before and still now. Chassis isnt perfectly square from an old off....its within 5mm all round though. Not sure that would be enough to affect anything.
Checking the efficacy of the hydraulics can be done by inserting a pressure sensor in the caliper bleed hole.
This will tell you the forces available at each corner.

Next, check the corner weights and suspension corner spring rates. If possible, they should be the same for front corners, and for rear corners. A difference in corner weights, along an axle, indicates the center of gravity is off-center, causing a diagonal weight transfer under braking.

Street cars are rarely square which requires the use of different spring rates at each corner to balance weight transfer.
chimpvalet
Pro
Pro
Posts: 447
Joined: Wed Jun 04, 2014 2:02 am
Location:

Re: One wheel always locking before the other

Post by chimpvalet »

Long ago a buddy had a brand spanking new X1-9 with slider calipers all around. Had a nasty propensity to lock up unevenly at the front under hard braking in the dry. Not my cup of tea.

Cheers
Steve
AC sports
Pro
Pro
Posts: 360
Joined: Mon Jun 03, 2013 6:25 am
Location:

Re: One wheel always locking before the other

Post by AC sports »

The pressure sensor is an idea I thought of, but need to find one. I'm sure they are pretty even though, having checked with wheels in the air, brakes applied to various degree and a torque wrench on wheels.
The corner weighting I didnt think about. I know for a fact there is a 3/4 inch spacer under the drivers side spring to bring the ride height to even. I know the springs are identical having replaced them recently and both were checked for poundage.
Passenger side I'm sure is lighter without my weight.
Given the steering box is on the drivers side also as well as water overflow bottle . Add the spring spacers on drivers side.... now I'm thinking this could be it. Need to cross weigh the car.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: One wheel always locking before the other

Post by David Redszus »

AC sports wrote: Sat Apr 03, 2021 3:20 pm The pressure sensor is an idea I thought of, but need to find one. I'm sure they are pretty even though, having checked with wheels in the air, brakes applied to various degree and a torque wrench on wheels.
The corner weighting I didnt think about. I know for a fact there is a 3/4 inch spacer under the drivers side spring to bring the ride height to even. I know the springs are identical having replaced them recently and both were checked for poundage.
Passenger side I'm sure is lighter without my weight.
Given the steering box is on the drivers side also as well as water overflow bottle . Add the spring spacers on drivers side.... now I'm thinking this could be it. Need to cross weigh the car.
Given equal hydraulic forces, the lighter weight corner will lock up first. But that means dynamic weight, not static.
The front axle weights should be the same with fluids and driver. If not, the spring rates must be changed; a spacer will not correct the weight transfer problem. Also, the corner weights will determine the longitudinal and latitudinal centers of gravity.

A hot brake corner often indicates a dragging brake pad, due to a stuck pad or piston.
hoodeng
HotPass
HotPass
Posts: 1092
Joined: Sat Mar 04, 2017 6:53 pm
Location: South Australia

Re: One wheel always locking before the other

Post by hoodeng »

Can you find a place that has a 'roller brake tester'? [Crypton do one] this will test the brake balance/effort on the car regardless of the chasis' influence.
It won't fix the problem but it will tell you where to start looking.
AC sports
Pro
Pro
Posts: 360
Joined: Mon Jun 03, 2013 6:25 am
Location:

Re: One wheel always locking before the other

Post by AC sports »

Calipers are a spare set that I rebuilt so definitely not them, as it was doing it before with the old calipers and now with these.
Would the dynamic weight transfer issue cause a hot disk though?
There is a local garage that has a type of rolling road to check brakes . I'll see if I can get it there to rule that out.
I know preload on the sway bar can cause one side to lock up.....but would that disc run hotter though?
I'll remove it and check.
hoodeng
HotPass
HotPass
Posts: 1092
Joined: Sat Mar 04, 2017 6:53 pm
Location: South Australia

Re: One wheel always locking before the other

Post by hoodeng »

Best money you will spend to eliminate or confirm a problem.

One test is worth a thousand opinions! have been there, mine turned out to be a slight caliper misalignment, and it turned out the upright had been machined like it.

Cheers.
Geoff2
Guru
Guru
Posts: 1980
Joined: Mon Nov 09, 2015 4:36 pm
Location: Australia

Re: One wheel always locking before the other

Post by Geoff2 »

Have you replaced the brake hoses? I have had a hose block up with rust/sediment.

Is there a distribution block that divides L & R front brakes? If so, it should also be checked/cleaned out.

Is there a separate outlet from the m/c for each front brake? If so, maybe one is partially blocked causing residual line pressure to build up.
emsvitil
Pro
Pro
Posts: 292
Joined: Thu Mar 19, 2015 7:47 pm
Location:

Re: One wheel always locking before the other

Post by emsvitil »

Might be the sliding parts of the sliding caliper binding up.
Ed
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: One wheel always locking before the other

Post by David Redszus »

Take the car out to a smooth asphalt road. Apply the brakes sufficiently hard to lock up all four corners.

Now inspect the skid marks. Which corner locked up first, second, etc, can be determined by the locations
of the skid marks.

Braking force is determined by hydraulic pressure acting on the caliper pistons to create a braking force.
The grip is determined by the vertical loading of the tire. Static corner weights do not reveal the
dynamic weight transfer that actually occurs under braking.

When the hydraulic brake force exceeds the dynamic corner weight x tire Mu, the wheel will lock up.
You can reduce the brake force, either hydraulically or by use of a brake pad with a lower coefficient
of friction, or you can increase the vertical corner loading by adding weight or spring stiffness.

What are your static corner weights with fluids and driver?
AC sports
Pro
Pro
Posts: 360
Joined: Mon Jun 03, 2013 6:25 am
Location:

Re: One wheel always locking before the other

Post by AC sports »

David Redszus wrote: Mon Apr 05, 2021 11:21 am Take the car out to a smooth asphalt road. Apply the brakes sufficiently hard to lock up all four corners.

Now inspect the skid marks. Which corner locked up first, second, etc, can be determined by the locations
of the skid marks.

Braking force is determined by hydraulic pressure acting on the caliper pistons to create a braking force.
The grip is determined by the vertical loading of the tire. Static corner weights do not reveal the
dynamic weight transfer that actually occurs under braking.

When the hydraulic brake force exceeds the dynamic corner weight x tire Mu, the wheel will lock up.
You can reduce the brake force, either hydraulically or by use of a brake pad with a lower coefficient
of friction, or you can increase the vertical corner loading by adding weight or spring stiffness.

What are your static corner weights with fluids and driver?
Yesterday I put 45lb of ballast on the passenger side floor and put the old man in the passenger seat.
Went to the local test track armed with the ir thermometer. Much much better now. On a few occasions they locked simultaneously, other times one just before the other. Ir thermometer revealed more equal rotor temps.
Seems I have a weight distribution issue. I'm baffled why the lighter side front runs hotter without ballast though. I cant get my head around that one.
Anyway, I need to corner weight the car and check for sway bar preload. The sway bar isnt shimmed but the drivers side of the car has a shim under the coil spring.
enigma57
Guru
Guru
Posts: 1989
Joined: Sun Oct 07, 2007 11:59 pm
Location: Galt's Gulch

Re: One wheel always locking before the other

Post by enigma57 »

Just a thought...... In this situation, what about adjustable proportioning valves for each wheel? Then experiment and fine tune until you have even braking with the car weighted to suit track conditions.

Good luck,

Harry
Post Reply