mark kibort

After taking the calipers off the Hobert racer, it was easy to see the upgrade process to make any S4 system, very close in performance to the GTS Big reds.
(i.e. 322mm rotors).

There are two challenges. spacing out the calipers to meet the larger diameter of the GTS rotor, and assuring there is no rubbing of the new larger diameter rotor, with the smaller radiused calipers.

First, take a look at the pics. Seems like that only a 9mm washer should do the trick. I was thinking that the entire caliper met the hub over 4-5" of mating surface. it doesnt. all that makes contact, is a small .75" diameter spot that surrounds the bolt hole. Certainly, a longer caliper bolt will have to be used . The last picture is the spacer installed but the stock rotors from the S4 are still there too, showing how far the caliper moves out with the pad.

I wont know what the caliper vs rotor will look like until i put it all together, but i would bet it will be close and maybe require some grinding on the outer edges of the rotor relief area on the caliper.

I cut an old aluminum lug nut at the thickest point and made a washer to size things up. Ill see if I can find a washer company that has a 9mm thick washer and the proper diameter bolt hole.

Now, you can have most of the benefit of the GTS braking system, but still using the stock S4 calipers. since rotors are no more than $25 more expensive, its a no brainer.

Next will to be to use some REAL racing pads. Pagid blacks or Hawk HT10/14s

mk

Mrmerlin

those calipers look cooked you might try sourcing a new or rebuilt set as these look like they are ready for replacement. I would also put on new lines

mark kibort

look at the heat vapor trail on the trailing edge of the pad on the caliper. 1200 degrees?

Yes these calipers have seen some action. It might be time for some replacement scheme

mk

Mrmerlin

I wonder how often the Nascar racers replace their brake calipers,
Oh Mark if you do go the, replace route, dont forget to order new Caliper bolts, new lines, banjo bolts and sealing washers, the whole system should be brought back to standard.
If you have the ABS still connected you should also clean off the sensors and the generator rings they look dirty. Good luck

PorKen

3/8" might be easier to find.

Lizard928

I dont know why MrMerlin is suggesting that they be replaced as if they arent leaking then they are fine, IMHO. However you may consider replacing the seals IRREGARDLESS as to if they are leaking or not and pull out the pistons to inspect them for any damage/fatige. If they are fine then there is NO need to replace them.

The calipers have not seen anywhere CLOSE to 1200 degrees as well. At that temp the fluid boils well before that point (even full race fluid), and then it doesnt work any further. Plus on top of that all the seals would have completely melted as would the pistons and calipers as most aluminum melts abit before that point.

The only other thing is that it is NOT a good idea to use those washers you have made, they would deform, by that I mean crush around the bevel, leaving the bolt loose which will then work its way out and lets just say that could lead to a VERY messy situation!

Mrmerlin

Colin, odds are these calipers have been on the car since it was built, i am sure a newer set of calipers that have been rebuilt will work better than the ones on there now,
They do look cooked. whether or not the seals are leaking shouldnt be a reason to replace a part thats highly stressed. and has seen a lot of heat cycles.
I guess that Mark could also rebuild the calipers he has now........ Same thing goes for a hi po engine they still get rebuilt even though they are still running.
Good point about the stand offs deforming.

mark kibort

I know the calipers are not at that temp, but the vapor trail could be much hotter than the fluid inside the calipers. we see glowing red hot rotors at the track on some turns. just at the very end of turn 2 at laguna for example. I know the pagid oranges resist fade until 1000 degres, and the blacks near 1200 that probably is rotor temp. the calipers are protected by the thickness of the pads and backing plate. But, that fire off the rear of the pad is scorching the caliper on the trailing edge of the pad.

I wasnt going to use those washers made from lug nuts. it was just for a mock up. the problem im finding now is that with the calipers made for 12" vs the rotors of 12.7", there will be about a 2-3mm difference out at the edge that might need to be ground off. is that materil of the caliper at the ends, cut out for the rotor solid, or is there fluid in there? I need to see one taken apart before i start grinding on the calipers in that area. Here is a pic of the gap on a stock caliper and rotor set up. its about 3mm tops.

as far as the washers are concerned, it might be good to use steel, what do you think. 3/8s" might do it (.375). .021" of the pad might be left exposed at the top, but that might help the rubbing area at the ends of the S4 calipers on the 322mm rotors.

mk

belgiumbarry

i would make a lasercut 1 piece steel or aluminium ? template with the 2 holes in it and using ALL the contact surface ( which is not a circel ! ) ... sorry, i would not thrust 2 separate washers.... the bolts are getting also a bending stress in that way , not only shear stresses... no ?

