996 hubs + boxster brakes/rotors fit under 15" Fuchs?

[Maxhouse97]

Crazy question - I used Instant-G's 996 brake kit to upgrade my stock Carrera brakes to 996 brakes. Had to go to 17" wheels. Great setup and feel. Believe he uses early 944 hubs machined to fit the rotors.

However, if I ever wanted to backdate the car, one thing I would want is 15" wheels. I suspect stock first generation 986 rotors would bolt right up, which I believe are 298x24mm, as opposed to my current 319mm fronts. If I could get the adapter right and mount the caliper lower, do you think these would fit under 15s? Aren't the 930s 300mm as well ... they fit under 15s.

I realize that offset and backspacing of the caliper may be a problem, as well as caliper height, but have no idea how (hence the question). I looked on the 944 boards and some guys have had success fitting 951 brakes under 15" phone dials.

What sparked this was the most recent issue of Excellence where a R Gruppe car was listed as having 986 brakes under 15s (don't know what rotors they used though).

[reddogmotrsprts]

Boxster calipers fit under 15" wheels.

[Maxhouse97]

Great - with how much clearance though? Stock 986 calipers and rotors would be a slight upgrade over Carrera 3.2 (extra 5-8mm I believe) but now I am getting greedy and wondering if 986.2 rotors (2005+ non-S) will fit.

Also want to make sure when folks say boxster calipers will fit they mean with boxster rotors or if they mean with Carrera rotors. Thanks.

[Bill Verburg]

don't for get that rotor specs include other factors besides diameter

for instance thickness, 996/Boxster S calipers want rotors that are 28mm thick, Boxster are 24mm. If you use these w/ 996 calipers you are starting w/ rotors that are less than spec.

[KTL]

The Boxster brake kits like TRE and Rebel offer have Carrera rotors in front, SC rotors in rear. The SC rotors are used at the rear because the Boxster rear caliper is originally designed to use a 20mm thick rotor. But if somebody wants a bit more rear rotor thermal duty, we can grind the pad abutments/posts inside the caliper and fit the wider 24mm thick Carrera rotor.

brake system components Rebel S Racing Products

[Maxhouse97]

Thanks Bill I understand. I can switch to boxster calipers I guess. Was trying to make sure they would fit without changing the hubs back, or if I could take advantage of slightly larger rotors (2005 boxster) since I don't have the limitation that most carrera owners have (that being easier to stick with Carrera rotors).

Basically looking for good, cheaper options for brakes that would fit 15" wheels than 930s.

[KTL]

Originally Posted by Maxhouse97

.

Basically looking for good, cheaper options for brakes that would fit 15" wheels than 930s.

Carrera brakes?

[reddogmotrsprts]

Everything is cheaper than 930 brakes.

Are you tracking this car? What does it weigh? Can we assume you have a 23.8mm master cylinder and a 5.8x pedal box?

[Maxhouse97]

Yes my stock brakes would work, but then I would have to go back to bleeding them after every DE run...

Track - yes, but only DE couple times a year. Probably weighs 2600ish. Yes on the m/c, dont know what you are talking about with the pedal box. It is stock.

[reddogmotrsprts]

The 5.8x refers to the amount of mechanical amplification of pedal pressure you get. The earlier pedal set yields only 5.2x. It's useful to know so that we can guess at what changes will feel worse.

Boxster fronts with the Carrera rears will give you good bias as far as piston pressure goes, but you'll have more friction up front due to greater pad area. At the same time, your rotors will be of equal thickness at 24mm. This suggests to me that you will overheat the fronts well before you overheat the rears, and that therefore, your rear rotors are too thick.

Your car isn't super heavy, so I'd recommend going back to the Carrera brakes and working on the fluid problem instead. Your money is probably best spent on high-quality fluid, brake ducts, and insulators -- in that order.

This to me seems like the cheapest and easiest solution which puts you into 15" wheels without any troubling performance compromises, but I defer to those with much more experience.

