GTD Spindle / upright impact on braking performance

I am still not happy of the braking on my GTD (Wilwood 4 pots on front and single pot Scorpio rear). During the 2 laps at Le Mans Classic my pads smelt burnt odour at the first chicane in Mulsane straight, ok it was a big brake at 160 Mph but this should work correctly normally.

I spoke with Mark Sibley (MDA GT40) who explained me that the original GTD spindles & front uprights are not stiff enough and allow the calipper to move from the disc with a consequence of a bias wear of the pad, i verified and my pads are effectively bias weared /ubbthreads/images/graemlins/frown.gif
Anyone has some comments or experience about that?
Any solution would be welcome ...
thanks for your help

What I think you talking about is Spindle Flex leading to pad knock back. The standard GTD front spindles sometimes did and sometimes did not suffer from this problem. GTA917 races his 40 on these spindles and during my conversations with him I do not recollect him saying he had this problem. Also Julian KS doesn't suffer from it either. and he also does competitive events on slicks. However I did suffer from it and I know of others too who had this problem. Therefore to me it is a lottery whether you get it or not!

I never noticed it causing uneven pad wear and don't see the connection. As you are cornering the spindle flexes and pushes the pistons back into the caliper. Unless you are hard on the brakes in a corner (not recommended!) the force required to do this is really quite small. It shouldn't show up in uneven pad wear especially when considered against wear from 160 mph stops such as you have done.

The classic sympton of spindle flex is that when you go for the brakes, in a straight line after some cornering, you have a long pedal that pumps up on one push. I remember having to left foot the pedal stiff on straights before corners during competition to save my braking distances getting too long!

From this problem replacement front uprights have been used by those that lean their cars into corners. I understand that Roy Smart can make these for you from the GTD (Poole) factory designs plus subsequent improvements that have been incorporated. Paul Thompson can say more on this as he is more aware on what is on offer.

If you are gettind wedge shaped pad wear then check your brake calliper mounts are mounted equally around the disc ie they are not at an angle before you even start to use the brakes. I made my own bracketry and found that I had to be really carefull in the alignment of the calliper to the disc.

Another solution to knock back is to use anti knock back springs. These little springs sit inside and behind the brake caliper piston and exert a small level of force permanently on the piston pushing the pad back onto the disc. I use these on my Alcon calipers with satisfactory results. I have not noticed shortened life expectancy of the brake pads either!

I still use the single pot caliper at the rear on a vented disc. Are your rear brakes vented? They need to be! I have found that with Alcon 4 pot fronts and the single rear pot, I get a good balance on braking and can brake as much as I ever need to in competition.

How big are the pads in your Wilwood calipers? If small in surface area then they could overheat quicker than a larger pad of equal material, wear faster and be less effective. Multiple piston calipers may sound great but they have to be of the correct size to be better than a lessor pistoned but correctly sized caliper. Maybe the fronts are working too hard? Do you get brake fade?

Silly thought? Could the smell actually be the clutch?

Anyway I hope this helps.

Re: GTD Spindle / upright impact on braking performance
As nobody else has I'll ask the obvious (and maybe spurious) question :-
Given the apparent flexibilty of the component, what is the upright breaking performance on impact? /ubbthreads/images/graemlins/wink.gif

Fred,

what type of pads do you have?
I have the green stuff wich is in between normal road usage and the the racing (carbon) ones.
Could this be the cause as I have same brakes as you.
RGds
fred

Whilst I have heard of the spindles flexing, I have never heard of one breaking.

to answer Fred before, i used both solutions with the pads and no better results from one to the other (racing and road).

Malcom,

i run no vented disc at the rear (yes i know it's a shame)
I think the calliper clearance won't allow a vented one, is there any spacer to put in ?
I am 100% sure that the odour is not from the clutch.
I ran several times the car on track days and the braking performances decreases more and more even with a pad change.
Last time at Monthléry i had what you explained: a long pedal after several corners and brakes but i thought it was due to vapor lock and i bleeded the brakes, i also removed the servos but the result was the same, excepted a stonger pedal with a better feeling on it.

My advice is that my rear brakes are not strong enogh, this car with a central engine needs to have a good balance between the font & the rear, Fred you use HiSpec callipers on the rear which might change everything in comparison to my setup.
I think this is the first way i must explore before changing all the front drive trains.
But the uneven pad wear on the front will remain an enigma ...take a look to the disc surface, strange wear isn't it ?

Calliper mounting / disc (seems to be ok)

The wear marks on your rotor suggest that the rotor is slightly thicker at the outer dia or that it is warped a little. Did you mic the rotor to check that it is the same thickness at the major and minor dia's of the brake swept area of the rotor. If it is warped or tapered a clean up cut on a lathe should fix your problem. My setup is very similar to yours (except that I have 13" rotors front and rear), and with heavy use of the brakes I experience very little fade even when they get almost red hot.

