HubDrum

[IslipMinor]

For a Minor with 2 people, full fuel tank and no luggage, the main table shows, for all the listed combinations of front and rear brakes, the resulting front/rear brake balance and the pedal pressure required for a 1g stop

This is what it shows. A comparison of front/rear brake balance and brake pedal pressure between all the front and rear brake combinations in the table.

The potential longer pedal stroke required with the smaller bore m/c is more or less offset by the smaller rear cylinders, the net effect is just 6% more stroke and a lighter pedal. If the earlier larger rear cylinders are retained, the result is a 17% longer stroke, an even lighter pedal, but very over braked rear, which is why BMC reduced the cylinder size to avoid rear lock-up.

Even with the 71/29% balance we have, the rears locked up under heavy braking, so have fitted a Mini rear pressure limiting valve (modified to be adjustable). Have to explain it to the MOT man, as the handbrake sails through the MOT and the rear foot brake is a bit marginal!!

[silloyd]

Interesting.

If I've read that correctly:

1. Given that I currently have 8" front and 7" (3/4" slaves) rears with a 13/16" M/C, I currently have a 74/26 split with a brake pressure of 119 for a 1g stop(?).
2. Switching to the Rileys (0.75" slaves) will give (also using a 13/16" M/C and 7" (3/4") rears) a 70/30 split with an equivalent brake pressure of 138.

I would end up with a reduced front/rear balance and 16% higher pedal pressure for the same braking effect(!?!?).

I can't refute the data; however, logic was suggesting to me that with 3/4" (0.75") cylinders front and rear the same pressure is being applied at both ends. With the wider shoes and larger diameter of the Rileys I would have expected the fronts to have been far more effective. But that's just my logic for you

Whilst I'm not exactly small and can easily find the extra leg effort I'm beginning to wonder if this is worth the effort or whether I should stick with the 8" drums for now, flog the Rileys and look for Wolseys(??). It begs the questions:

1. What is the optimum front/rear balance, does the AP boffin suggest a design target?
2. Are reduced risk of brake fade and lower wear rate (both as a consequence of much greater shoe width) the only positives of this 'upgrade'?

(Note I'm looking at general and moderate road use only and not racing)

BTW do you have some sort of calculator for this?

[bmcecosse]

The real point is - that your Riley brakes will keep on stopping you long after the standard brakes have faded away to jelly..... There is heresay that the R brakes are a bit 'heavy' - but I for one will be interested to hear how you find them..

[IslipMinor]

Yes the AP man does state the optimum, based on the legislative requirement for new cars from the late 80's, which is 80% front and 20% rear, but most modern cars have a load sensing proportioning valve that maintains the appropriate balance. He adds a caveat that this is likely to overbrake the front at lower speeds on older cars without the valve, so it seems that 70/75% front is more likely for a fixed proportion system.

The Riley 3/4" front cylinders are just too small to get the pressure on to the shoe linings without high line, and thus pedal pressures. The large wide shoes will be more resistant to fade, but from personal experience with a friend's Riley 1.5 whilst at university, not immune!! At that time most medium sized cars had pretty high pedal pressures, and like Roy I also had the joys of DS11 pads and VG95 rear linings to contend with - mine were on a Sunbeam Rapier, with no servo, but also no fade.

The calculations all come from AP and I have built them into an Excel model, so that by simply changing parameters, the results for any brake combination are obtainable. The data in it is for a Minor, with the brake options listed in the table. You are very welcome to a copy of it, along with the original article from AP. If you would like either/both please let me have your email address.

[silloyd]

Thanks IslipMinor, PM sent.

Now if only I could find some slightly larger diameter slave cylinders that would fit reasonably easily...

[silloyd]

Well, finally got around to fitting them and the new master cylinder and the end result is good.

I can't say that the overall pressure on the pedal under normal braking is massively different but it is a tad harder when doing the emergency stop. I have more pedal play but I suspect that I need to tweak the adjusters on the front, now that the pads have bedded in a bit; never gets near the floor though.

The replacement M/C has also eliminated the slowly sinking pedal (separate post). I haven't stripped the old one to check for scoring but given that it had had a new set of seals only about 18 months ago and it had never been right since I'm not sorry to have changed such a critical component...even if it is a really frustrating job.

I have never used W brakes so I can't compare but I would say that this has been a worthwhile upgrade.