Putting the big calipers on my 82 ( set from M928) i needed to space out the rear ones to get the rotors in the middle of the calipers... now Carl advises also 2 washers to be used but i didn't go that way... again to hold perfect the contact faces and not stress the bolts in any other way.... so i laser cut aluminium spacers ( think it was 4 mm ) to be placed under the rotors... the wheel bolts are still long enough ( use with steel nuts ) and i just see the ebrake pads protruding the rotors... but that isn't important i think.

so , instead of placing the calipers more inside , i placed the rotors more outwords.

jon928se

I hear what Barry is saying and he has good arguments

Gut engineering reaction is that so long as the spacers have the full contact area ( I know you aren't goint to use sawn off lug nuts as spacers) they have in the stock oem scenario and you use new Grade 12.9 bolts you won't see any problems.

Reasoning behind this is
You can probably lock the wheels and/or exert maximum forces on the bolts now with the stock S4 setup when the brakes are not tooo hot.
Thus moving the calipers outwards on a bigger dia disc brings two benefits The pads don't get as hot as they don't work as hard and the rotational force applied to the pads by the disc is less. The max retardation being governed by the tyres (which havent changed) not skidding on the track surface.

The other factor being that the stock factory setup was designed (by default) to last 20+ years without inspection. I hven't put numbers on it but I'ld hazard a guess that the caliper bolts are over designed by a big factor, and the limiting factor is likely to be the threads in the hub and not the bolts.

Now if you said you were going to use 30mm spacers (to use a 380mm rotor) I would probably not comment at all until I could put numbers to it.

to be safe I would probably recomend changing the bolts every season.

mark kibort

I think you have a different set of forces with the axial mounts of the early cars, vs the S4 which the bolts are used in a radial mount. (spacers put the calipers out at a further raduis to match the larger diameter of the larger rotors) the offset is perfect, so you dont need to use any spacers on the roto to change the offset.

I think you are right noticing that my spacer mock up could be an issue, but if you look at the caliper and hub pictures, you see the contact area is not that great. so a larger diameter spacer washer of near .35" would be used. Probably steel. I cant imagine any significant increased shear forces. Ill have to pass that by someone that can confirm that.

Im now leaning to wait until i try the new racing pads. I cant imagine that .5" of radius could help as much as I was expecting it to. I think its around 10% more braking force at first glance.

as far as braking force comparisons go, the only issue i have had recently was braking from very high speeds to slow corners. before 120mph to 40mph was good. 130 to 40 is a little tougher to modulate the pedal near the end of the braking zone. you can tell, the pads are failing. however, if i needed to stomp lockup the fronts, i could do that. You loose the modulation with brake fade. the larger diameter rotor gives the same braking force with less pressure. The power (heat) should be the same for a equal decel rate.

mk

Lizard928

Mark to answer your question, there is NO fluid in the area directly above the rotor, the two banks are connected with the small hose at the bottom.

I now understand what you are talking about with the heat, and thinking about it more it would be a good idea to take a thin piece of stainless and put it between the two outter parts of the caliper and the rotor (where you want to cut down), this could help in cutting down on heat transfer.

mark kibort

Thanks. Not knowing what the caliper tops look like on the inside, was making me question whether this could be cut down by a couple of mm. as you get toward the apex of the rotor caliper, this distance goes to 0 vs the stock S4, so its only at the very ends that Ive calculated about 2-3mm of difference. Inother words, if the pads are in the same position at the edge of the rotor, the ends of the calipers are going to probably hit now. cutting out that clearance is the goal.

So, what are you talking about with the stanless steel "insert"? If i understand you correctly, why would you want stainless there? to protect the calipers in that area from heat? If so, i dont think that would work as the calipers now are aluminum and get rid of heat fast, where steel doesnt. I dont know what the GTS calipers look like at the ends, but i imagine there is a similar air gap.

mk

belgiumbarry

i think Colin meant the heat from the rotors would be more reflected, radiated back and not so much accumulated in the aluminium calipers.. no ?

PS Mark, before such works.... high pressure clean that junk, and steam it out !

mark kibort

those calipers have been exposed to some major heat and corrosion. If i remember, my very first pictures of the cars underbody were shots of the brake area and it looked similar to this. I wonder if it was from the original salt exposure from the Bonneville speed runs? the calipers are pretty clean, just stained and there is a lot of heat marks on them. Scots are even worse. He is kind of keeping up, and doing it with the smaller rotors and the single piston caliper S brakes. . He turns those pagid orange in to moon dust.

mk