Fluid dynamics - changing wheel cylinder bore size

[California Dreamin]

Well, it's all done with new pads, shoe linings and the new smaller wheel cylinders and there is no noticeable difference in the braking. I just have to get all the friction surfaces bedded in then I can try a couple of emergency stops. Thanks for all the thoughts on this.

As the rears are only likely to contribute around 35% of the total braking effort the 13% reduction in force applied to the rears is a 'relative' figure as 13% of approximately a third just under 5% overall.

However the 'ratio' in braking effort between front and rear will now be reduced .......might only be a 70:30 split.

A lot of commercial vehicles fit brake compensators to increase rear brake effort as load is added to the vans payload.....the difference in weight of an unladen van compared to a fully laden van can be dramatic, so to prevent the rear brakes locking up all the time the compensator increases the rear braking effort as weight is added over the rear wheels....a very good system.

LT's use this system and have a lever and spring attached to the back axle which pulls tighter as load is added, letting more pressure through the compensator to the rear brakes.

Martin

[Oldiebut goodie]

Yep - there are various systems under there including the load compensating one, that's the only one my MoT guy had a good look at when he was under there although the payload is pretty constant in a camper except for fuel and water compared to a freight carrying van. The valves that permit the rear shoe brakes to make contact before the front discs may also help to mitigate any losses in pressure. It really needs testing on the dyno now to check that it is satisfactory which is what my last step is now before I attempt the Scottish mountains again .

[blacky]

Hi OBG. For what its worth when the mk1 golfs were about there were 2 sizes of rear wheel cyl with 2mm difference in bore. I have interchanged them in pairs as have other garages with no discernable effects in driving or on rolling road.This was the same with fiesta's also, they had 2 dia's. It's like tyre speed ratings some are for 120mph max, some for 140 mph max, What the blithering heck. Don't know if anyone remembers but early mini coopers had a brake intensifier under o/s front wing consisting of a stepped cylinder. On another note brother is a builder and years ago had a renault 4 van with load proportioning valve in rear brake circuit, it went haywire and was repiped without it in circuit until another could be sourced,needless to say one never was and in following period till van was scrapped it travelled with stones,cement mixers and allsorts of crap or sometimes totally empty. Neither way felt different to the other. V.W. passats used to have a heavy balance weight rubber encased moulded to o/s driveshaft which would seperate from shaft and slide down shaft making a racket. the budget repair was cut it off with grinder. It never gave another problem and you couldn't tell difference. What I'm trying to say is maybe we over analyse.

[tobydog]

Just an observation, strange how the cylinder bores equate to imperial dimensions. Do they have an American or British heritage?

[Oldiebut goodie]

Yes - the bores are originally quoted in 1/16ths - I think that the Yanks still use imperial for them.

[AdrianC]

On another note brother is a builder and years ago had a renault 4 van with load proportioning valve in rear brake circuit, it went haywire and was repiped without it in circuit until another could be sourced,needless to say one never was and in following period till van was scrapped it travelled with stones,cement mixers and allsorts of crap or sometimes totally empty. Neither way felt different to the other.

I used to have a Citroen Mehari - tupperware 2cv jeep - which'd been swapped onto a later chassis from under a 2cv car. Originally, the Mehari would have had a load proportioning valve. The car didn't.

One day, I was approaching a roundabout in the peeing wet a bit sharpish, braked a bit hard... and the back locked up. Not the front. The back. In the wet. Turning. The back just stepped right around. Have you ever sat under a frame tent, in a cat litter tray, and stared out of an almost opaque side window at the 38 tonner you're about to reverse into? I have. It's not fun. Somehow, by luck, I missed everything.