Brakes, old or new

BruceB said:

A question for Nigel or anyone else: with the dual AP caliper setup, what M/C are you using?. From what I gather from a brief search, the correct M/C piston diameter should be about 5/8". Although there seems to be quite a bit of differing opinions on this.

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Bruce, I use an adjustable Brembo radial master cylinder. Kinda the opposite of ‘period’ really !

Ah right Nigel, I saw that you had a radial M/C on your Seeley, didn't realize on your Commando too. And definitely agree that the feel from a radial M/C is brilliant.
Another question just came to mind about using dual PR sliders. As best I can tell neither the right side nor left side sliders come with spindle clamps (presumable they assume there is a regular slider on the other side). So how do you secure the spindle on the non threaded side? Or do you use a different spindle setup?

Don Pender master cylinder kit with stock Lockheed caliper and Norton rotor. I have no complaint. Never could understand what “wooden” feel meant. Maybe I do have that Scotish grip. My grandmother was a McClane

BruceB said:

Ah right Nigel, I saw that you had a radial M/C on your Seeley, didn't realize on your Commando too. And definitely agree that the feel from a radial M/C is brilliant.
Another question just came to mind about using dual PR sliders. As best I can tell neither the right side nor left side sliders come with spindle clamps (presumable they assume there is a regular slider on the other side). So how do you secure the spindle on the non threaded side? Or do you use a different spindle setup?

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I use the same MC on the Seeley and Commando. Doesn’t seem to work quite as amazingly with the AP callipers vs the Brembo’s, but still bloody good!

My sliders are from Emery, the sliders, hub, spindle, spacers, disc, etc all as a kit. It all fitted perfectly, even the spacers put the wheel dead centre. I was seriously pleased with it all, and their service.

The spindle arrangement is different to stock as it clamps the sliders. IMO that’s a more solid arrangement than the pinch bolt AND removes the potential for it to be broken via over tightening.

Fast Eddie said:

Bruce, I use an adjustable Brembo radial master cylinder. Kinda the opposite of ‘period’ really !

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The master cylinder on my Seeley 850 is the one which was normally used to operate a single Lockheed AP calliper - it now operates two. It is one-finger operation, and for racing, it needs to be like that. I use a quick-action twin-pull twist grip and operate the brake with my fore-finger. That way, everything is nice and quick. One thing I never do is grab a big handful of brake. On any bike, that is dangerous. You only need to chirp the front tyre to be in business. If you don'tget your hands off quick enough, the result is usually being flicked over the front. Hydraulic steering dampers are essential - if you lose the front, they straighten the bike out, if a tank-slapper starts. It is all quite safe.

My mate built my Seeley with a Laverda 750 SF motor. But when I got the bike, I could not get the motor - so I bought a Commando 850 motor. The bike had a single chromed Suzuki GS1000 disc with one AP calliper. It was nowhere near enough brake. As soon as I rode the bike, I immediately got into a corner too hot. I swapped a GP carb for the second AP calliper. Even then disc pads were still a problem. My discs are mid-70s Suzuki on CB750 Honda hubs. My brake lines are braided steel and came from a hot-rod shop.
When I built the Seeley 850, none of it was rocket science. Virtually, all I needed to do carefully, was make a set of engine plates.

I suggest some people might sweat a bit if they get into corners too hot, and start running wide. You need to wash off speed, but if you use the front brake - as the front depresses, the bike runs wide faster. You need to trail brake, until you judge you have lost enough speed, then accelerate.

1970s brakes where not good all the way around. I rode a BMW R90/6 for years with a single disk factory setup. It had a cable operated master cylinder located under the tank. You got cable flex and hose flex before the brake would bite (if it ever did.) It was the most useless brake of modern times.

I had gone back to the shop for a visit and the store owner threw me the keys to a BMW K100RS. He said "go ride that." I did and at the first corner I grabbed a handful of brake like I always did on my R90 and found myself looking at the ground and the front wheel. Took me three more savage stops to unlearn a bad habit.

'What the mind does not know, the heart does not grieve over'. I rode motorcycle on public roads for ten years before I road raced. The first time I raced I found myself at the end of the front straight at Calder Raceway, and the bike was not stopping. So I held the brake on as hard as I could. It suddeny grabbed and chucked me up the road at about 90 MPH. My two young sons were standing there watching. The good thing was, I never had to watch them riding motorcycles. Later one of them had a Kawasaki ZZR750. He was very careful, but still found himself one day, sitting on his bum in the middle of an intersection with traffic buzzing all around him.

