Date: Fri, 26 Feb 1999 11:51:11 +0200
From: "Roy Björkstrand" <>
Subject: Re: 86 Saab 90 problem with breaks

Oliver Klotz wrote: > Hi, > half a year ago I have changed the front break disks. And now sometimes > when I´m slow down there is some vibration in the breaks and in the > panel also there is some low noise. This appears only by speed below 70. > I´m wondering about because this appears only after driving more the 20 > km. If a drive only low dinstance the vibration don´t appear. > > Do you have an idea? > > Best regards > Oliver I don´t exactly know what it might be. Is there any heating at the disks after normal driving (no hard braking) ? DON`T touch the disks. Put your fingers into water and splash some water drops onto disk (though holes on rims) - any "boiling"? If disks are too hot your calipers might be (are) stucked. After brake disk (and pads - you did change pads I hope) replacement calipers are in different position than they used to be and "stucking" appears. Although this might (in worse case) appear also during driving not only when braking. But your vibration, this could be wrapped disks. Although vibrations should be worse on high speed, try to do a "stronger" braking at 100km/h+ (check the rear view mirror) - vibrations? Yes, you have new ones but... Brake disks are produced by molding (I sorry for not-so-correct terms - no dictionary). When mold cools down there are some internal strains. These are releaved by heat treatment (most of them). Then disk is machined - this removes some material that "stabilises internal strains". Strenght keeps the disk straight but when it heats enough - it wraps. And when it wraps it starts to heat more on one spot "pushing" the disk even more wrapped (heat expansion). These kind of problems come usually with "cheap" disks. There are two reasons: 1) mold is inaccurate and material to be removed is great. 2) heat treatment is poor. When replacing the disks you have to make sure that you follow the instructions carefully and brake softly at least for first 1000kms. If the vibrations are not coming from brakes, it can be anything from cv-joints to wheel bearings. Keep in mind that because the problems appears after long drive it seems to be heat related. Thank you for reading this long story, I hope this helps you to narrow your "troubleshooting". Roy 99 -82 900 -85 900 -91

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