[Maxhouse97]

No that was one of my first ideas - if I can get the car light enough (which its at least 100 lbs lighter than when I last tracked it with stock brakes) than the Carrera brakes might be ok. Honestly the only thing I wasn't looking forward to was sticking the heavy iron ATE caliper back on for unsprung weight. And loosing the cool look of drilled rotors

I already have all the parts though so this should be easy. Could also go with boxster calipers at that time as well (with adapter).

At that point it would be balancing looks vs braking performance. Looks wise I wouldn't need this until/if I backdated the car. Thanks.

[KTL]

What pads do you use for the DE track days? We in our group of guys tracking the old cars got by OK with PF97 (no longer available) or Hawk HT-10 for many years with our stock brakes. Didn't need to bleed them every session. Just before each event. Our most frequent track is Blackhawk Farms a fast short track that's hard on brakes. Seven turns, five of which are hard applications of the brakes. Not much time in between turns, except for before & after T1, for the brakes to cool. Recently a switch to Raybestos ST43 has been a good choice. Point being is a pure race pad will negate much of the stock brake issues. However you still have calipers that don't like the heat and the pressure seals eventually leak.

Pad area does not necessarily effect friction. F = mu x N, and there's no area component to that equation. Larger pad area provides a wider platform for application of the braking force and more heat insulation.

[reddogmotrsprts]

Originally Posted by KTL

Pad area does not necessarily effect friction. F = mu x N, and there's no area component to that equation. Larger pad area provides a wider platform for application of the braking force and more heat insulation.

Right, my comment wasn't very precise, but if you increase the pad area, you are altering the ratio of heating (due to friction) to cooling (due to airflow) area on the rotor.

I hadn't factored in greater heat insulation from pad area in any of my brake calculations. This seems like an obvious oversight now that you've mentioned it. Do you know significant this is versus rotor area, or how we might weight this value?

I suppose that we're trading fluid fade in favor of rotor fade -- which might be a net win for the OP -- and I'd just like to know if there's an accessible way to estimate this trade-off. I expect that it varies within some range depending upon pad compound, area, volume, etc.

[KTL]

You're right that the larger pad has some consideration for affecting absorption versus how it is also covering more of the rotor and thereby reducing the rotor's ability to shed heat directly to the air. But it seems that it's generally accepted to negate that effect. See Stoptech's statements on that around 1/2 down the page under "Pad Area"

Brake System and Upgrade Selection

Pagid talks about pad area Technical info: PAGID RACING - Motorsport and High Performance Brake Pads

Wilwood says the same thing Wilwood High Performance Disc Brakes

So the pad's larger surface isn't really a primary consideration for how much heat it takes away from the rotor. But what is a consideration is how the larger pad is a larger insulator in terms of shielding the caliper from the heat generated by friction, as well as reducing pad wear. By using a better pad compound that is more heat tolerant, it should insulate the caliper from heat (fluid fade) and also not be inclined to experience rotor fade, caused by pad breakdown

[reddogmotrsprts]

Originally Posted by KTL

You're right that the larger pad has some consideration for affecting absorption versus how it is also covering more of the rotor and thereby reducing the rotor's ability to shed heat directly to the air. But it seems that it's generally accepted to negate that effect. See Stoptech's statements on that around 1/2 down the page under "Pad Area"

Thanks for the links. Here is the comment from Stoptech that I think you're referring to:

Pad area and geometry are however important for several reasons [... including ...] heat dispersion and dissipation over a larger surface area and greater mass. Although in the case of a larger pad, the pad masks a larger portion of the rotor face, absorbing more radiant energy and shielding the area from cooling that may cancel any actual benefit.

It seems to me that the first sentence giveth and the second taketh away.

My interpretation is that "a larger pad [... cancels] any actual benefit [of heat dispersion and dissipation over a larger surface area and greater mass]".

This seems to agree with my statement. I only scanned the other two links, but they seem to simply state that a larger pad absorbs more initial heat and wears better, which is pretty self-evident, I think.

