Blue spot rear pads ok for track temperatures?

[thompster]

The rain finally stopped long enough for me to try out my new Wilwood/DS2500 front brake combo over the weekend. Following some 'spirited' braking the front discs were nice and hot but the standard rear calipers running EBC Ultimax (bought without thinking) barely got past tepid.

That leads me to think that the rear friction material isn't doing too much so time for a change.

 

I know that the Delphi 507CC pads aka blue spots are supposed to give good balance so they must be about the right friction coefficient and are far cheaper than 1144s or similar. Friction levels aside though do the Delphis cope with track temperatures or am I better with a race pad in the back?

[John Vine]

I have Delphi 507CCs on the rear of my R400D (AP 4-pots at the front), and I've found them to be fine on road and track.

JV

[John Vine]

aka blue spots

I forgot to add that the current 507CCs (at least, those sold by CC Parts) no longer have the blue spots, as CC have changed their supplier.  Details > here.

JV

[Richard Price]

Thompster,

if your rears are not getting hot, then they are not working, which is likely to be because you've improved the front braking power! You need to improve the rear braking power by a similar margin.

my own experience is that, with track day use, the blue spot pads overheat and crumble.

I think that some braking "improvements" are far from that. Retaining a reasonable balance is easily as important at "braking power". 

Similarly, some "better" pads need heat in them before they work. Therefore, you need the characteristics of both axles to change at the same rate, otherwise the balance will suffer (potentially very dangerous on a road car where they need to work with good balance from cold!)

Your current set up sounds safe, but, if the rears  are cold, they are not doing enough work (don't underestimate how much stopping power actually comes from the rear brakes). You're likely to be able to lock the fronts easily, but, not actually have improved stopping power!

it sounds like you need pads with a higher coefficient of friction to make them work better for the same hydraulic pressure. Or, increase the rear pressure (which you can't do with a standard brake system).

if you want to use unproven set ups then you really also need dual master cylinders and a bias bar, song garbage balance can be optimised.

 

[John Vine]

my own experience is that, with track day use, the blue spot pads overheat and crumble

Clearly, I don't go fast enough!  And thanks for the technical bit -- I've learnt something there.

...song garbage...

Sounds like the Eurovision Song Contest.

JV

[thompster]

Hi both, thanks for replies.  The blue spots not being up to the temperature was my worry particularly as I'm a late/heavy braker.

The DS2500s have a fairly consistent co-efficient of 0.42. Most of what I've read on here says the blue spots give decent balance with 4-pot fronts but I can't find a friction specification for them.

 My slight worry was that by fitting the same to the rear particularly given the large pad area at the back I'd have an over-braked back end.  Dual masters and balance bars sound complicated - I do have a bias valve already though so maybe over-braked wouldn't be a bad place to start and then dial some of it out.  Downside of course is that the blue spots can be had for 13 quid a set.. DS2500s not so much..!

Cheers, James.

[MADMALC]

Hi T,

I may be under thinking this but my observations are:

Even poor pads will get hot if being worked so if you are sure the rears are not getting hot it means they are not being used and changing friction material will not change this.

As you say you have a bias valve why not take some bias off the fronts and put more on the back to get the rears to do more work.

My SLR had the adjuster and this what I did but be cautious as the last thing you need is the rears doing excessive work when it is wet and slippy. A bit of cautious trial and error with the bias valve would be my first action.

Once you have experience you can then have a wet and dry setting for the bias valve if you so wish.

Only when the rears are actually trying to do some work and being over faced with heat would I look at upgrading the rear pads.

[Richard Price]

A bias valve in the rear circuit will only reduce rear brake power further

[MADMALC]

As I am reading it the bias valve is adjustable and it is not indicated which line it is in nor is it stated which way the adjustment is at the moment

If the bias was currently limiting rear pressure it could be adjusted to increase rear pressure.

Not withstanding any of the above if the rear brakes are not getting hot then something is stopping them from working.

[Richard Price]

thompster,

i don't necessarily advocate spending a fortune on pads, but, I can't see the point in upgrading one end and ignoring the other.

why did you feel the need to "upgrade" the front brakes? What symptoms did you have that you wanted to overcome?

if your blue spot pads are not getting hot, than yet are not working effectively enough.

your revised front brakes may produce better stopping power for a lower hydraulic pressure. But that means that the rear brakes are not working as effectively anymore (because it takes less hydraulic pressure to lock the fronts), so your overall stopping power is reduced. Assuming you bias valve is in the rear line, you need to ensure that the valve is not limiting rear pressure at all. If you already have maximum rear pressure, then you need pads with a higher coefficient of friction.