Fast Eddie said:

I use the same MC on the Seeley and Commando. Doesn’t seem to work quite as amazingly with the AP callipers vs the Brembo’s, but still bloody good!

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The AP's pistons have a different surface area than the Brembo 30/34 so with the same master cylinder piston to caliper piston the AP will always have a different lower ratio. So to get a proper comparison you need to reduce the piston size until the ratios match.

Surface area's

AP's times two 2676mm2 = 5353mm2

Brembo 30/34 times two 3228mm2 = 6456mm2

So for the magic 27:1 ratio on a standard Master Cylinder its 17mm for the Brembo's and 15.875mm for the AP's.

Radial masters have a different calc for the magic piston diameter but the same principle means the diameters need to differ too.

kommando said:

The AP's pistons have a different surface area than the Brembo 30/34 so with the same master cylinder piston to caliper piston the AP will always have a different lower ratio. So to get a proper comparison you need to reduce the piston size until the ratios match.

Surface area's

AP's times two 2676mm2 = 5353mm2

Brembo 30/34 times two 3228mm2 = 6456mm2

So for the magic 27:1 ratio on a standard Master Cylinder its 17mm for the Brembo's and 15.875mm for the AP's.

Radial masters have a different calc for the magic piston diameter but the same principle means the diameters need to differ too.

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That’s good info, thanks

"Radial masters have a different calc for the magic piston diameter but the same principle means the diameters need to differ too."


Please say more on this.

The lever pivot orientation to the piston, also part of the "mechanical advantage" equation, I assume.

concours said:

"Radial masters have a different calc for the magic piston diameter but the same principle means the diameters need to differ too."


Please say more on this.

The lever pivot orientation to the piston, also part of the "mechanical advantage" equation, I assume.

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I forget a lot lot as I move from project to project so its a bit hazy, I did investigate Radial master cylinders as part of upgrading an Aprilia RSV1000 and there are calculators out there for selecting the right piston diameter which are more extensive than the simple 27:1 ratio from Vintage Brakes. These take into account the length of the lever, pivot distance from centre line of piston etc.

I think it was this one I used, its a linked xlr file in the first post.



Master cylinder piston diameter 14.0000 mm
Caliper large piston diameter 27.00 mm
Caliper mid piston diameter 0.00 mm If there is only one size of piston enter zero in this line and the line below.
Caliper small piston diameter 0.00 mm If there are only two sizes of pistons enter zero in this line.
Number of large pistons 4
Master cylinder lever pivot to finger 90.00
Master cylinder lever pivot to piston 25.00 mm
Single or double acting? If single enter 2 in this row. If double enter 1.

This also may be behind the AP racing lever RGM sell, it did improve feel when I fitted one many years ago.

BruceB said:

But is a dual disk overkill?. "Thomasa" mentioned that he was happy with the stopping power of a single AP caliper, can anyone else comment on it's effectiveness? Where does it lie between the original caliper and a Pender or CNW setup?

One downside of the dual AP caliper setup (apart from cost!) would seem to be the added unsprung weight. Can anyone comment on how noticeable that is?

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I cannot comment on the original brake setup (was badly gummed up on my bike when I bought it), nor on Brembo calibers, but I can confirm that a single AP caliper and a 12 or 13mm master cylinder plus the RGM floating disc works very well, even at Spa trackdays (which are scary fast!!)
It's perfectly possible to chirp the front tyre while still keeping enough feel to avoid locking the wheel.

If you look through the archives , you will find several discussions with Storm42's Seeley project, where he started with a twin disc and went to a single disc due to a combination of "what's enough braking" and unsprung weight.

Mind you, I'm also running a twin AP caliper setup on a bevel Ducati, and I would not change that,

With the CNW Brembo master cylinder, SS lines, and high-friction Ferodo pads, I can chirp the front (stock size) Avon tire with the OEM caliper. But my rotor is not OEM. It was on the bike when I bought it in '06 and is ventilated and not chromed so it's aftermarket or a modded original. It might be that the rotor is much better friction-wise than the OEM rotor but I never had any way to compare OEM with my rotor. IOW, whether the present rotor is better for braking power than the OEM rotor, I have no idea.

I have a cast iron disc with a floating Honda CB750 calliper on the rear of my Seeley. I don't know if it works or if I ever use it. It needs to be replaced. Cast iron discs are really stupid.