Why does my OSF brake lock up?

[John Vine]

Folks, I could do with some advice please. The spec is: 1999 1.8 Supersport-R, Caterham 4-pot/pads, standard rears, wide track, adjustable platforms, 185/60-14, A021Rs at 18psi all round.

 

When I brake really hard, the OSF wheel always locks before the NSF, wet or dry (in fact, I have trouble getting the NSF to lock at all!). This has been a feature of the car pretty well since new (when it had standard track, brakes, suspension etc).

 

I've attended to all the usual things: tyre pressures, flat-flooring, caliper bleeding, but the problem persists. What could be causing it?

 

JV

[stevefoster]

Barring mechanical faults, brakes, tyres etc...

 

I had this in to a couple of corners - Druids at Brands and North Bend at Lydden.

Try 0.5 turn more weight on the offending corner. More if needed until resolved.

 

Hants (North) and Berkshire Area club site here

My racing info site

here

 

 

 

[Nick Woods]

Do you have the corner weight figures from when it was flat floored ? If the OSF is carrying less weight than the NSF this might explain it, although this is not too likely if you are the only person on board.

 

Alternatively, there might be something wrong with the NSF brake. My car had a spell of pulling to the right, which we eventually traced to a slightly sticking piston in the NSF caliper causing the pad to rub and glaze. Once we'd deglazed the disk and pad with some emery cloth it pulled up ok again.

 

You could also try taking it to an MOT testing station and getting them to test the brakes for you, I beleive they can tell you if one is working better than the other.

 

Nick

Red and Black 1.6K supersport

visit Carrotland.co.uk

 

[John Vine]

Thanks for the speedy response, chaps!

 

Steve: when you say "more weight", do you mean reduce or increase the spring length, and how much more weight would 0.5 turn add (see weights below)? I agree about Druids, btw. The last trackday I did there, I turned in in a haze of tyre smoke! Unnerving for me, but worse for the guy (very close) in front!

 

Nick: I do. They were: LF - 144.0, RF - 144.0, LR - 186.5, RR - 196.0, including an allowance made for me as sole occupant. Perversely, I fitted adjustable platforms and had the car flat-floored in an attempt to fix the problem! I don't suspect the NSF brake at the moment, as the locking-up occurred both before and after fitting the 4-pots. The brake test suggestion is a good one.

 

JV

[Nick Woods]

Its unlikely to be unequal front weights then

 

To increase the weight on a spring you need to turn the adjuster so it compresses it, i.e.so it pushes the car upwards - think of it as a table under one leg of which you've just put a beermat.

 

If your shocks are mounted the 'correct' way up this means raising the collar.

 

Nick

Red and Black 1.6K supersport

visit Carrotland.co.uk

 

[oldbutnotslow]

Nick, I and 8 others have shares in a set of computer scales and we have gone through a very steep learning curve regarding corner weighting and believe me when I say its not as simple as putting more weight on one corner by increasing the spring tension. If you alter 1 all the rest will move as well, so be very careful about messing with this. If you have equal fronts I would leave them alone.

I would look at the calliper as Nick has suggested as a first port of call. Also the discs can be rubbed down but with Garnet paper NOT wet and dry and not Emery cloth. I would check the pistons first.

 

 

Grant

 

Black and stone chip.

here

[stevefoster]

Hmm. Assuming brake fault then it would pull as well I would think.

 

Is getting perfectly equal fronts the right thing to have in an offset driver position car?

I found using the method of calc in a freely available spreadsheet and that described in an article in LF by Roger Swift was the best for me with the odd 0.5 turn extra at some places.

 

If you mail me i can send you the spreadsheet and you can compare.

I guess you could work out what is needed to be changed from your set up to this one and if it doesn't work - put it back

 

Hants (North) and Berkshire Area club site here

My racing info site

here

 

 

 

[shn7]

Another thought to add to the pile. Have you tried swapping the tyres around? Are they all the same compund? Could one be offering more grip than the other simply due to differences in the tyres?

 

Steve.

[Colin Mill]

Hi John

 

Have you considered swapping the disks between left and right? It would at least eliminate one possibility.

 

 

[Nick Woods]

Is getting perfectly equal fronts the right thing to have in an offset driver position car?

Steve, FWIW I asked the same question a while ago here and although I didnt get many replies we decided to go for equal fronts on the basis that (in theory ) it ought to give even braking at the expense of some slight handling balance.

 

We now have our own cornerweighting spreadsheet which shows the difference between 'equal fronts' and 'equal diagonals' For Johns car, the 'diagonal' figures would be

FL FR

142.0 146.0

RL RR

188.5 194.0

, which is only 2Kg per corner away from the current values. and results in a FR-LR cross weight of 49.9% rather than the 49.3% at present

 

Which is 'better' . I dont know and I doubt if I'm a good enough driver to tell either 😳. If a car is set up with 'equal diagonals' then has an extra half turn added to the FR then even more weight will be added to the FR. Maybe it something to do with the way the weight is transferred forwards under braking ? Mine is set up with equal fronts but although I was braking hard at the Academy day I cant say that one wheel was noticably locking up before the other.

 

I think I'll stop now because I'm starting to confuse myself as well

 

 

 

Nick

Red and Black 1.6K supersport

visit Carrotland.co.uk

 

[John Vine]

Thanks, chaps. Lots to think about now.

 

Colin: I haven't considered swapping discs. If I did so, presumably I'd need to swap pad sets as well? And what effect, if any, would the change of rotational direction have?

