Why Does My Car Shake When I Brake? – Reader’s Questions

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Jim asks the question “why does my car shake when I brake?”.

Welcome to the first Reader’s Questions! If you have a car related question, please leave it in the comments at the end of this article or email it to dan@dangerneering.com.

Often a car can develop a shake or vibration when the brakes are applied. This can occur at different speeds, and may only be noticeable when the brakes are applied a certain amount. Regardless, all of the symtoms generally come down to the same common issue.

Glazed Brake Discs (rotors)focus-st-brake

Rust can occur on the surface of the brake disc (rotor). A small amount is quite normal, the surface needs to be bare metal, and bare metal left in a typical open environment or after you wash the car will begin to rust. A small amount of surface rust (orange in colour) is nothing to worry about, as it will be cleaned off when you drive the car. Sometimes though, this surface rust can become corrosion, this is a darker brown. This means the metal is becoming pitted, and corroded. Depending on how badly set in the corrosion is, when the car is driven and the brakes are applied it can polish up the surface of the rust rather than cleaning it off, causing glaze. Brake disc (rotor) glazing is actually quite common, and could be down to driving style. If the driver brakes often and drags the brakes for a long distance; brakes lightly over a very long braking distance; or is frequently on the brakes but not really slowing down, then this can cause glazing. Brake disc (rotor) glazing can be removed at home with basic tools, so long as it is not too badly set in. You would be best advised to look up the specific removal and refitting for your car, but the general steps will be:

  1. Remove the brake disc (rotor) from the car.
  2. Remove the glaze using sanding paper in a swirling motion, working your way around the whole braking surface, on both inside and outside.
  3. De-glaze the brake pads in a similar way.
  4. Refit the brake components.

If the brake discs (rotors) do not appear to be glazed, or after de-glazing them there is still a wobble when braking, the brake discs (rotors) may be warped.

Warped Brake Discs (rotors)

Warped brake discs (rotors) will occur when the brakes have become too hot and overheated. The excessive heat build up in the metal of the brake disc (rotor) has caused the braking surface to no longer be flat, to warp. You can sometimes see a blue hue or bluing of the braking surface. This is a good indication that the brake discs (rotors) have been subjected to extreme heat. You can use a DTI (Dial Test Indictor) to measure brake surface warpage, often a workshop manual will show you how to do this and what the allowed tollerance is. If your brake discs (rotors) are warped you can get them machined at a dedicated machine or autoparts shop. However, please read the comments at the end of this article on brake disc skimming. If you do have a warped brake disc (rotor) you will need to find out why, or it will just happen again.

Sticking Brake Caliper

There are a number of components in a brake that need to be able to move easily, and should be lubricated with the correct type of lubricant and deffinitely in the correct places. Generally these will be:focus-st-brake-pad

  • The brake caliper sliders
  • The brake pads where they contact the caliper carrier
  • The brake caliper piston(s)

If any of these parts are siezed they will need to be freed off, cleaned and lubricated with a dedicated brake components grease. If the corrosion to the parts is significant they will need to be replaced. Sometimes it may not be economically viable to clean up and use the parts even if they can be saved. For example, often the brake sliders will come as part of the caliper carrier, and so will be a considerable price, therefore spending some time getting them working properly again is worth it. On the other hand, a sticky caliper piston will generally mean stripping the brake caliper down and replacing the piston and seal with a new one. It may not be that much more expensive to just replace the whole caliper with a remanufactured part, and save yourself the time and hassle of rebuilding the stuck one. It is worth comparing different prices here.

Other Possibilities

A shake, wobble, or vibration when the brakes are applied will usually be a brake related issue. However, there can be instances where it may be caused by other parts of the car, such as the wheels and tyres (tires), suspension, and the engine. It is well worth checking for any thing obvious in these areas too before deciding it is a brake issue and getting in depth in to removing brake components. It should also be obvious, but maybe it needs to be said, the engine, transmission, suspesion, and wheels should all be looked after just as much as the brakes. If you have any questions related to brakes or the other areas mentioned here, leave a comment below or email me on dan@dangerneering.com and I will do a Reader’s Questions on it.

Brake Disc Skim or Replace?

It might not always be wise to have a brake disc (rotor) skimmed, some reasons would be:

  • the disc would be too thin after it has been skimmed
  • if it has taken significant heat it will be suseptible to overheating again
  • replacement discs (rotors) are not much more expensive
  • sometimes you can get a deal on discs (rotors) and pads bought as a set
  • it seems like a hack and you should just buy new parts
  • (and my personal favourite) it’s a good reason to replace them with some upgraded parts

So Jim, I hope this has answered your question and helped you to fix your car.

If you have a question you’d like answered in Reader’s Questions then please leave it in the comments below or you can email it to me directly on dan@dangerneering.com!

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