Steering/Suspension/Brake Clonk on reversing

[peteoddcar]

After a drive usually when I reverse into the drive way - I get a clonk as the clutch bites sounds as if its the left side - I feel it through the steering wheel just gently just happens once cant repeat it unless I drive for a mile or so and repeat.MOT man had a real good look nothing obviously loose worn.All visible bolts checked and tight.
Had new drive shafts about 2000 miles ago, new calipers, discs, dampers - it did it before they were fitted, I am puzzled not too bothered but theres something loose some where - longeron, gearbox mount?
Any ideas?

[Jeemy]

I am sorry if this sounds patronising - its really not meant to be! Have you checked the seat bolts? I tried for ages to track down a clunk at exactly this point, reversing onto the driveway. It was the seat bolt which was loose; the bolt clunked to the opposite pole on a dead stop and because the whole car stops/rocks, you don't feel the movement, which is slight anyway.

[Brit-Car-Nut]

Several forum members have found that their raft bushings had perished. They are very difficult to test as there is no easy way to stress them when jacked up. Bushing #51 in the parts guide differs between several models and that is the one most say needed replacement. I would expect to replace #50 and any others you find that are wearing out.

[hatters]

I am sorry if this sounds patronising - its really not meant to be! Have you checked the seat bolts? I tried for ages to track down a clunk at exactly this point, reversing onto the driveway. It was the seat bolt which was loose; the bolt clunked to the opposite pole on a dead stop and because the whole car stops/rocks, you don't feel the movement, which is slight anyway.

I had exactly the same problem with mine ,it took me ages to find out the seat was loose

[peteoddcar]

I like the seat bolt theory - I'll check it at the w/e and report back - bush #51 is also a good lead also. Thanks guys

[peteoddcar]

Seat Bolts re tightened - no knock yet...

[GoM100Go]

Mine does the same thing! Been doing it since I bought it. I thought it was a Lotus quirk. I have have both my seats removed for reupholstering and it did it befor and after. Both seats are bolted fine. The clunk definitely comes from the breaks, but it doesn't effect breaking ability s just clunks at random. Not everytime, and rarely enough to couse me a panic thinking I backed over something or someone Feels like I drove off the curb, -KLUNK- only in reverse and at random. I have had my mechanic check the breaks, of course it did not do it when she drove it... Breaks great, she says. UGhhh embarrassing. Any ideas.

James

[nicowalker1]

I also get a clunk when first applying the brakes in reverse. It is just a small abount of play in the pads...

[dapinky]

I also get a clunk when first applying the brakes in reverse.....

I think that's just a blue car reversing into you while you try to get away

[nicowalker1]

I also get a clunk when first applying the brakes in reverse.....

I think that's just a blue car reversing into you while you try to get away

Ouch!

[Brit-Car-Nut]

I also get a clunk when first applying the brakes in reverse. It is just a small abount of play in the pads...

There should NEVER be that much play in the fit of a pad!

[Fourdoorcat]

There should NEVER be that much play in the fit of a pad!

I suppose that brings up a couple of questions:

1. How much play in a brake pad is too much and
2. How much play in a brake pad is enough to be noticed under the conditions mentioned

If the actual movement of the pad is "too much", then it is a problem if the clunk is heard or not
If the actual movement of the pad is within whatever are "acceptable limits" and the clunk (or clonk) is noticeable, then it is benign.

I would guess that pushing back the caliper piston and measuring the free play of the pads would produce a number. (I haven't messed with the brake pads, so I am flying a little blind). Once this number is known, is there any specification that would apply?

Ken

[Jeemy]

Surely this refers to lateral, rather than inny-ooty, movement of the pads? If a pad moves outwards enough to clunk I wouldn't feel safe driving the car. It shouldn't move laterally but at least there is a reasonable explanation for that.

Where the little hook teeth locate in the carrier, there could be play, this play should be minimal enough to cause squeaking and eradicated by copper grease. If the piston allows the pad to move inwards or outwards enough to be audible under use then the piston is surely no good?

A clunk I would be more liable to attribute to the carrier bolts. I've found these are under a usually insane amount of torque, fair enough when you consider that when the braking wheel is on the outside corner, the whole weight of the car can be pushing against them.

[Brit-Car-Nut]

The play that might cause a "clunk" is lateral or rotational. The pad gets pushed against the disc and the friction pushes the pad to the end of it's rotational clearance, thus creating the "clunk".

Several issues are of concern: If there is enough play to cause a clunk, the pad is actually NOT stopping the car while it is traveling with the disc. When it reaches the end of the clearance, it then starts sliding along the disc creating friction that stops the momentum. The transition often manifests itself by making the car "pull" one way or the other. Also, the physics of the pad moving is akin to hitting a piece of metal with a hammer - it will quickly damage and misshape the metal (caliper) where it is being stopped. There is also a chance the pad lining could get torn off the backing by the sudden energy transfer when it runs out of room.

In 40+ years of servicing cars, the most common cause of a brake clunk (after loose bolts) is a cheap, off sized pad.

Anyone that has a "clunk" when they step on their brakes right after changing direction needs to eliminate a loose caliper, a loose wheel (lug bolts not tight, allowing the disc to rotate on the hub), a damaged caliper (should be evidence of hammering in the pad area) or improper fitting brake pads.

How much play should there be between the pad side and the caliper? one or two thou at the most. Should just be enough to set the pad in place. If the pad is twisted while installing it, it should not fit.

[Fourdoorcat]

Great response John. It's very inclusive and well done. One or two thousands of play, I would expect, should not produce an audible clunk (or clonk).

Jeemy...we are referring to radial movement of the pads due to friction against the rotor. Yea...in and out would be really bad.

Ken

[Jeemy]

I thought so, it was just when you said pushing back the piston would let you gauge the movement that I wondered if you meant movement outwards/inwards was acceptable or expected!!

The clunk could also be related to the wheel shifting on the hub, or the hub shifting on the axle?

When James said the clunk was as bad as dropping off a kerb I found that quite concerning. I'm learning more and more not to be so apprehensive but I think a degree of caution, especially with brakes, is probably wiser than the opposite.

If the clunk occurs when shifting direction then something in caliper, carrier, hub, axle or other place on the car is moving into position before doing its work. Enough play to be that audible, surely indicates this is a situation that will exacerbate itself over time.

I have a creak now somewhere on the car which I am attributing to rear brakes not being used for almost a year. It wasn't there when I did my first 20 miles and disappears again after a few seconds of driving, but its there and vibrational on first movement. At present my windows are loose, most panels and wheel arches are off and she is only moving short distances....I just have to hope that when everything is put back it fixes itself because having somebody else drive her while I feel about is not lending me any clues - I'm pretty sure its the undamped window.

[nicowalker1]

My calipers are fully refurbed (inc pistons), checked by a lotus specialist. Brakes pull evenly when tested on rollers. Just to add I have a brand new car, this also occasionally as a slight clunk when braking in reverse. It will definatley be a rotational thing although only happens in reverse.

[Tannerman]

It is the movement in the calliper not the pads if you loosen off the pads the standard callipers move up and down and side to side.
I have the upgraded brake kit and it doesn't do it now.

John

[lotuswills]

Just bought an M100 SE there seems to be a clunking noise from the off side rear (occasionally) as if one were driving off a kerb type clunk, not there every time and not heard it reversing (yet). Its worse when two up and wondered if its shocks related ?

[Jeemy]

It could (I don't know how the Elan suspension works exactly, just trying to be helpful) be a sharp, chipped or broken spring end turning in the cup on compression.