Megane RS250 track car - vibration under braking when hot

[southpaw]

Hoping someone can point me in the right direction on an unusual braking issue. I have a highly modified Megane RS250, build for track use running c.365bhp and weighing under 1,200kg.

On the road, the brakes are absolutely fine. On track, after around 3-4 laps, I will suddenly get a very harsh vibration from the front end as soon as I touch the brakes, which will happen on heavy or light braking. It only happens once everything is hot, and if I do a cool down lap and then press on again, I’ll get another couple of laps before it returns.

The usual source would be pads/warped discs or pad deposits. I had two piece discs with PFC pads and have just changed to single piece high carbon discs and Winmax 6.5 pads and had the exact same issue first time out on track following very light road use, so I’m thinking it may be suspension related?

Has anyone had something similar or could point me in the direction of where to start? Given it’s a track only issue it’s very hard to diagnose!

[Smint]

Are the rear brakes doing enough work, maybe check all is well there, is there a load proportioning valve that might have been dislodged or need repositioning
.
Presumably the car is road legal, have you tried coasting into a MSA or uphill dual carriageway layby after cruising a good few miles, not using the brakes at all to slow the car down then have a feel all round for unexplained extra heat? i use a laser thermometer for this type of thing, maybe you've got a bearing on the way out.

If nothing doing there maybe try repacking all the CV joints with fresh grease.

Just off the top of my head what i'd try.

The only two cars i've had this type of issue, a 71 Mustang V8 fitted with unservoed drum brakes all round, braking from any 3 figure speed caused instant awful brake fade and violent shaking.
The other one was a manual Rover 827 ex police driving school, the brakes simply weren't big enough for the quite eye opening speed this car could maintain, it's has gone through umpteen sets of rapidly warping brake discs and pads whilst a training car and that continued during my ownership.

Edited by Smint on Saturday 10th July 11:12

[southpaw]

Smint said:

Are the rear brakes doing enough work, maybe check all is well there, is there a load proportioning valve that might have been dislodged or need repositioning…

Thanks Smint, the coasting idea is a good idea, will have to work out a way of getting the car up in the air quickly.

I was thinking a bearing on the way out worsening with heat, just hard to work out which!

[GreenV8S]

southpaw said:

I was thinking a bearing on the way out worsening with heat, just hard to work out which!

It doesn't need to be on the way out to loosen with heat. You may be able to feel it if you can get the car stopped soon enough, although of course that's a terrible thing to do to hot brakes.

I suggest you get some temp tabs on the rotors and pads and see how hot they're actually getting. It may be that you simply need more cooling.