Originally Posted by KTL

So the pad's larger surface isn't really a primary consideration for how much heat it takes away from the rotor. But what is a consideration is how the larger pad is a larger insulator in terms of shielding the caliper from the heat generated by friction, as well as reducing pad wear. By using a better pad compound that is more heat tolerant, it should insulate the caliper from heat (fluid fade) and also not be inclined to experience rotor fade, caused by pad breakdown

So, I don't agree with this (again, based on my limited experience and understanding). It seems to me that whatever heat is generated must be shed, and while I'm interested in how the choice of pad and pad area may insulate the fluid from this heat, I don't think the pad can perform any magic here -- either it insulates and thus denies the flow of heat through itself and the caliper, thus increasing heat in the rotor (rotor fade), or it helps absorb that heat (fluid fade) and thus reduces the temperature of the rotor.

[KTL]

I agree the first sentence indicates the benefit of the larger pad is nearly counterproductive since more pad in contact with the rotor provides a larger area to absorb heat.

Make no mistake about it, pads do indeed absorb some heat. I'm not trying to imply the best race pads do a super job of blocking heat. The pads obviously create heat by nature due to their friction characteristics. But the pad also has to insulate the caliper from heat as much as reasonably possible. The pad material itself must be highly heat tolerant to survive. It survives by being heat tolerant and by shedding itself.

That said, pads can only deal with the heat so much and the heat doesn't just magically flow away- it has to go somewhere. And as the pad gets thinner, they have less mass available to deal with heat and more heat will transfer to the backing plate, then the pistons & then the fluid. If the pads were so good at insulation, what would be the need for high temperature fluid? High temp fluid like Castrol SRF can handle up to 600 degrees and brake temperatures get into the 1200+ range. Well if the fluid can handle 600, where's the other 600 going? Pad is preventing some of that heat from transferring thru to the pistons and fluid but not a whole lot.

The links indicate the larger pads absorb more "initial" heat "The size of the pad matters in terms of heat capacity and wear rate. A larger pad will absorb more initial heat and has better wear characteristics " whatever that means. As I understand it, we don't go looking for bigger pads to manage heat. We go looking for bigger rotors and as a result the pad typically grows in size to fit that rotor? Maybe, maybe not. I'm not 100% sure on that. But typical rule of thumb is larger rotors carry larger calipers which in turn carry larger pads.

The larger pads do indeed collect more heat, in terms of heat volume because they have more mass to gather heat, but I don't think that's a shortcoming compared to smaller pads? I would suspect the average heat level in a small pad is greater than a large pad, on account of the force being more focused on a smaller pad. In the end, the rotors are the bigger heat sink than are the pads.

When the rotor's thermal capacity is peaked, where does the heat go? It goes into the pads because where else will it go? Path of next least resistance, which is the pads and then pistons. Dunno about the specifics of pad metallurgy but its very detailed with many different materials, some of which are intended to combat heat transfer. I do know the pistons have serious consideration for lack of heat transfer. That's why actual racing pistons are made of materials like stainless steel and titanium because they are less conductive of heat than plain steel or aluminum. I've seen pistons that have ceramic inserts (GT3 cup car brakes) to block heat and some people choose to use "shims" made of titanium to block the heat. If the heat didn't transfer thru the pads, those inserts and shims wouldn't be necessary.

I think you're referring to pad fade as rotor fade? Rotors don't actually fade. It's the pads that fade which in turn can physically affect the rotor. If the pads go south far enough, they can get hot enough to create hot spots on the rotor that collect more pad material in those areas and that creates "warping". Once those hot spots are there, the rotor has been permanently heat treated and those spots aren't going away. Not even if we were to cut the rotors, because the metal has been permanently altered on a material level. Plus we don't want to cut the rotors anyway since that takes away thermal mass to absorb heat. The rotors are then junked. Look there, more talk of thermal mass, which is a good thing, and the same applies to the pads?

