Steering, Suspension, & Brakes

Turns out that wasn't a regular seller on the caliper and he only has the one side so the cost goes up a bit. Maybe I should look a little more closely at the Toyota calipers as they may be a better match on the rotor thickness anyway. I recall you said they are aluminum but a couple pounds heavier?

JB

OK after some review maybe not. Does the Miata use aluminum calipers? Those Lexus calipers were aluminum weren't they? Best not forget the early GM hubs either, two spacings (drum and disc) and fairly light. Not aluminum though, I have sent an inquiry to Bill on his hubs and will report back.

I think the speedway rotors and hats are a good option if the 3" backspacing will work. Soon as the car goes back on the lift I'll look into that. A lightweight (alloy) 4 pot caliper with a 7/8" rotor slot (for .81 rotor) and pinned rectangular pads (removable without unbolting the caliper) would be optimal and we're close with the RX7 caliper. That may be as close as we can get, but maybe not. I think we still have a few options to look into.

JB

RX7 calipers are most common, but also 300Z and Mitsubishi 3000(also Dodge Stealth) used aluminum 4 pot calipers. The Lexus one is huge. I think they all use a 30mm thick rotor(1 1/4").They are all rarer to find used and more expensive new. The 300Z used 3.5" mount spacing, Mitsubishi used wide pattern like Mazda. You already know about the aluminum twin piston slider Camaro and Corvette. Newer Mustang also had a similar design. Miata is single piston slider. I think The RX7 is your best bet. The picture on the 4 lug thread shows the earlier one on a rotor/hat setup. The newer one looks a little more modern, but uses the same rebuild kit and pads. The one in the picture is spaced out for a 1.25" rotor. I don't see any problem using shims for .81 rotors.RX7 stock used 22mm(.88)rotor.Maybe they were thinking about going to 24mm and thats why the wider slot. The wider ears on the calipers actually make it easier to make an adapter bracket.Check out rockauto.com for specs on rotors and calipers. I'm not as weight conscience as you. I think the thicker heavier rotors are best for people who use their cars as every day drivers. The Toyota calipers are iron and the larger ones weigh 9-10 lbs but have excellent pads and 4 large pistons for clamping the rotor. They can also use WRX and 300Z performance pads by grinding a tab off. The small Toyota weighs about the same as stock MGB. Lighter is better for racing but I'm more concerned with stopping.



Edited 5 time(s). Last edit at 02/02/2011 04:22PM by mgb260.

For those obsessed with aluminum,( count me in), said RX 7's have a very nice, forged alum. lca. Mid 80's "Supra" has a beefy, alum., uca. I suspect, both will "fudge" into Mustang II spindle.This makes front/rear steering , manual/power racks-doable. Cheers, roverman.

Art, I have a picture of that setup somewhere! Might be on other computer.Found it! It is a Datsun 510 with Nissan V6. It uses the RX7 upright though.




Edited 1 time(s). Last edit at 02/02/2011 09:07PM by mgb260.

Relative piston size is not an intuitive thing and it makes a lot more difference than you'd think whether you have 1, 2, 4 or 6 pistons. You can't just add them up because hydraulics is based on surface area of the piston, so you'll come out with square inches or square centimeters. I'm more familiar with doing these calculations in English so I start by dividing the piston diameter by 25.4 but it's not necessary. The formula is "Pie are square" (cobbler) The stock 54mm pistons then, 54 / 2 = 27 x 27{squaring} x 3.14{pi} = 2289 sq mm X 2 pistons = 4578 sq mm or 45.78 sq cm.

Here's why I like working in inches. 54 / 25.4 = 2.126... / 2 = 1.06... x 1.06 = 1.13...x 3.14 = 3.55... x 2 pistons = 7.1 sq in. You can almost estimate it in your head. To me they are just easier numbers to work with and to visualize. But there is an extra operation.

So the twin piston 54mm caliper has about 46 sq cm of area. Working backwards for a 4 pot caliper we get a 38.18 mm piston, nearly the same size as the RX7 caliper, only .18 mm or about .007" smaller. So the stock master cylinder and rear brakes should work fine with this one. If anything the pedal may be a smidgen firmer though in reality you'll never even feel it.

What about 1 piston calipers? Those used to be sized in inches and it would take a 3" piston to match the stock area or about 76mm. That's fairly big. My Mach 1 Mustang brakes had a 2.38" piston or 28.7 sq cm which explains why the balance bar had to be adjusted almost all the way to the front to get balanced braking. (don't forget when you are converting area the conversion factor has to go through twice)

You can calculate the increase you need in the MC and rear slave sizes too, although converting from drum to disc is not as straightforward because drum brakes are self-actuating among other things. Still, a 10% size increase at one end, coupled with a 10% increase at the other and a 10% increase at the MC will work out just fine. Where you get into trouble is when the percentages quit matching, you go to rear disc, or something along that line.

