Brakes juddering when warm

[kingzetts]

Currently touring in the Pyrenees and the mountain hairpins have brought to the fore a problem I've put up with for a while.

When cold, the brakes work fine - pulling straight and no judder. When they get warm, I get quite severe side to side juddering of the steering wheel. This then goes away if the brakes get past warm to hot at which point they go back to being judder-free but just need a bit more pedal pressure. Get out of the steep stuff, get the brakes cooled down, and they go through a juddering period as they cool but stop when cold.

Car has Zeus front calipers and standard 3.8 solid discs.

My theory is that the discs must be warping as they warm up but trueing themselves up when they get hot enough - but I'm not 100% convinced. Anyone got any thoughts?

[Heuer]

Warped discs would be my guess. If you are going to make a habit of mountain roads and Alpine passes you need to consider some ventilated front brakes. What pads are you using? Greenstuff?

I presume you have checked the spinners are on tight, wheels balanced and tyre pressures correct?

[kingzetts]

I agree, David - vented fronts are (as of earlier this week) on my winter to-do list. I'll be researching which supplier to use when I get back.

I am using green stuff pads, and the fronts were off for a tyre change and for the tracking to be adjusted just a few days ago so I'm confident pressures are OK and spinners tight.

[christopher storey]

I have some doubts about the idea that the discs could warp and unwarp like this , although I suppose it is not impossible. I think it is more likely that the pad material ( or indeed the disc surface ) is going through phases where at certain temperatures the coefficient of friction becomes erratic, probably because of glazing , which will cause judder by grabbing certain parts of the disc but not others. You could try removing the front pads and rubbing them on some fairly coarse , say 120 grade, aluminium oxide paper, but this is rather laborious when on a trip , or rather more riskily you could give the brakes a really good work out for a short period , but after this keep running for quite a few minutes with little braking so as to cool in particular the rear discs and thus minimise the risk of cooking the rear oil seals

[chrisfell]

Things that cause brakes to judder (amongst many other things).
1. Rotor out of true (side to side).
2. Rotor out of true (not centred on the hub).
3. Pads loose on their pins or pins loose in their calipers.
4. Rotor contamination (rust, oil or grease).
5. Cracked or split rotor (hairline or thunderingly great big one).

(This one was very obvious!)

[Heuer]

the fronts were off for a tyre change and for the tracking to be adjusted just a few days ago

Alarm bells ringing. Now that could be the source of the problem - suggest you try swapping the front wheels to the rear and see if that makes a difference as I suspect wheel balance. Front to back is easy to do with the standard jack and should only take you 30 minutes (or 1.923 seconds if you have an F1 pit crew handy )

[kingzetts]

Thanks all of you for the replies. I'm confident it's not wheel balance, as it was also doing it before the tyres were changed and the symptoms just don't fit. As we are now on the homeward stages, there won't be any mountain stages so,I will live with it and investigate further at home.

David, do you have vented fronts on your OTS? If so, which supplier did you choose?

[Heuer]

Crikey I am not sure, they have been on there for 15 years but I seem to recall they are S3 vented discs, cross drilled. Worth looking at offerings from AP Racing and Fosseway but don't go mad or you will end up needing spacers and having to upgrade the rears to maintain balance.

[1954Etype]

Not saying this is your problem but we have removed a lot of greenstuff pads due to glazing. It looks like the driving style of some drivers is too light on the brakes which was causing glazing. As a result, we fit standard pads now with no issue. You shouldn't need vented discs for normal road use but might in the mountains. Have you got the thin 3.8 discs on the front?

[kingzetts]

Angus, you could be right - at home I usually drive fairly sedately around my local area. Do you find that glazing effects come and go with temperature, though - I can certainly see glazing causing variations in friction but would have thought that if the judder went away that would suggest the glazed areas had been worn away?

I do have the standard 3.8 discs on the front.

Thanks, David. I may upgrade the rears as well, but lots of research to do first!

[1954Etype]

John, can't really be sure but for the price of a set of pads combined with some spirited jaunts up and down the Malverns it might prove the point☺

[christopher storey]

John : unquestionably coefficient of friction varies with temperature. This is why cold brakes ( discs ) don't work as well as warmed up brakes, and why in the old days of drums, fade occurred with high temperatures ( as well as the long pedal effects of expanding drums ) .

[kingzetts]

Christopher,
Indeed CoF varies with temperature. Where my understanding runs out is this; to get juddering, I think the CoF needs to be different at different points around the disc, so that as the wheel turns the braking effort at a given hydraulic pressure alternately increases and decreases as the pad moves from a lower to a higher friction area and back to a lower. So as the brakes warm up, the magnitude of the CoF changes between high and low areas must initially be increasing, to cause judder as the system warms, then decreasing again as the discs get very hot.

Or am I just overthinking the whole thing?

[christopher storey]

John : I'm not an expert on this , but from the glazed pads I have seen , they seem to have patches which look different and which show uneven wear characteristics, so that presumably different parts of the pad provide a different coefficient at different parts of the disc's surface thus setting up a vibration . Why this only happens at certain temperature bands I do not know . Empirically speaking, I am fairly sure this is a characteristic which has increased with non-asbestos pads , as I do not remember its being a problem in the old days

[JagWaugh]

Are the discs new? What manufacturer?

When you brake you generate heat at the ring which is the contact area. With a hatted disc, if the hat is cool, too thick, not evenly torqued to the hub, or of uneven thickness the heated annulus may be constrained and cannot expand evenly outwards in a plane, so that it ends up either going wavy, or tilting. (I've seen this in industrial brakes a few times).

The fact that your brakes go from not juddering to juddering then back to not juddering may just be that the heat that you are putting into the system takes a while to distribute itself more evenly.

Mountain driving is an acquired skill, in spite of having worked a lot on industrial braking systems it still took me about 2 years to learn to brake like the locals when I moved to Switzerland.

I would suspect runout on the discs, spinners not being tight (get the brakes/hubs warmed up then give the spinner a gentle whack), a crack, or interrupted areas of different color on the braking surface, then I would check that the pads aren't hanging (lubricate the pins), change the pads, then the torque on the fasteners between hub and disc and readjust the wheel bearings.

It could be a great many things.

[kingzetts]

JagWaugh,
Some good information in your post, thanks. The discs are not new, purchased from Barratts 6 or 7 years ago during major restoration. No idea of which brand.

I'm intrigued - what is the technique the Swiss locals use for braking?

[JagWaugh]

It is most obvious at night. The alpine inhabitants rarely use their brakes unless they are in traffic, speeding, it is a hairpin, or they are actually stopping. During the day you see that the non Swiss plated cars invariably brake before each and every corner. When you are going uphill just lift off before the corner, going downhill use the transmission. When you do brake, don't trail, check your mirror, sharp and short.

If you are used to flat roads, or mostly motorway driving, the alpine roads seem very narrow and the turns all seem like hairpins. The 300m drop on one side of the road can be a bit distracting too.