Ideally we don't want the heat to get beyond the rotor all that much and we should cool the rotor as best as possible. For whatever reason most people don't want to go there and instead they'll throw better pads and better fluid at the problem as crutches. Those solutions deal with the problem OK but its still not dealing with the problem at the source- rotor cooling. If we put on a larger set of rotors, or rotors with better cooling vanes and get some ducted air cooling them, the pad and rotor life goes way up.

[Bill Verburg]

Pad mask is a non issue
for an SC it is ~2.5% f and ~1.6% r
for a 993RS it ~3.7% f and ~4.3% rear
for a 930 it is ~ 2.2% f/r
for a 964 it is ~2% f/r

the bigger thicker rotors more than make up for any additional masking effect, + the curved vane internal ducting of the late big rotors is far more efficient at pumping air through the rotor than the straight vane Sc and 964 rotors

as far as mechanical and thermal stress the larger are over which these are distributed leads to longer life

@70bar
SC pads experience 19n/cm^2 f & 18n/cm^2 r
993RS experience 15/13
930 21/13

pedal ratios were mentioned earlier these are pretty much fixed
thru '76 no boost it's 5.5
'77-89 boosted it's 5.2

[reddogmotrsprts]

This is a pretty interesting discussion. I hope I'm not coming across as gratuitously argumentative.

Originally Posted by KTL

I agree the first sentence indicates the benefit of the larger pad is nearly counterproductive since more pad in contact with the rotor provides a larger area to absorb heat.

Actually, I think Stoptech is making the same assertion that you did -- that larger pads can offer more insulation. But they are saying that those benefits are outweighed by the loss of exposed rotor surface, which is what I was basing my recommendation upon.

Originally Posted by KTL

Make no mistake about it, pads do indeed absorb some heat. I'm not trying to imply the best race pads do a super job of blocking heat. The pads obviously create heat by nature due to their friction characteristics. But the pad also has to insulate the caliper from heat as much as reasonably possible. The pad material itself must be highly heat tolerant to survive. It survives by being heat tolerant and by shedding itself.

That said, pads can only deal with the heat so much and the heat doesn't just magically flow away- it has to go somewhere. And as the pad gets thinner, they have less mass available to deal with heat and more heat will transfer to the backing plate, then the pistons & then the fluid.

Sure, this is all a given. But look at it this way... If pads were more efficient at shedding heat than rotors, then we would run steel pads and rotors made of pad compound. Okay, that's silly, but I think you get my point.

Originally Posted by KTL

If the pads were so good at insulation, what would be the need for high temperature fluid? High temp fluid like Castrol SRF can handle up to 600 degrees and brake temperatures get into the 1200+ range. Well if the fluid can handle 600, where's the other 600 going? Pad is preventing some of that heat from transferring thru to the pistons and fluid but not a whole lot.

I don't understand. It is preventing heat from transferring to the caliper/piston; in your example, it's preventing 600°F of heat flow. But remember that the caliper/piston/fluid represent a usable thermal mass as well. Although the rotor may absorb and shed heat most efficiently, we don't mind heating the other components up as long as performance isn't affected, right? Let me put it this way: if no heat transferred into the caliper then we'd be wasting the capacity of those components to absorb and shed heat.

Originally Posted by KTL

The links indicate the larger pads absorb more "initial" heat "The size of the pad matters in terms of heat capacity and wear rate. A larger pad will absorb more initial heat and has better wear characteristics " whatever that means. As I understand it, we don't go looking for bigger pads to manage heat. We go looking for bigger rotors and as a result the pad typically grows in size to fit that rotor? Maybe, maybe not. I'm not 100% sure on that. But typical rule of thumb is larger rotors carry larger calipers which in turn carry larger pads.

I think what they're talking about here is spreading the friction out over a larger surface, thus preventing greater heat in a smaller area, which would destroy the pad faster. More pad material equals longer wear, and we can either run a thicker pad or a longer/wider pad.