My next task it to determine the piston size of the Jaguar XJ6 rear brake caliper and the piston size of the XJ6 front brake caliper piston. With those dimensions in hand I can determine the appropriate piston surface area for the 4 pot front calipers, and the balance for that system in an MGB should not be much different than what is used in the stock XJ6. Any error can be easily taken out by the balance bar.

The 1-1/4" thick Speedway rotor is the same price as the .81" version and 8.7 lbs vs 8 lbs so not a real severe weight penalty. With the steel hat that's 10-11 lbs each. This opens a wider range of calipers as usable.

JB



Edited 1 time(s). Last edit at 02/03/2011 11:38AM by BlownMGB-V8.

Very scientific , Jim! I based my estimates on master cylinder size on what was used in the stock vehicles.RX7 used 15/16",small Toyota used 7/8" and large Toyota used 1". MGB stock 3/4". The Nissan Z guys like the 15/16" 79-81 ZX master with the small Toyota calipers,probably for a firmer pedal. They also go to 1" master when using the large Toyota calipers because excessive pedal travel. Jim, check out the 90-92 Nissan Z calipers for 1 1/4" rotor or Mitsubishi 3000 GT (Dodge Stealth) or Lexus LS400. Rockauto.com has caliper specs. Not as common but meant for thicker rotor.You can always use a spacer in the more common Mazda RX7 caliper.



Edited 3 time(s). Last edit at 02/04/2011 12:18AM by mgb260.

Jim, I did some research for you, all these calipers are meant for 30mm thick rotors except the Lexus LS400 which is 32mm. 91-93 Mitsubishi 3000 GT VR4 and Dodge Stealth Turbo have huge 2.25" pistons. The 95-2000 Lexus LS400 has 1.68" pistons, 90-92 Nissan 300ZX Turbo have the smallest pisons at 1.59".



Edited 2 time(s). Last edit at 02/04/2011 12:31PM by mgb260.

I got the piston sizes for the XJ6, 48mm front and 43 for the rear. Front is 4 pot, rear is 2. The area of the front pistons is 11.2 sq in (72.26 sq cm) and the rear is 4.5 sq in (29 sq cm). The area in metric in this case gives a near perfect snapshot of the front/rear proportionality at about 70/30 give or take a point or two. Weight distribution is identical to the MGB. Readily available big piston aftermarket calipers max out at about 1-3/4" or 44.5 mm which would shift the balance towards the rear a bit, 68/32. Use of the RX7 calipers would give 60/40 which I've seen quoted as generically ideal.

So there's the range. Anything between the RX7 caliper and the big piston aftermarket units is going to work, and any of those should be within the range of adjustment and use of my balance bar/dual 3/4" MC setup. (Roughly equivalent to a 1-1/16" single cylinder except in total volume as that is evenly split front to rear.) In all probability the smaller one will have to be balanced towards the front and the larger towards the rear but in either case probably not as drastically as it is now as both those proportions should be within a workable range. My gut tells me to go for the higher distribution to the front. Time to shop for calipers.

JB

picture of caliper spacers:

I may need those. I bought the Outlaw calipers off ebay with 1-3/4" pistons and set up for a 1" rotor. But 1" rotors are about like finding hen's teeth so I'll either have to use spacers or mill them down for .81" rotors.
[cgi.ebay.com]
But the pads might be expensive. Still have to get rotors too.

JB

Jim,You Outlaw you! I think some of those use the same pads as Wilwood. You could copy the pads for stainless shims and use the .81" rotors or make spacers easily for the 1.25". 3" hat would be good for caliper clearance but might make rotor close to tie rod end. There are smaller ones available or Heim joint. Remember to use red Locktite on hat to rotor bolts. Or 88-94 C4 Corvette Heavy Duty 13" rotors are 1.1" thick but have a shallow hat for the 2 piston slider calipers. They are left and right different because of curved vanes. Heavy though(20lbs).Here are specs:




Edited 2 time(s). Last edit at 02/04/2011 08:46PM by mgb260.

Yes I think they do use the Wilwood pads from what I've seen. There is a Sportsman class 1" rotor but it is just 10-1/2" diameter and not a bargain item, about $90 each plus the hats. This is why I got the calipers so cheap, $110 shipped for NOS. With good pads a 10-1/2" ventilated rotor is all the MG will ever need, still, if I can find a bargain on the rotors I may still be able to bring it in under $300 total which is in the ballpark of what we are shooting for with this thread and the other two. These calipers weigh around 3-1/2 lbs each plus pads so something in the neighborhood of 4-5 lbs ea.

For anyone using the stock rear brakes the RX7 caliper would be the way to go, or in an aftermarket 4 pot caliper a 1-1/2" piston size, either of which will have the same piston area as stock. For a disc brake rear axle I'd be looking for something with a 48 mm or 1.9" single piston caliper with built in e-brake or maybe a bit larger but not over 2.4" or 60 mm. There should be a stock caliper like that available.