Yes. Going to Dual master cylinders is a lot of work. But, adjusting balance to suit different tyres, different camber settings, and different conditions is then very quick and easy.

for what it's worth, I now run AP's on the front and standard rears, with 1144s all around (but I've run dual master cylinders for  almost ten years now). When I first did the pedal box conversion. I was amazed how much I could mover the bias to the rear, and how much better the car stopped.

[thompster]

Yup - the bias valve only dials out rear brakes, you can't bias towards the back.  At the moment its all the way forward and in the rear line which I think (hope!) means it's not restricting the rear.   Rear pads at the moment are cheapo EBCs rather than blue spots.  They get warm, whereas the fronts get hot.  My take on that is that they are scrubbing off less energy because they create less friction for the same amount of pedal pressure- something like that anyway.

As to why I went 4 pot at the front?  When I bought the car it had standard front calipers with unknown pads.  I've never been happy with brake feel or performance and they overheat on track.  I could have just bought some better pads but lets be honest, where's the fun in that?   After all, ownership of a Caterham itself shows a certain  disregard for practical and necessary!

[thompster]

 

And just in case I've had it backwards all this time.. here's my bias valve pointing to the front of the car. I hope it's in the 'not restricting anything' position....! (The white stuff on tunnel is gaffer tape residue)

[Richard Price]

When you say "they overheated" what do you mean? 

Did the pedal go soft, or, did the pedal remain hard, but become ineffective?

the rear calipers are prone to fluid boil due to the design where the heat is transferred into the fluid rather than the calipers.

i've heard several times where a soft pedal had been cured by fitting big front brakes... However, the "cure" has moved the bias forward, so takes load of the rear brakes. A more effective cure for a soft pedal, is a racing fluid with a higher boiling point.... (And much cheaper!)

My sons car had standard brakes, mintex f6r front and 1144 rear pads, and good fluid. It was dangerous until the front pads were warm, but then coped with running on slicks on track days without problem (neither of us hang about either). But, for the road (or sprint/hillclimb) use, you need pads that work from cold with no surprises (which is why I run 1144s)

 

 

[thompster]

Previous overheating was a long pedal travel problem - made worse after standing for half an hour.  It had new decent fluid (forget what, not motul but something decent) but gave all the symptoms of fluid boil.  At the time I assumed it was front but you're probably right that it was the rear.   I wonder if something like 3m pad backing material would slow down heat transfer ot the caliper. As above I've fitted the wilwoods because I wanted them though, not as some rationally thought out cost-benefit exercise!

Having stuck that photo on I've managed to find a tech manual for the bias valve - first thing is it looks like I'm one click off full rear brakes.  Second thing is that even on fully forward (Position 7) it reduces rear pressure by 25%.. 100 bar input reduces to 73 bar.  Based on that it seems that if you have a bias valve you need to overbrake the rear to get back where you started.

[Blue C7]

Just a thought. Do you have a friendly mot garage nearby that would check what brakes are actually doing at each wheel.

[Ivaan]

Make up, or get made up, a short piece of pipe with the fittings, to bypass the bias valve. Looks like you could do that without taking the valve off. If it helps, then permanently remove it.

The rear facing brake pipe looks very close / touching the tunnel as it goes through the hole. Could wear a hole in the pipe!

[Richard Price]

Thompster,

The symptoms you describe are typical fluid boil.

if you want to rule out fluid boil, use Castrol SRF. Don't mess about starting any lower or with other measures (other than slowing down for a cool down lap, and not using the handbrake when parking in the paddock)

you may find that your bias valve is not actually doing much anyway. I use a datalogger and have a transducer measuring brake hydraulic pressure. When sprinting, the highest pressures I see are around 30bar.

if using any sort of valve in the brakes, you really need to understand the pressures involved, and the characteristics of the valve, to understand the effect they have. Unless the valve has been custom designed for a specific brake system, it's unlikely to offer anywhere near the usable settings that you may think it does. 

Most propper (open formula) competition cars use dual master cylinders so that brake balance can be optimised without the work that major manufacturers put into optimising brake systems for production cars.

Modt Caterham race cars don't use dual master cylinders because they are not allowed to by regulation.

 

[Paul McKenzie]

Thompster,

Get rid of the EBC pads-they're c**p