Front brake calliper upgrade

[Scud]

I'm no expert, but I think, yes, you should expect the small ones to move first. And if memory serves me, when rebuilding the calipers, the small ones usually pop out before the large ones. I read somewhere (can't recall the source) that the small piston on the trailing edge was intentional as you described, with the addition idea that if it were reversed, the leading edge of the pad could bind. I don't know if that is theoretical, or a real concern, but that's what I recall. Anyway, it seems like Brembo was aware of something with the 34/34 calipers as they used one pad per piston instead of one pad per pair of 34/30s. Maybe someone more knowledgeable will be along to explain... or maybe we can get @Chuck to look in his mechanical engineering books...

[Lucky Phil]

6 minutes ago, docc said:

Alright, so . . . when cleaning and "equalizing" the piston movement during brake service (on the original V11 caliper pistons), the smaller pistons, closest to the brake line input, should be expected to move first (if I understand this technology, now).

And that, this not so much to put "more" pressure on the trailing side of the pad, but to even out the pad wear and contact over this rather long pad (of the original V11 single pad Brembo) due to the "quite different thermal and therefor frictional conditions" between the leading and trailing edges of the pad.

Yes?

 

In "theory" docc the leading small pistons will move first due to smaller piston size and probably less seal friction due to smaller circumference but in reality there are variables at play. 

When the lever is pulled there will be more pressure on the trailing area of the pad due to the greater sized pistons and this compensates for the less effective friction coefficient on the trailing area of the pad due to increased disk temp compared to the leading end of the pad and "gassing" of the forward pad friction material under heavy load which reduces the effectivity of the trailing end of the pad. It's a consequence of long banana shaped pads and callipers to some degree to fit into small radial spaces created by rim size and large brake rotors. The later 4 pad design suffers less I suppose so they go back to the even piston sizes.

Phil  

[Lucky Phil]

8 minutes ago, Scud said:

I'm no expert, but I think, yes, you should expect the small ones to move first. And if memory serves me, when rebuilding the calipers, the small ones usually pop out before the large ones. I read somewhere (can't recall the source) that the small piston on the trailing edge was intentional as you described, with the addition idea that if it were reversed, the leading edge of the pad could bind. I don't know if that is theoretical, or a real concern, but that's what I recall. Anyway, it seems like Brembo was aware of something with the 34/34 calipers as they used one pad per piston instead of one pad per pair of 34/30s. Maybe someone more knowledgeable will be along to explain... or maybe we can get @Chuck to look in his mechanical engineering books...

The small pistons are on the leading edge scud.

Phil

[docc]

40 minutes ago, Lucky Phil said:

The small pistons are on the leading edge scud.

Phil

So I have it backward, saying that the trailing (top) piston is the smaller? (This is very hard to see with the brakes mounted.)

[Scud]

Ok - you made me go searching. I found a discussion on an R3 forum with post that seems credible. There's a good illustration there that explains the reasoning for the smaller piston being on the leading edge. Maybe @docc could figure out how to borrow that picture and post it here for us?

 

 

[Lucky Phil]

3 hours ago, Scud said:

Ok - you made me go searching. I found a discussion on an R3 forum with post that seems credible. There's a good illustration there that explains the reasoning for the smaller piston being on the leading edge. Maybe @docc could figure out how to borrow that picture and post it here for us?

 

 

So you didn't believe me Scud and went looking on a Jap bike site that "seems credible"  The only assumption I can make is my explanation lacks credibility 

Phil

[Lucky Phil]

4 hours ago, docc said:

So I have it backward, saying that the trailing (top) piston is the smaller? (This is very hard to see with the brakes mounted.)

Yes docc you have it in reverse. I actually know this stuff and I have a calliper in my hand but you and scud might want to check some Yamaha or Honda forum or something. 

