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MG Midget and Sprite Technical - 4 Pots

1971 midget fitted with KAD four pots, new master cylinder, braided flexs and green stuff pads, brembo disks.
Braking is not as good as I feel it should be with these calipers, should I change the pads or change the size of the master cylinder?
d brenchley

Send them to me, and I'll take a look.

A
Anthony Cutler

A, will you pay postage (estimate £500). LOL
d brenchley

Don't worry about posting - send your post-code and I'll collect...

A
Anthony Cutler

Anyway - back on topic??
d brenchley

Can you lock the front wheels?
Toby Anscombe

Toby, not easily - have bled the system several times, no servo but the "effort" to stop seems high (it passed the MOT by the way).
d brenchley

Have you adjusted the rears? If they are loose then all the pressure goes to the back which means a lack of front pressure....
Toby Anscombe

in a hydraulic pipe system with no check valves I fail to see how the pressure can be anything but the same throughout.
What calculations did you do before fitting the 4 pots, how did you size up the master cyl?
David Smith

I don't know what you were expecting, but you should be able to lock up the fronts with a really good stab of the pedal. I just installed green stuff pads and new brembo rotors on my car and the feel is great. I haven't really tried a hard stop yet but normal driving is much improved from before. Perhaps its the 4 pot calipers, but I would think those would improve the breaking not reduce it. There is the total piston area to consider though, it should be equal to or greater than the original calipers to be of any improvement. A smaller total area and you'll get less effective braking for sure.
Bill Young

If you have MORE slave piston area with the 4 pots than with with Spridget (or MGB) calipers the pedal should be longer and softer.

So, either you have LESS slave piston area (you'll need to measure the pistons to check) or some other fault.

I don't suppose you have the brake torque figure from the MOT? If not they might be able to find it from an electronic record.

Assuming you have less piston area and this has given you the harder pedal you'll either need to fit a servo or you might get away with using the late brake pedal (the hammer head) which is cranked such to provide more leverage than the earlier pedal.
Daniel Thirteen-Twelve

Pistons are 36mm and the pedal could be described as longer and softer. KAD recon you can get away using the original master. I've got a set of 1144(?)pads in the store somewhere so will try those at the weekend.
d brenchley

By the sound of it you need a larger bore M/C. This should solve your problem, but if in any doubt why not give Tim a call at Frontline and ask for his views. He also does a for pot, albeit from Hi-Spec (not a good product or a good customer service (personal experience)).
But Tim Knows his stuff and is worth talking to.
I have 6 pots and had to go up to a 1'' bore IIRC.
brian s

OK, getting the calculator out and assuming midget pistons are 51mm the area for both 2 and 4 pots works out very nearly the same, ?
So its either more air in there somewhere or....
d brenchley

Area is 3.14 x dia-squared... so the area of 2 and 4 pots can be very different... worth doing the arithmetic properly.

A
Anthony Cutler

3.142 x r squared

3.142 x 25.5 squared x 2 =40.86 (2 pot)
3.142 x 18 squared x 4 =40.72 (4 pot)

Unless my brain has finally collapsed ;-)
d brenchley

The rear adjustment as Toby says is a relevant factor. Pressure is equal throughout the system (assuming no pressure limiting valve on the rear) but if the rears are adjusted too loose, then all of the fluid movement in the system goes into applying the rears so that all the pedal travel is used up before pressure on the front disks can be fully applied. Pressure throughout the system increases once the movement of the rear shoes has been is completed, and only then is full pressure available to the front disc brakes.

That is why rear disks are an effective improvement - not because one needs greater braking power on the rear but because rear discs "absorb" very little of the fluid movement unlike conventional rear brake shoes.

Maybe the pads just need bedding in a bit more?
Guy Weller

I would agree with Guy - 200 fast road miles minimum before making any decision. fist 50 miles gentle braking , then gradually increase. I did about 50 road miles bedding in the other day and they felt ok until i got them on the track. they were not anywhere near ready!
d cusworth

Danny here is a braking specialists' bedding in procedure:

"Mintex M1144, M1155, M1166 and M1177:

1. Clean discs with brake cleaner or other approved solvent. Fit your new Mintex C-Tech Racing Pads
2. Apply 2-3 light applications of the brakes at 30mph down to 0mph
3. Apply 3-4 steady applications of the brakes at 70mph down to 30mph
4. Leave as long as possible to cool down. They will now work from cold. Do not left foot brake"

Sounds like you glazed yours before you hit the circuit..


David Smith

I also have the same problem with the KAD 4 pot calipers (using the later 1500 M/C) it passed the MOT but unable to lock them and pedal is soft.

Brian what M/C did you fit for the 1" bore and pedal box?
K Harris

So, without any real increase in piston surface area to gain more mechanical advantage why should you expect the 4 pots to work that much better than the two pots other than maybe the larger surface are of the pads themselves. The pressure on them is still the same. I was under the impression that you wanted to gain piston surface area in order to multiply the force per square centimeter and thus increase the overall force pressing the pads against the discs. That requires a trade off, increased volume in the calipers results in a longer pedal travel. If you exceed the pedal travel then you have to move up to a larger master cylinder which then reduces the piston area difference and effectivly reduces pressure on the pads. It's a pretty carefully engineered system to get everything right.
Bill Young

The advantage of 4 pot calipers instead of 2 pot calipers is that you get a more even application of the pad on the disc.

With the KAD calipers you also have a very useful weight saving which is an unsprung saving.

The input pressure first comes from the right foot.

It could simply be pads don't have a lot of grab and bite.
Daniel Thirteen-Twelve

This thread was discussed between 11/05/2009 and 12/05/2009

MG Midget and Sprite Technical index

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