What spec Hydraulic Oil are you using ? Just a thought you may be exceeding the temp limits which may be disguising any other problems.

Fred,
Hard to tell from a photo, but it looks like hard spots in the rotor casting. This is not uncommon with some cast irons. Typically the hard spots look more shiny than other parts. The hard spots generate a lot of heat compared to the rest of the surface and from the photo it appears that this is the case. Hard spots are best removed by grinding the rotor rather than machined in a lathe.

There may be some caliper flex. Not unknown with Wilwood calipers.

Tim.

The marks on the rotor look like hot spots to me. But as you run single pot solid disc rear brakes your fronts will be heavily overworked. GTD dropped the use of solid rear discs after a while and only sold vented rears. My car originally had the solid rear discs and they eat up pads with little braking effect. My record was set of pads gone in 80 miles in one day at Goodwood, lots of brake dust!

From seeing your front brakes my suggestion is to change your rear brakes to either more Wilwoods to match the front or just to the vented single pot caliper set up typically found on GTD's. Your existing caliper will not be alterable to suit a vented disc, discard it! However it is not just a matter of changing calipers and discs over, you also need to mod your rear upright so that the new position of the caliper is better for brake bleeding etc. The steel uprights you need are available from Roy Smart I beleive.

My standard recommendation to owners with cars like yours is to always upgrade the brakes before the engine. Keep your stopping and go power in balance and you will live!

Thanks guys for your comments,

The engine was never touched since i own the car for 7 years now, but i must agree that the 350 hp it gives are quiet hard to stop sometimes ...
I think a grinding of my front discs could be a no wasted time.

Malc have you any ref of part n° to submit for the rear calipper & vented disc ? I wonder otherwise if i won't go for the Hispec with the handbrake system which seems to be really good value. I know that i'll have to modify the rear uprights to fit them which it is not an easy job, especially to have the calipper bracket in the good line with the rotor. Having good brakes is the most important before any other improvements i totaly agree with that too !

Fred, I think that roaldin has touched on a likely issue here. From the strange pad wear, and the picture of your disc, it is possible that you have to much flexibility somewhere in the caliper mounting. This could be in any adaptor component used to mate the caliper to the upright, or in the caliper mounting on the upright itself.
From what I understand, too much flexibility in this area has two effects, first, deflection and twisting of the caliper relaative to the disc under steady braking loads, which causes the biased pad wear, and second, brake judder which causes the hotspots on the disc.
I would look carefully at the caliper mounting for any flexibility due to insufficient robustness, or poor mounting due to any additional spacing washers used to position the caliper.
I have seen some brake upgrade kits which look very suspect in this area.
So, did you notice any judder while braking hard? because those heat markings certainly suggest it.
What is the pattern on the inside face of your disc?
Does the caliper use a mounting adapter and any spacing washers?
regards
Dave

Interesting, the calipers are the same as mine - an early willwood superlight. I assume you do not have bridge bolts across the caliper which will allow an amazing amount of flex and uneven pad wear along with a long pedal. I had this trouble and fitted the bolts both front & rear. this made a huge difference to braking ability, pad wear & pedal travel.
rgds,
RV

Dave, Rick,

Your comments are very interesting, the callipers are fitted with a kind of anodized bracket on the upright.
Rick i had some troubles to understand exactly what you ment (don't forget i am French !!!) Which bolts do you spoke about ? A pic would be welcome to understand ...
Because all these bad effects are those i have !(uneven pads wear / long pedal / poor efficiency ....)
Which rear brakes do you use Rick ?

Fred,
I am using superlight front and dynalight rear calipers. Both are 4 spot calipers. You will find that the earlier versions of these calipers had the pads held in place with split / cotter pins.
If you have a look at the wilwood website you will note the current superlights have bridge bolts fitted which prevent separation of the caliper halves which causes the tapered pad wear and long spongy feel at the pedal. You should find that the superlights already have 1/4" holes already drilled for these bolts to be fitted. These are located on the side that the pads are inserted from and either side of the cotterpin.

On the dynalight (rear) calipers the hole in the caliper halves for the pad retaining split pin was drilled out to 1/4". You will also need to drill out the cotterpin hole in the pads to an oversize 1/4" to allow the pads to slide properly.

Make sure you use very high tesile bolts and also a compression tube between the caliper halves so that the bolts can be torqued correctly without closing the caliper halves.

You will be amazed at the difference.
regards,
RV

Rick,

I understand what you mean by bridge bolted, it is a good lead to follow. I am not sure to be able to use my old pads with a such modification but it is worth trying ...
Thanks you all for your help & comments, Ron did a great thing with this forum

No Worries Fred.
If your pads are tapered throw them away and get replacements. I am waiting on delivery of some hawke "HP plus" pads. These are supposed to be very good agressive pads suitable for the street (ie no heating required) and be able to be used in relative anger for track days etc.
The picture of the wilwood caliper is exactly what I was trying to explain.
please post the outcome of your mods.
Cheers,
RV