Phil

[docc]

I did figure out a way to display this illustration, but not from the Triumph site; from the source they referenced on Quora. Seems Lucky Phil knew all of this already. No surprise, it's all news to me . . .

https://www.quora.com/Why-do-BMW-brake-calipers-have-multiple-pistons-that-are-different-sizes-in-the-same-caliper/answer/Vk-Gopalakrishnan

[Scud]

7 hours ago, Lucky Phil said:

So you didn't believe me Scud and went looking on a Jap bike site that "seems credible"  The only assumption I can make is my explanation lacks credibility 

Phil

I believed you, I just wanted to learn more about it. As they say, sometimes a picture is worth a thousand words. And I had to "unlearn" some things I thought I knew. 

[Pressureangle]

I was roadracing at the advent of differential size caliper pistons. It's simply to mitigate wedge wear, as Docc illustrated above. The mechanism is simple, the larger piston has more surface area, multiplied by the hydraulic pressure so it pushes harder against the pad. It's nothing to do with gassing, it's simply a counteraction to the natural twisting forces applied to the pad, forcing the leading edge into the rotor with greater wear.

[Lucky Phil]

8 hours ago, Pressureangle said:

I was roadracing at the advent of differential size caliper pistons. It's simply to mitigate wedge wear, as Docc illustrated above. The mechanism is simple, the larger piston has more surface area, multiplied by the hydraulic pressure so it pushes harder against the pad. It's nothing to do with gassing, it's simply a counteraction to the natural twisting forces applied to the pad, forcing the leading edge into the rotor with greater wear.

Using the term "I was road racing at the advent" as some form of knowledge based technical qualification is amusing. A bit like "I'm a pilot so I know the engineering technicalities of my aeroplane". Some of the least technically knowledgeable people I've ever met are motorcycle racers and pilots.

There are many detailed reasons for differential pistons sizes. Some are more obvious and of greater influence and some less so. Some are mechanical, some are thermal and others chemical. They all add up to create the reasoning behind the design. As is usual when you start drilling down into the details there is rarely a simple singular answer.

Phil 

 

[docc]

Um, so . . . when performing the routine maintenance of cleaning the pistons and "equalizing" the piston movement, the trailing/ larger pistons should be expected to move first (during maintenance with the pads removed, watching the movement of the pistons while applying pumping action from the master cylinder)?

[Lucky Phil]

3 hours ago, docc said:

Um, so . . . when performing the routine maintenance of cleaning the pistons and "equalizing" the piston movement, the trailing/ larger pistons should be expected to move first (during maintenance with the pads removed, watching the movement of the pistons while applying pumping action from the master cylinder)?

Who knows docc. The theory is the smaller pistons will move farther for a given input pressure but with less force. In reality there are seal frictional considerations to take into account. From memory it's the smaller ones that usually move first. It's not really relevant in any way that I can see. You just pump them out until the pads touch then lever them back evenly until you can force the calliper back over the disk.

Phil

[Pressureangle]

13 hours ago, Lucky Phil said:

Using the term "I was road racing at the advent" as some form of knowledge based technical qualification is amusing. A bit like "I'm a pilot so I know the engineering technicalities of my aeroplane". Some of the least technically knowledgeable people I've ever met are motorcycle racers and pilots.

There are many detailed reasons for differential pistons sizes. Some are more obvious and of greater influence and some less so. Some are mechanical, some are thermal and others chemical. They all add up to create the reasoning behind the design. As is usual when you start drilling down into the details there is rarely a simple singular answer.

Phil 

 

Well Phil, why don't you spell them out for the stupid among us? Listen, everyone tolerates your irascibility because you know your stuff, but you would do well to internalize the fact that you're not some oracle of knowledge. I doubt that there's anything you know about brake calipers that I don't, and that I simply stated the core purpose without flinging academia and gratuitous erudition to people who haven't asked and probably don't care. 

Let's not turn this place into Wildguzzi, ok? Slow your roll.

[gstallons]

Tranquilo hijo