Observations for the record:

• Both hubs came off the stub axles easily and without separating the inner bearings, one by pulling only and the other with the aid of a large screwdriver - some kind soul had applied copper grease on the shafts and on the back plates to stop them rusting together;
• The oil seals were in good nick and not particularly damaged but changed them any way as preventative medicine as they are relatively cheap and it's a pain stripping it all down in future, bearings were all fine;
• The Morris flexi pipes are only just long enough as they attach to the slave cylinders at right angles rather than the slightly upwards incline of the Morris slaves;
• I found RILEY RM 1 I/2 1952/54 FRONT WHEEL BRAKE CYLINDER and RILEY 2 1/2,1952/53 FRONT BRAKE WHEEL CYLINDER that look incredibly similar and have 7/8" and 1" bores respectively (no part numbers sorry). Which suggests that there may be other slave cylinders for the Rileys that are direct replacements, or at least relatively easily adapted, that have larger pistons (than the standard 3/4") and may improve the front/rear balance and pedal pressure. I'm afraid that I'm not prepared to spend £120 on a set to find out though!
• The new (Lockheed) M/C was much wider than the old and a b***h to fit as a consequence. With hindsight I wished that I'd measured the old one and ground a bit off the side of the new one to match first. Still, with such a snug fit it ain't going anywhere! Should have listened to advice on the forum!!

[bmcecosse]

Well done - and thanks for coming back to us. Referring to the example noted above (and reading it again with a clear head this time) I can't for the life of me see how fitting larger brakes to the front and retaining the same rear brakes and master cylinder, can possibly reduce the front rear balance - ie the suggestion is that this would give LESS braking at the front and more braking at the rear (all supposedly for a 1g stop - which the Minor could never achieve in the original road tests). The 9" Riley front brakes have MUCH more friction area - hard to see how this would reduce the effectiveness of the front brakes compared to the original skinny/smaller diameter 8" brakes. Just being larger diameter in itself gives the brake shoes greater 'leverage' against the spinning drum surface! I rather suspect this may have been left out of the original calculations.......

[IslipMinor]

The 9" Riley front brakes have MUCH more friction area

Yes, true, but MUCh less cylinder area, therefore MUCH less pressure on the shoes. The net effect is that for the same pedal pressure the braking effect with the Riley setup is the same as the 7" drums and less than the 8", so reduced front/rear balance. All the factors of lining area, drum diameter, CoF, 2-leading shoe and a lot more are taken into account in the AP calculations.

Retaining the standard Minor 3/4" rear cylinders, but with 7/8" front cylinders and 9" Riley front brakes, the pedal pressure comes down to virtually the same as the Minor 8", with similar brake balance, and with 1" cylinders the pedal pressure reduces to 95lb and the front/ rear balance goes up to 80/20%, which the AP man states is too front biased for a brake system with no rear brake load sensing valve.

The bigger brakes retain the advantage of being less prone to fade in normal use.

Roy, you have the full details of the calculations, so can check this for yourself.

[bmcecosse]

I'm too busy at the moment Richard to study it in detail, but surely in that example the R brakes have 3/4" cylinders - as do the Minor 8" ??

[IslipMinor]

Roy,

The Minor wheel cylinders are 15/16" diameter, against the Riley's 3/4" - the difference is 56% more area in the Minor cylinder. thus 56% more pressure on the shoe linings for the same pedal pressure and the same master cylinder diameter.
That is only one factor, as the drum diameter, lining area etc. are all part of the total picture, but rather like engine cc, there is no substitute for bore size!

[bmcecosse]

The chart for some reason doesn't give the standard Minor cylinder diameter - although obviously hints at it with the Wolseley option - my own personal choice ! The proof of the pudding of course is in the eating - and I have found that the difference between sets of shoes is very considerable.

[IslipMinor]

Roy,

Thanks, updated chart with standard Minor front wheel cylinder diameter added.
[frame][/frame]

[Declan_Burns]

Richard,
Do you have the values for the Ford disc brake set-up as a matter of interest?

[IslipMinor]

Declan,

I don't have the dimensional data, but if you can give me the piston diameter, number of pistons, distance of the piston centreline from the stub axle centreline and the pad area, I can work it out!

[Declan_Burns]

Anybody have this info? I don't really want to take everything apart at the moment. It was just as a matter of interest.