Pads only have to grow to fit larger rotors in one dimension, so they could have the same surface area regardless (with some obvious limitations).

Originally Posted by KTL

The larger pads do indeed collect more heat, in terms of heat volume because they have more mass to gather heat, but I don't think that's a shortcoming compared to smaller pads? I would suspect the average heat level in a small pad is greater than a large pad, on account of the force being more focused on a smaller pad. In the end, the rotors are the bigger heat sink than are the pads.

When the rotor's thermal capacity is peaked, where does the heat go? It goes into the pads because where else will it go? Path of next least resistance, which is the pads and then pistons. Dunno about the specifics of pad metallurgy but its very detailed with many different materials, some of which are intended to combat heat transfer. I do know the pistons have serious consideration for lack of heat transfer. That's why actual racing pistons are made of materials like stainless steel and titanium because they are less conductive of heat than plain steel or aluminum. I've seen pistons that have ceramic inserts (GT3 cup car brakes) to block heat and some people choose to use "shims" made of titanium to block the heat. If the heat didn't transfer thru the pads, those inserts and shims wouldn't be necessary.

Agreed. That's why I included thermal insulators in my list of suggestions. I think Fenn Lane's pistons are pretty fairly priced, but I had the idea that a complete set of shims would be less expensive.

Originally Posted by KTL

I think you're referring to pad fade as rotor fade? Rotors don't actually fade. It's the pads that fade which in turn can physically affect the rotor. If the pads go south far enough, they can get hot enough to create hot spots on the rotor that collect more pad material in those areas and that creates "warping". Once those hot spots are there, the rotor has been permanently heat treated and those spots aren't going away. Not even if we were to cut the rotors, because the metal has been permanently altered on a material level. Plus we don't want to cut the rotors anyway since that takes away thermal mass to absorb heat. The rotors are then junked. Look there, more talk of thermal mass, which is a good thing, and the same applies to the pads?

Sure; the point is that the rotors get toasted and not the fluid. I was contrasting the two, since the OP's issue stemmed from the latter and not the former.

Originally Posted by KTL

Ideally we don't want the heat to get beyond the rotor all that much and we should cool the rotor as best as possible.

If that's the case, then having more surface area in contact with the rotor means more heat could flow into the pad.

Originally Posted by KTL

For whatever reason most people don't want to go there and instead they'll throw better pads and better fluid at the problem as crutches. Those solutions deal with the problem OK but its still not dealing with the problem at the source- rotor cooling. If we put on a larger set of rotors, or rotors with better cooling vanes and get some ducted air cooling them, the pad and rotor life goes way up.

Agreed; hence my second suggestion. Again, I'd start with the $40 worth of fluid before springing for $100-400 brake ducts.

I'm assuming that the insulators offer a poorer cost:benefit than the ducts, but I'm happy to be schooled on that point.

[reddogmotrsprts]

Originally Posted by Bill Verburg

Pad mask is a non issue
for an SC it is ~2.5% f and ~1.6% r
for a 993RS it ~3.7% f and ~4.3% rear
for a 930 it is ~ 2.2% f/r
for a 964 it is ~2% f/r

the bigger thicker rotors more than make up for any additional masking effect, + the curved vane internal ducting of the late big rotors is far more efficient at pumping air through the rotor than the straight vane Sc and 964 rotors

as far as mechanical and thermal stress the larger are over which these are distributed leads to longer life

@70bar
SC pads experience 19n/cm^2 f & 18n/cm^2 r
993RS experience 15/13
930 21/13

pedal ratios were mentioned earlier these are pretty much fixed
thru '76 no boost it's 5.5
'77-89 boosted it's 5.2

So the mask is basically insignificant to any real world calculation, assuming the rotors are identical? What do you make of Stoptech's comments?

Is there a pedal box combo that yields 5.8x? I'm wondering where I got that number.

[Maxhouse97]

Wow love where this thread went. I used Hawk pads - can't remember which ones. I'm sure I didn't have the best braking technique either.