Our customers ask us a lot of questions about problems they’re having with their car’s brakes. We take them very seriously, as they’re such an essential part of your vehicle’s safety. From the most basic, ‘what does brake mean’ and ‘what does the brake warning light mean’, to more sophisticated questions about brake pad compounds, ABS and driver-assisted braking systems.
That brings us to brake judder.
At some point in almost every motorist’s life, they encounter brake judder.
What is brake judder, you ask? Well, it’s the vibration caused by something amiss in your braking system. The vibration travels throughout your car’s components and features most strongly in the steering wheel and brake pedal.
Exactly what it is and why it happens, is what we’re going to look into today. We’ll explain the myths and mysteries, and how to get everything working as normal again if you’re unlucky enough to encounter the issue.
Brake hub or disc run-out
Run-out is caused by ill-fitting discs. If the alignment isn’t spot on with the hub or the brake calliper, then there’s a good chance the connection between them under braking isn’t going to run smooth, materialising in the juddering we’ve been talking about.
Mechanics use a special tool measures run-out. A run-out gauge measures tiny gaps, often tenths or hundredths of a millimetre, between the contact points of your brake pad and disc.
When could hub or disc run-out happen?
The first option is that dirt or rust has gathered on the wheel’s hub.
Anything that causes poor contact between surfaces can throw the alignment off, creating strong and weak points in braking during the wheel’s rotation. Where this is the case, the discs need dismantling, and all surfaces cleaned to replace the components in perfect alignment.
The second option is down to over-tightening positioning screws.
This sometimes causes distortion to the contact surface. Such distortion will, again, cause misalignment that can lead to vibration when the brakes are applied. If this happens on your car, you may have to replace your brake discs. Be sure the replacements aren’t over-tightened when fixed into place.
Is the hub itself damaged or distorted?
Wheel hubs can warp or shift in shape for a variety of reasons. If your wheel’s hub isn’t the precise shape that it needs to be to operate safely, then you have a couple of options. Measure how far out of tolerance the discs are and then re-fit them in a new position to see if you can achieve a better and acceptable run-out tolerance.
If that doesn’t work, then you’re going to need to address the problems with your wheel’s hub, with either some form of repair or replacement.
Another common reason for run-out is incorrectly fitted alloy wheels.
Alloy wheels can add value and performance to your car, as well as to its appearance. They can also add judder, due to run-out, if not fitted correctly.
To keep production costs down, many alloy wheels fit a lot of different vehicle models, using spacers and spigots to get them to sit in exactly the right place. If the spacers or spigots are lost or damaged, or if they’re not sat as flush and firmly as they need to be, then the alignment of your brake’s components can soon slip beyond their acceptable tolerance limits.
Severe disc overheating, causing distortion
Any two separate surfaces moving at high speed and connecting under pressure will create friction. Friction causes heat, especially when the two parts are generally metal compounds.
Depending on how and where that heat impacts your brake discs, it can result in what mechanics call hot spots. They’re usually caused by intermittent contact repeatedly happening in the same area, and if so, it will often leave blue spots on the brake disc surface.
Repeated heavy use of the brakes can be a cause of overheating and its subsequent damage. A more obvious indicator of overheating brake components is the smell they give off. You could notice that burning smell long before you feel any juddering.
If you find those blue spots on your discs, then the damage could already be done. Often, the only resolution for distortion is to replace your brake discs and pads.
Disk thickness variation (DTV)
A deformed brake disc will, without a doubt, cause juddering under braking. Any disc with an uneven thickness appearing around its diameter will create different spacing between the brake disc and pads. Consistent spacing is required for the brake pads to apply the same pressure around the full rotation of the disc, to slow the car safely and smoothly. DTV causes a reoccurring loss