Vibration/judder on braking - help!

[revesby37]

I drive a Marlin Sportster, which I've had for a little over a year. I've had a recurrent problem with a very severe vibration when I brake. Only at 50mph - no problems at lower speeds or above 60. The thought of keeping my speed below 50mph does not appeal, so I have to cure this.

I changed the front disks last year and that seemed to cure it for a while. The front RH disk was a little out of true, although not much. Then more recently it recurred on the LH side, so changed that disk and again cured the problem. Now it's back again. On the RH side I think. The disk is true. I've checked the suspension strut and joints - all fine. I dismantled the caliper, which seems fine, all 4 pistons move. Everything seems tight, including wheel bearings. Am running out of ideas.

My sportster has Cortina uprights & hubs, Capri ventilated disks and Princess calipers. All have seen little service, the car having done 7000 miles on new equipment.

[robgee1964]

revesby37 said:

I drive a Marlin Sportster, which I've had for a little over a year. I've had a recurrent problem with a very severe vibration when I brake. Only at 50mph - no problems at lower speeds or above 60. The thought of keeping my speed below 50mph does not appeal, so I have to cure this.

I changed the front disks last year and that seemed to cure it for a while. The front RH disk was a little out of true, although not much. Then more recently it recurred on the LH side, so changed that disk and again cured the problem. Now it's back again. On the RH side I think. The disk is true. I've checked the suspension strut and joints - all fine. I dismantled the caliper, which seems fine, all 4 pistons move. Everything seems tight, including wheel bearings. Am running out of ideas.

My sportster has Cortina uprights & hubs, Capri ventilated disks and Princess calipers. All have seen little service, the car having done 7000 miles on new equipment.

I've just had a similar problem on a jag, turned out the problem was uneven disk surface, due to pitting. This meant the friction level was not constant as the disk rotated, thus causing variation in force and hence judder. Problem was cured by a disk skim, also I was very careful when refitting, and kept repositioning the disks on the hubs, until the run out was within a few thou.

Lack of pre-load in steering rack and/or ball joints would also make the thing more susceptible to wheel shimmy, what sort of shape is your rack in? An MOT worthy rack could still have insufficient pre-loading.

[revesby37]

Finally I sorted the problem on Friday. As with you (robgee1964), a disk skim was the answer. I went for the on-car skim (using Protec lathe) which also takes into account any hub or bearing run-out. Found lots of places with these machines via simple internet search - one just up the road.

Now very smooth but I'll wait for the disks/pads to bed in for a few hundred miles before I use the brakes in anger - unless forced.

Thanks for the reply.