 

Steve N: I'll try swapping the tyres round. But I'd expect them to be equal as they are both A021Rs, equally worn, and fitted correctly rotation-wise.

 

Steve F: Re spreadsheet, YHM.

 

Assuming that equal front weights are a Good Thing, I'd begin to suspect the calipers/pistons/pads/discs. BUT, it seems to me that the uneven braking is the same now (after fitting 4-pots and vented discs) as it was with the original 2-pots and solid discs.

 

Something else I've noticed: When I jack up the front centrally, under the cruciform, the OS wheel is about 1" off the ground when the NS is still just touching. Could this be due simply to the lack of driver weight, or might the chassis be out of alignment? If the latter, would this be enough to produce OSF lock-up, and if so, shouldn't flat-flooring have corrected this?

 

JV

[Don Cook]

Is it the same when you have a passenger in the car ?

[John Vine]

Good question, Don, but I don't know. I rarely take a passenger (my wife doesn't really like the 7 much). And when I do, I guess I'm a little lighter on the brakes. But I'll give it a try if I can find a volunteer....

 

JV

[stevefoster]

That jacking result seems very odd. Mine will come up eveningly.

Check your damper / wishbone isn't sticking and that all the tubes look right.

 

Hants (North) and Berkshire Area club site here

My racing info site

here

 

 

 

[Jason Plato]

go to an MOT station and put the car on the brake rollers . Test if the brakes are the same power each side .

 

Was the flat floor done with you actually sat in the car??? I read

 

"They were: LF - 144.0, RF - 144.0, LR - 186.5, RR - 196.0, including an allowance made for me as sole occupant"

 

allowance as sole occupant sounds a bit dodgy to me ??

 

AP calipers also require regular cleaning of the pistons , how long has it been since this was done ?

 

As a last resort , try lowering the drivers side rear platform by 3 threads .......

 

 

hope you manage to fix it

 

 

 

 

[Colin Mill]

Hi John

 

Yes, I was wondering what I would do about the pads. On balance I would swap the pads over with the disks as the pads & disks are already bedded in together. Also, if the problem turns out to be some kind of contamination at least you won't be spreading it around. I guess that the surface of the disks will have some directionality from having been used in their current home so i would guess there will be a period of resettling.

 

It will be very interesting to know what's causing it.

[John Vine]

Thanks for the extra comments.

 

Steve F: Dampers extend to their full length when I jack up the car, and I was most particular to tighten the wishbone bolts with the car on the ground. Chassis tubes seem fine too, but if there were some misalignment, would it be obvious to the naked eye? (And thanks for the spreadsheet.)

 

Dave J: The flat-flooring was done by Millwood during my recent annual service. When I booked the car in, they asked me how much I weighed, so that they could "load" the car in my absence. Re cleaning, I've never done it myself, but I'm assuming that it would have been done during the annual service (but then again, the lock-ups also occurred prior to 4-pots). Looks like a brake-power test will be revealing -- I shall arrange one.

 

JV

 

 

[John Vine]

An update....

 

I had the brakes tested at an MOT station yesterday. Result: equal each side, as near as makes no difference (if anything, the NSF was very slightly more powerful).

 

I'm about to re-bleed the calipers -- even more thoroughly this time, and especially the NSF. If that doesn't fix it, am I left with just chassis misalignment (twist)?

 

JV

[Graham Sewell]

The brake test at an MOT station does not test the limit of adhesion, only the amount of retardation prior to lockup. For that, I would expect to see near identical results for both sides, and bleeding the fluid will make no difference. What is happening is that the OSF tyre has slightly less grip - whether by geometry or weight loading - and hence gives up the ghost earlier.

 

As long as the car brakes in a straight line, get used to this quirk. In the end, one wheel will always lock up first, it is just useful to know which one!

 

Low tech luddite - xflow and proud!

[John Vine]

Graham,

 

Thanks for the logical analysis. The trouble is that I'm losing potential retardation, especially on wet roads. Assuming the problem is one of geometry, I had hoped that flat-flooring would fix it, and was puzzled (not to say a tad disappointed) when it didn't. Maybe I should be applying more weight on that corner?

 

Out of interest, the brakes worked perfectly at Brands on Sunday -- no lock-ups at all. But then, maybe that's just because I wasn't trying hard enough!

 

JV

[Colin Mill]

Hi John

 

I was wondering if you have any idea how much extra pedal pressure it would take to lock the other wheel as well?

[John Vine]

Interesting question, Colin, but I don't have a satisfactory answer as, when I do lock the OSF, I tend to ease off the pedal rather than press it down further! What was your line of thinking?

 

JV

[shn7]

John,

 

Did you get a chance to swap wheel/tyres? I think you've got plenty of us ondering what the cause is. There again it could be as Graham says.

 

Steve.

[Colin Mill]

Hi John

 

Just trying to get a feel for what size of effect we are looking for. One question I have been meaning to ask but keep forgetting (ano domini) - is the car live axle by any chance? I ask because the engine braking will twist the axle clockwise and weight the right rear and offload the front right. I have no idea if the effect is big enough to see but its the right way around.

 

Colin

[John Vine]

Steve: No, not yet, but I plan to do that over the next couple of days and report back. In the past, however, when swapping fronts round (from Yoko's recommended rotation) to comply with the MOT requirements, it was still the OSF that locked up first.

 

Colin: No, it's a de Dion.

 

I've asked Jon at Millwood for his advice too. I'll post anything significant.

 

JV