Shud­der on brak­ing

Car Mechanics (UK) - - Help! -

I have a 2005 BMW X5 3.0d Sport. For the past few months there has been a se­vere shud­der when brak­ing at 30mph and up­wards. The ve­hi­cle went in for its MOT a month ago, so I as­sumed the prob­lem would be picked up, but the car passed with no ad­vi­sories. I didn’t men­tion it to the tester, but per­haps I should have. Do you have any idea what the prob­lem could be? Ge­orge Gill As the ve­hi­cle has just passed its MOT, we should hope­fully be able to dis­card the pos­si­bil­ity of wear or play in the sus­pen­sion joints. Al­though the brake test in the MOT will show up any im­bal­ance in brak­ing ef­fi­ciency, it won’t nec­es­sar­ily in­di­cate the pres­ence of dis­torted brake discs or brake pads which have be­come hard­ened by heat or age. The dis­tor­tion to brake discs refers to a vari­a­tion in the thick­ness of the brake disc which can oc­cur over time. This can be caused by ei­ther the brake discs not be­ing seated cor­rectly or by hot spots de­vel­op­ing on the brake discs and caus­ing vari­a­tions in hard­ness that re­sults in the disc face wear­ing at dif­fer­ent rates.

The brake disc runout can be checked with a dial test indicator gauge (DTI) and this may be a good first move in di­ag­nos­ing the source of the jud­der. The max­i­mum runout of the brake disc is 0.08mm, al­though a more ac­cept­able read­ing would be 0.04mm. Brake pads that have be­come hard due to age or be­cause they have been sub­jected to ex­cess heat can also cause a jud­der at higher speeds.

The Sealey VS0290 brake disc runout test kit.

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