At one time there was a Mitsubishi Eclipse (I think it was the '91 with the base engine) that used a nice rear caliper with an external allen wrench screw head to retract the piston but I only ever saw that caliper on one car. Very nice and super easy pad change. But the piston is too small so you'd need something from a larger car. For a somewhat smaller piston figure the percentage and go that much smaller on the front. The pedal would be firmer but more precise. NASCAR has gone to 1" pistons and high line pressures but go too far and you'll need a smaller master cylinder. The reason for using a single piston slider on the rear is that it allows the incorporation of the e-brake cable. Stay away from the type where you have to spin the piston back into the caliper. On those it's often easier to swap calipers than to screw the piston back in, even if you don't destroy the seals doing it, which is a definite risk. The reason for this is that the internal adjuster has nothing to keep it from spinning backwards so instead of screwing the threads back together everything just spins and even the smallest speck of dirt or rust will frustrate your efforts. I'd recommend a caliper if I could, I know there has to be a good one out there. It is inconceivable that the Mitsubishi design was only used in a limited application on one model, it really was very, very good. A pad change took all of about 5 minutes. Might be worth lunch with a Midas employee.

JB

Anybody ? "Wilwood" likes this process best, but NOT cross drilling ! The RX-7 rear caliper has built-in e-brake. Why not a" line loc", for those without,(including stealth, theft deturrent)? roverman.

Art,

A line lock is not a legal parking brake most places. I always used a block of wood on my Camaro. ;-)

I was at a u pull it yard today and there was an rx7 turbo 2, so i got all the calipers off. What would be the issue of using the caliper withouta spacer for that .07 inch difference with a .81 caliper? Especially if you change it before it wears down to nothing. I know the pads wear down much more than that amount. The RX7 has some really nice calipers, these are very light for their size.

Robbie, Spacers are easy, just copy the pads backplate. Stainless works good. .035 shim each side. You are right, as the pads wear the pistons can bind or pop out. What are you going to use for rear rotors? The RX7 rear rotor is thinner than the front,20mm (.79) instead of 22mm..81 rotor and hat also. I don't think you need a shim there. When you get it all figured out should work excellent. Keep us posted.



Edited 2 time(s). Last edit at 02/09/2011 11:30PM by mgb260.

Well, This will be a slow process as my car has working brakes. The only reason I got these now is I was there and they were available. I already have rear discs, which are gm but i dont like them, with 1-1/4 rotors. I want to go with 10-3/4 in the front. Will probably keep my rear setup unless i have too large of a proportioning problem. Right now, I have a 3/4 master, so im not sure if that will work or if i will have to go to a 7/8 or 15/16.WIilwood has a .72 hat mounted rotor but is too large, like 11.66 for my 15 inch rims. There are still a lot of things I will have to work out, but im not in a hurry.[wilwood.com]

.035" per side should be in the working range, it all depends on how much gap you have between the caliper and the rotor on each side, and the thickness of the backing plate. Obviously you don't want to wear the pads down to nothing but if you do, you don't want the side of the pad eating into the corner of the caliper, potentially ruining it. I think most backing plates run close to 3/16" thick so if the nominal running clearance isn't too much the extra .035 should be no problem. But the stainless shims make great heat shields (even better if you can get titanium).

I just scored a set of rotors and hats:
[cgi.ebay.com]
It'll take a little work to make them fit. But absolutely killer pieces, 13" x 1-1/4" ultra light ceramic coated Red Devil, probably steel alloy. Should come in around half the weight off a comparable rotor, I'll know more when they get here.

I'll have to redrill the hats, redrill my hubs, and either make or buy spacers for the calipers to widen them to the 1-1/4" size. Cost with shipping, about $118, a very good price for these parts. So I'm approaching $250 and with pads will be right close to $300 which is sort of the goal. And if it all works as planned I'll have the lightest and most effective brakes almost anyone could want, at a price almost anyone could afford. It helps that I can do some machine work myself, but hey, this is what we're shooting for here.

I've decided not to chase after aluminum hubs.... though you never know what might show up on ebay. Mine are very strong and well made, only needing re-drilled to go back on the car. So I'll be giving up a pound or two there. My target is a finished unsprung weight about the same as what I had before, and about the same as stock. Once the brakes are fitted and I know exactly what I want it'll be time to get in touch with Greg Smith at Weld and talk options. I may call Centerline too, it's hard for me to believe they don't have some sort of a plan in place to make 17" wheels.

JB

Jim,Super trick rotors for cheap! Looking forward to the "How to do it". I think you won't have a problem with the rears over powering the front. Even with the balance bar setup, if you have excessive pedal travel you may have to upsize your masters.