MG Midget Forum

I did the usual search and found many threads of single versus dual and of course they are in reference to the Midget/Sprite.

As some of you know, my next project car will not be a Midget/Sprite, but will use front Midget kingpin/spindles, rotors and calipers along with the widened rear end housing with stock drum brakes.

I've read and posed the question of how well the stock brakes work and many here have said they work very well when in good condition with correct adjustments made. So rather than reinventing the system, I think I'll just use all stock, but new parts.

So that brings me to the master cylinder. As said above, I'll be using the stock front 8.25" rotor with new Midget calipers with the drums in the rear.

I've read here that the stock bore was 7/8" and that 3/4" is recommended. At least that's what I remember reading this past weekend.

My master cylinder does NOT have to "fit" in any MG pedal box as I'll more than likely use a "Wilwood" type floor mounted pedal box. It may be a dual brake cylinder with a bias bar application, but again, I need to start with the appropriate bore master cylinders.

My guess is that I will probably not need much bias front/rear as the original master cylinder for this brake setup on the Midget/Sprite was a single circuit system. Even with the same pressure, the disc front probably apply more braking force than the small drums on the rear which in function provides their own bias of sorts.

So what bore should I be looking for?

Or is there a dual circuit single master cylinder that everyone has been converting over to? I see some of the earlier Datsun dual circuit master cylinders are quite compact and use remote reservoirs, which will be required in my application.

Thanks again in advance for the help and I'm sure some great suggestions...

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Dan Lockwood
O'Fallon MO

Perhaps you should do some rereading. Gerard is the forum expert of disk brake conversions. Here is his page: http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm

7/8" is the stock master bore diameter for drum brake spridgets.
3/4" is the stock master bore diameter for disk brake spridgets.

However, the above dimensions only apply to a stock master and pedal box setup. Once you start playing with non-stock parts, pedal locations, stroke length, etc, all bets are off regarding what the bore diameter should be. I'm no expert, but it seems intuitively obvious that careful design is needed to ensure that the pedal stroke length corresponds with the proper amount of fluid displacement for both front and rear brakes.

Dave, thanks.

I do understand that the pedal lengths and ratios do need to be calculated as to the correct stroke for proper fluid displacement.

The term "disc brake conversion" has a suggestion to a car guy like myself, but not experienced in Spridget talk, to mean all wheel disc brakes. Gerald uses the term as well when referring to his 7/8" to 3/4" resizing. I am of the belief though that in early Spridget talk it means only updating the front wheels with disc brakes and leaving the rear as drums. Is this correct?

With all things remaining the same, a smaller bore diameter will increase the pressure to the slave cylinder pistons, but will require a longer stroke to provide the same amount of fluid. As long as the pads/shoes are in good condition and the rears are adjusted properly, bore is more important than available stroke.

Like I said, my application will NOT be a normal Spridget setup other than I'll have the same wheel brake components.

I want to keep it simple and low cost if possible. My thoughts were that there "may" be a dual circuit master cylinder that is upgraded to in the Spridget application and it may be adaptable to my application.

Thanks again for your thoughts and information. Gerald's information was a good read, thanks.

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Dan Lockwood
O'Fallon MO

Given the uncertainty regarding what "early Spridget talk" means, I would say that in general you are correct. I know nothing about converting rears...seems like a lot of hassle for minimal gains.

I was all set to do larger front rotors and upgrade the rear end housing I have to disc, but quite a few guys suggested just making it as new and that should be good. My car application will probably end up coming in under the weight of a stock Spridget and it's only going to have 4.00x19" rubber on all four corners, so the brakes will very likely overpower the skinny tires in stock configuration.

Keeping everything stock is a bunch cheaper than upgrades too. Although I will do the slotted and drilled rotors, if nothing more than for the looks; can't hurt and I need new rotors anyway.

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Dan Lockwood
O'Fallon MO

You are correct in saying that the talk about upgrading to disc brakes is only referring to the fronts. The Bug Eye had drum's all around and unimpressive brake performance so switching to front discs from a later car is common. The disc brake cars had a single circuit 3/4 master. Soooo, never having any dual piston brake masters, does one go with two 3/4 pistons or two pistons that amount to about the same area as as one 3/4 inch piston??? I would think the latter.

Kurt

I have dual master cylinders with a bias bar on the racecar. I built the box with the same pedal ratio as stock (my notes say ~6.5:1).

The front master cylinder is 5/8" diameter and the rear is 3/4".

Isn't the master in the 1500 dual.

I'm curious....is the bias bar 50/50?

Kurt.

Close. The bar is a little to the front but that is more for the sake of rear brake life than correct balance.

John, good morning.

I'm sorry I didn't pay any attention at the time to your name. But I see now that we share the same last name it's not a very common name.

I did the Nat Geo DNA testing a few years ago and signed up for the DNA Tree program. Every week or two I get a spreadsheet with DNA relative, some with Lockwood as the last name and plenty more with other sir names.

It's always nice to see another Lockwood from time to time.

Have a great day...

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Dan Lockwood
O'Fallon MO

Andrew, good morning.

Wilwood had a couple pedal boxes in the 6.25:1 range for the ratio. This would quite close to an original setup of the Spridgets from your notes.

I see from your avatar that your little red bomb is a racer, or at least one of them are. I would guess the bias bar setup is for the one you race. In that car are you running the stock brake setup at the wheels, 8.25" disc front and drum rear?

If that's the case, a 5/8" front and 3/4" rear may be a good starting point for my project car.

I've gone back and forth on the dual master cylinders versus a dual circuit master cylinder. If I went with the dual circuit it would be a 3/4" and I would then just use Wilwood's adjustable proportioning valve setup. In a previous life, I had Wilwood 4-piston polished calipers on all four corners with 12" rotors up front and 10.5" on the rear. I had shoe expanding park brakes inside the rear rotor top hat. I ran this without any boost. I also had very heavy front weight bias on the car and ran 185/70-15" fronts with 6" rims and 285/60-15" rears with 10" rims. I had to do quite a bit of tweaking to get the brakes to work, but a nice gravel road is your friend on setting the brake proportioning valve. During a testing session with Rod & Custom magazine I stopped 60-0 in 161'. Others in the group were in the 130' range, but for a 3280# car with most of the weight on skinny front tires, not too shabby...

My project car will only be running 4.00x19" tires so I'm quite sure the stock Spridget brakes will be more than adequate for my needs.

I really do appreciate all of the input on this site. You have all been a BIG help and very friendly to a non-British car owner. At least I can say that I owned a '62 Sprite back in '68. Bought it for $425.

Thanks again guys!

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Dan Lockwood
O'Fallon MO

Wow, Dan. How big of a V8 do you have to put in a Sprite to make it weigh 3280#!?

Kurt.

Kurt, something like this...

But not a MG, a '40 Willys coupe.







Dressed as it is in the first picture, it weighed in at around 850#. Then add trans etc and the weight just keeps going up. I should have weighed the front wheels and then the back when I was on the scales. With half a tank of gas it weighed 3280#. And like I said before, it was probably 65% front weight.

That's about the only car of the 65 I've owned in my life that I have seller's remorse really bad...

Or you could do something like this.

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Dan Lockwood
O'Fallon MO

Oh my!!