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MG Midget and Sprite Technical - 1.5' (front Mini) brake shoes

This *might* interest some, now or in the future.

I fitted the 1.5" (front Mini) brake shoes to my '73 midget rear drums (and replaced the drums (not Mini items!)) instead of the standard Spridget 1.25" rear shoes to even out a bit the improved braking I get from Mintex 1144 pads at the front.

When the 1.5" shoes turned up I found the linings were deeper to match the extra shoe depth but not as long, that is the linings didn't go as far round the arc of the shoes (see yellow lines in photo).

I checked and these were the correct shoes - BUT - 1.5" shoes seem to be available with the linings going further round the arc as with the 1.25" shoes.

On a quick rough measure I found the 1.5" shoes only had a small increase difference in lining surface area compared with the 1.25" shoes but of course a wider band of contact to the drums.

I'm not able to say if the now improvement in braking is down to the new shoes and new drums, or theses plus the extra shoe width, but on three braking tests (on a suitable and empty road) the braking seemed more even front to rear than before.

What difference the missing lengths of the wider brake linings would have made I don

Nigel Atkins

I imagine one advantage of having wider drums, but with (roughly) the same lining area, is that the drum surface area is much greater so the cooling would be better?
J Smith

You need to bear in mind that generally the force due to friction is independent of area so increasing the area of the friction material on the shoes won't increase their braking force.
David Billington

'bear in mind that generally the force due to friction is independent of area'

Counter-intuitive, but true!

The same force is applied to the 1.5" wide shoes, so less per sq. in., so less heat build-up, but the braking effort is the same.

Mintex 1144 pads have quite a high 'u' value (coefficient of friction), so can put more braking force into the front brakes, which can also have the effect of unloading the rears.

We have Mintex 1155 pads in the Metro 4-pot calipers (virtually identical total piston area to the standard Midget calipers), and we need to have a pressure limiting valve in the rear brake line to prevent rear brake lock-up under heavy braking.

With the same rear drum diameter and brake cylinder bore size, the brake lining material is likely to influence the braking force more than anything else.

Richard
Richard Wale

Interesting stuff, thanks

Nigel when I fitted the 1.5" shoes to Lara it was in order to get the benefit (claimed) of extra area to aid my handbrake to get a grip

The car regularly gets parked on a 15 slope at home and any extra help the ridiculously inefficient handbrake can get is worthwhile

That said the braking was never an issue, step on the pedal and very effective retarding happened

For the handbrake I ought to undertake Paul Walbran's little but effective changes, maybe after Le Mans this year and when I fit my wavy washers to the slave cylinder


(note to self: order said Belville washer set)
Bill sdgpM

Thank you all, some interesting posts.

Yes I can follow this, similar but not the same as what I say about wider tyres, wider shoes may have less lining but it is of a more friction value. The 1.25" didn't have anything printed on them and I can't remember what was printed on the 1.5" shoes so when I get a chance I'll try looking the Mintex 1.5" shoes details.

The front to rear braking is more even now and overall the braking is good (good tyres help of course) so I'm satisfied.
Nigel Atkins

Bill,
I don't know what the differences are between the 1275 handbrake and the earlier models but my 1275 handbrake is good, it was certainly better than my wife's brand new Vauxhall handbrake and remained so.

The handbrake on my Midget had started to go off a bit so when I was looking at the rear brakes at the start of the job (last November!) I saw the clevis pins were scored so I replaced them and their washers and this and general cleaning and lubricating brought the handbrake back to where it had been.

I did wonder about replacing the balance lever (for the rods) as I never seem to get any grease in it when I grease the handbrake but as the handbrake works well and I'm totally fed up with seeing the underside of the rear of the car I've not bothered.
Nigel Atkins

With you on not wanting to dash under mine for quite a while, yetawhile ;)

I have done all the refitting with close fitting clevis pins bit, adjusting up nicely but without the essential 'bite on a slope'

Paul sent the drawing of the minor mods he dose to increase the overall efficiency and they are on hold but definitely in the plan for the future
Bill sdgpM

In that case in the meantime can I suggest the only good use of a computer tablet and/or "smart" phone, to wedge them under a rear tyre and turn your front wheels appropriately towards the kerb, leave the car in an appropriate gear - but by far the most important part is the placement of the computer tablet and/or "smart" phone.
Nigel Atkins

Nigel,

What do you say about wider tyres as I must have missed that in the past. Re tyres they are more complicated than straight friction as there is also a mechanical component to the grip as well due to the tyre conforming to surface irregularities in the road surface.
David Billington

David,
just a general similarity about wider tyres not necessarily offering greater grip, I usual put that tyres are very complex components (but that's not including the interaction with the road surface).

What has baffled me is that in the thread title I definitely typed 1.5 inches yet it shows as 1.5 feet.
Nigel Atkins

Bill,
Fit the 1500 handbrake cable system instead like I did to mine. Much much better. That'll hold you on "the slope"
Geoff Mears

This thread was discussed between 23/05/2018 and 26/05/2018

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