Multi Piston disc brake Calipers

[oldfart2]

Good day all,
I have just joined your group and have a question or point I would like clarified.

In multi piston, ie. 4 or 6 spot disc brake calipers, is there a ratio or formulae for the difference in piston diameters that is prevlent in some of the OEM and aftermarket sets available.
I have posted this in 2 other forums, with no response yet.

Thanks in Advance

Paul
Brisbane
OZ

 

[c2s]

Usually the piston diameters and number of pistons are a function of packaging and master cylinder sizing (for the correct hydro/mech ratio).
Kevin

 

[oldfart2]

Thanks Kevin,
I am sort of fishing for something I don't think really exists.
I am aware of the master cyl dia relationship, but in our case master cyl diameters is not an issue for this application, we can change both of them as the need suits.
We want to replace some heavy castiron 2 spot calipers with some aluminium 4 or 6 spots (semi race application) units. All the overpriced commercial calipers seem to have piston diameters that suit stock bar sizes rather than a specific ratio or formula.. ie. 1 3/4", 1 1/4" & 1" or 1 1/2", 1 1/8" & 7/8" and a few metric size of similar value.
These are just a sample of the assemblies we have considered.
The suppliers all qoute "special" design parameters for their piston sizing, but I beg to differ.
I/we ( I am part of a small design group) now want to machine our own caliper bodies to suit commercially avail pads, a new lightweight rotor assy as well.
This is what prompted my initial posting.

Thanks Again
Paul H
Brisbane OZ

 

[patprimmer]

Only a guess as this is not my field, and you are not getting many replies.

I think the piston sizing on the centre piston will be the maximum size that can fit in the avaliable space with consideration to maximum rotor size within the practical avaliable rim sizes.

on a 4 spot design, I think the 4 pistons should be the same size, and if they are not, I agree it is to get the required area from stock size raw materials. Not a good enough reason in my unqualified opinion.

On a 6 piston design, the smaller leading and trailing pistons will be about pad and caliper flex. I would like to see a finite element analysis of the structure before deciding on final dimensions.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[oldfart2]

Thanks for the responses, from a bit of further research it seem the smaller leading end pistons are to stop the pads wearing unevenly (on a taper, front to rear), this is is a problem, particularly on the longer, narrow style pads used on performance brake systems..ie. large diameter, narrow friction areas.
I guess I will research a little more, if no obvious corelation or ratio shows up, then I'll probably follow the current experts and go with OEM/non genuine spare part piston & seal kits..and fit these into my design housings to suit an "off the shelf" type pad that fit my design hat & floating rotor.

Thanks again,
Paul H

 

[JakubMech]

I think this may be to do with the pad area being taperd towards one end. Im a bit hazy about this but iv'e seen this in a few texts ( I think i might have seen it in a carrol smith book). It may be trying to achieve the same effect with a non tappered pad (even wear?).

Jakub

 

[spdingtkts]

Alot of the newer stuff has gone to individual pads for each piston.

This would prevent the backing plates to bend. This would also produce more even brake pad wear.

 

[bigtomwcp]

the sizing of the master cylinder and the piston is pretty simple. a massive master cylinder with massive pistons will stop a vehicle. but it will require much more pedal pressure. you want to design something comfortable for the driver and balanced at all four wheels and for the right weight of the vehicle. the length of the pedal is important. as you may have realized it is a package deal.

i would say

1 find the limitation of pedal length
2 select a caliper resonably sized for the application
3 select a mastercylinder to provide the appropriate pedal pressure and throw to operate the system