Spridget Rear Brake Rotor Question

The SCCA legal rear brake diameter for a Spridget is 9.12 or 231.6mm. That's the size of largest allowable brake rotor on the front.

So I found a brake rotor that has an outer diameter of 231.0 mm. The brake rotor I have from WC is just over 9" base on my tape measure. The Height is 49.6 which is just about the same size as my WC brake rotor. It measures 2". The rotor thickness is 12mm with a min. thickness of 10.mm. The Wilwood brake rotors that I have, has a spec of .380 in Rotor thickness which translates to 9.65mm. So would it be ok to turn the rotor down to 9.65mm since the car its coming off of has a curb weight of 1984. So if you add a 200lb driver, its 550lbs more than what my race car weighs with me in it. We're talking .35mm less than the min. thickness or 9/64ths smaller.

I can get these rotors for $15.95 each is the cheapest price I've found. In Autocad, I create my 4x4 lug pattern with 7/16 studs, then overlayed the 4x100 mm with the 12.5mm lug nut size. It looks just like my WC brake rotor. I thought about buying 1 just to see how it fits over the hub because I'm not sure what its like inside.

Darryl,

I think everyone should be hesitant about making a recommendation because to keep the rotor centered between caliper halves, you'd have to take material off of both sides and where the hat section meets the rotor disk, the radius might get thin. I'd think about taking all of the meat off of the back side of the rotor and shimming the caliper to center it so that the hat to disk radius is un-touched. Just a thought. Basil 707.762.0974 basiladams@yahoo.com.

Sorry, that's what I meant. If I cut if off the back, I can adjust the caliper to be centered on the rotor. Since I'm moving the rotor from the front of a heavy car to the rear of a lighter car by 500lbs would going just below the min. thickness requirement cause any problems? As long as I keep an eye on them for cracks.

Darryl,

Are calipers single piston? Dual piston? 4-piston?

Dick

The calipers are the Wilwood Dynalite Series Brake Caliper (part # 120-4062) 2 Calipers.

I was thinking about John's post on the Prod site where his rotor was cracked. My setup was for a GP Spridget and the rotors were just a touch bigger. I'd like to have some spare rotors.

Darryl,

With a 2-piston caliper and the modest difference in rotor thickness, I'd think keeping the rotor centered would not be critically important. The hydraulics won't know whether the two pistons are extended out equal amounts. And, given the fact that you're unlikely to let pad wear reach an extreme level, I wouldn't think the pistons would ever extend enough to where they'd come out of the caliper or come out far enough to expose a seal. So I think your concern about minimum rotor thickness is valid, but I don't think I'd be too concerned with having the rotor perfectly centered.

Dick

Basil,

Certainly more than one way to skin a cat, and I surely don't see a down side to the shims. OTOH, cutting a 12mm thick rotor down to 9.65 only requires taking off 2.35mm, which, if my aged brain cells can still do math... :S ... is 0.092, which means, if you took off the entire amount from the back side of the rotor, there'd only be .046 difference front side to back side, and I don't think that'd be too problematic on pads that are, probably, almost ten times as thick.

What'cha think?

Dick

Only time I'm worried about centering is if I put them on my WC setup. If I make my own backing/mounting plate then centering is not an issue. The main thing I'm concerned about is cutting the rotor back to where the thickness goes below the min. thickness requirement.

Rotor thickness is only a function of mass vs brake loading = more is better, and caliper geometry. Since you are using a different caliper, that's out. Using car mass as a determinant is worthless, since you are using it for much harder use in racing, or maybe much lighter use if it is a front disc going on the back. But, if it's worth going to all this trouble for "better" brakes, keep the rotor as thick as possible. You may be able to space the caliper itself for thicker rotor, or use a rotor at 10mm, or at 11.2, or whatever, or mod the calipers or pads. If you look around, you should be able to find the hub specs for the car the rotor fits, thus giving you the dimensions you need without buying one. Lots of specs re hubs and bearings - frequently integrated - out there.

FRM

Here's 2 different specs that I have. I'm not sure what Hug Reg is but I think its the hub face.

Bolt Pattern Diameter (mm): 100
Brake Rotor/Disc Bore Diameter (mm): 61
Brake Rotor/Disc Initial Thickness (mm):12.1
Brake Rotor/Disc Outside Diameter (mm): 231.2
Brake Rotor/Disc Overall Depth (mm): 49
Color / Finish: Grey Iron
Drilled: No
Lug Pattern: 4
Maximum Discard Thickness (mm): 10
Package Quantity: 1
Primary Bolt Pattern: 4x100
==================================================
Outer Dia: 231.0 mm
Height: 49.6 mm
Nom Thick: 12.0 mm
Min Thick: 10.0 mm
Hub Reg: 61.0 mm
Bolt Size: 12.5 mm
Bolt Circle: 4x100mm

So it is a Honda disc, Brembo 25095. "Hub reg(ister)" is the center bore and 61mm is Honda. And that means is is an outside mount disc. Problem you will have is the hat ID may be too small to fit over the axle end. I don't know the hat ID for Honda. Mazda is about the smallest that accommodates a 4x100 and it is too small to fit over a Spridget axle end.

I might be wrong on the app, but whatever it is, look up the front hub and see if you can find dimensions.

FRM

Sounds about right, have seen issue on backside of hats...some have a tapered bore too, where it starts ok, but doesn't fit over axle/hub od at bottom...

For 15-20 bucks worth buying one to look and see.

Bob L.

Thinking about this some more, without going into the cold dark night to measure parts...

The Spridget drum is located off the axle flange OD. That is the only determining dimension. So, the disc must also locate from that.

The Honda/Mazda etc disc locate off the center bore only. The ID of the hat is NOT a determinate dimension, but always provides some clearance to not fight with the center. My experience is with Mazda. So for example, the Mazda center is held at 55mm/=2.1654", The hub flange is 4.803", but the hat ID gives a small (indeterminate) clearance on this. Clearance varies by disc, and by manufacturer; rust may make it tight. Since as I said, the Mazda flange is about as small as you can get and still fit the studs on a 4x100mm pattern, you can be pretty certain that the Honda is that big or slightly more. I would not count on much more, since manufacturers are seriously scratching for space to fit all the parts on these assemblies.

You would need to machine an accurate recess inside the hat to fit the axle flange OD, or if possible arrange a center pilot, which you cannot do with OE Spridget shafts, but might with aftermarket ones.

FRM