Front brake issues

Hi all.
I wonder if anyone out there can offer any front brake advice? Here are my problems..
I have an RGM 12" disc conversion with an RGM bought Grimeca twin pot caliper.
I operate this on a Suzuki GT750 master cylinder. The output shaft to the cylinder measures about 12mm, where the lever contacts it, but I am not sure about the bore of the cylinder. The connection is the standard Commando rubber hose.

My problems are:-

1) The grimeca caliper makes a lot of rattling noises at slow speed, one of the pads is shifting about in the caliper. I have carefully oil-stoned the disc to ensure there are no edges or bumps on it.
2) The brake system feels abit 'wooden'. If I pull harder the brake seems to operate OK, but without giving me the feeling of being thrown over the bars. The lever feels 'solid' as I pull back, rather than a continuation of movement getting stiffer you get with a quality setup.
3) The lever is a long way from the bars when it contacts the piston, so the pull you get from your hand is reduced.

Are there any folk out there with similar rather disappointing experiences with front brake modification attempts?
I am not giving up yet...
Stu

Sounds as if the the piston bore of the master cylinder is too large. I don't know the diameter of the pistons in your caliper, but my guess is they are around 38 mm. The Vintage Brake chart shows that an 15 mm bore mastercylinder would give about a 12.8:1 ratio and be pretty wooden. Check out this chart and find a new master cylinder that gives a more optimum ratio. http://www.vintagebrake.com/mastercylinder.htm

I believe the RGM kit is a full floating disc, which will rattle on the pins and sometimes causes the pads to rattle a bit. Can you move the iron disc in and out on the hub center? If so, then it is a full floater. My Ducati 900 SP makes a lot of rattle pushing around the shop, but isn't noticeable on the road. (But maybe that is because of the Gia-Ca-Moto carbon fiber mufflers) :shock:

bigstu said:

The output shaft to the cylinder measures about 12mm, where the lever contacts it, but I am not sure about the bore of the cylinder.

Click to expand...

The Suzuki GT 750 master cylinder piston/bore size is 15.9mm.

My problems are:-

bigstu said:

1) The grimeca caliper makes a lot of rattling noises at slow speed, one of the pads is shifting about in the caliper. I have carefully oil-stoned the disc to ensure there are no edges or bumps on it.

Click to expand...

The Grimeca calipers use the same pads and pad retention system (long split pins) as the Triumph Lockheed calipers, so they also tend to suffer the same problem of pad rattles that can be heard at low speed as the Lockheed calipers because the pads tend to be a rather loose fit in both types of caliper, and some makes of pad are a looser fit than others.
One remedy is to fit ball point pen springs over the split pins between the pads . I've had limited success on both caliper types with that idea.

The most successful cure I've found so far, is the application of a small amount of silicone RTV gasket sealant to the leading and trailing edges of the pad backing plates, as the silicone acts as a cushion between the pads and caliper. This idea is still 'on test', so I don't know how long it lasts, but so far it has completely stopped the pad rattles.

bigstu said:

2) The brake system feels abit 'wooden'. If I pull harder the brake seems to operate OK, but without giving me the feeling of being thrown over the bars. The lever feels 'solid' as I pull back, rather than a continuation of movement getting stiffer you get with a quality setup.

3) The lever is a long way from the bars when it contacts the piston, so the pull you get from your hand is reduced.

Click to expand...

That's probably because you have a master cylinder with a 15.9mm piston diameter that's meant for the GT750's twin discs and twin piston calipers, not a single disc and a single two piston caliper, so the hydraulic ratio between the master cylinder and caliper is not ideal;-although the Norton/Triumph single caliper/master cylinder ratios were much the same, (5/8” master Cyl. piston) as apparently, they were fearful that the new-fangled disc brakes would be too powerful for most riders, otherwise.

I suggest you find another master cylinder with a 12 or 13mm piston size (try both if you can?), as they will give you a much better hydraulic ratio, and a much better feel at the lever as a result.

If you don't have the correct ratio between caliper(s) and master cylinder, the components themselves become less than effective. I run a 11mm master cylinder with a Grimeca 2P caliper and have a very good front brake. Nearly as good a stopper as my BMW K 75 twin disc Brembo setup. It's all about the ratio.

Thanks Guys
As for the rattle, the disc is a floater but that is not the problem, its the pads rattling in the caliper.
I will try the silicon gasket trick as a short term fix, but I am not convinced it will last long, maybe the gunk will stick to the trailing edge long enough...I will study other setups to see what they do with leaf type springs...IIf I have any success I will let you know.

The ratio is wrong, you are all correct, I hoped the Kettle master cyl would be OK, but I only measured as best I could from the outside. Thanks for the chart.
I think the ratio to aim for is over 20, but not over 25...Jim, I recon you are spot on with an 11mm setup, what's it off?
Does anyone know of 11mm jap setups I can steal from?
Stu

bigstu said:

The ratio is wrong, you are all correct, I hoped the Kettle master cyl would be OK, but I only measured as best I could from the outside. Thanks for the chart.
I think the ratio to aim for is over 20, but not over 25...Jim, I recon you are spot on with an 11mm setup, what's it off?
Does anyone know of 11mm jap setups I can steal from?

Click to expand...

Ron took a guess at the Grimeca pistons being "around 38mm" diameter, however after pulling a Grimeca caliper off my spares shelf, the pistons are 41mm, (but check your own to make absolutely sure that's the same) so for a 41mm two piston caliper, a 12 or 13mm master cylinder could be the better option (see the chart info.)?

bigstu said:

Thanks Guys
As for the rattle, the disc is a floater but that is not the problem, its the pads rattling in the caliper.
I will try the silicon gasket trick as a short term fix, but I am not convinced it will last long, maybe the gunk will stick to the trailing edge long enough...I will study other setups to see what they do with leaf type springs...IIf I have any success I will let you know.

The ratio is wrong, you are all correct, I hoped the Kettle master cyl would be OK, but I only measured as best I could from the outside. Thanks for the chart.
I think the ratio to aim for is over 20, but not over 25...Jim, I recon you are spot on with an 11mm setup, what's it off?
Does anyone know of 11mm jap setups I can steal from?
Stu

Click to expand...

The master cylinder is a Grimeca. It's from a MZ 250. It gives me a ratio of 27.79:1. 27:1 is what Mike Morris of Vintage Brake calls the sweet spot. I consider him the definitive answer for motorcycle brakes. He even made the sick rear drum brake on my Commando perform like a real brake.

Hi there,
all this talk about bore diameters etc I find a bit complicated,
I had the same problem with the lever being a long way from the bars so I filed a bit off the end where it contacts the piston, then I drilled a hole a put a thin screw in where the lever comes to rest when your not using the brake.
Now the lever sits closer to the bars when at rest & I can get a good grip on the lever without stretching my fingers to reach the lever.
Failing that you could put an adjustable lever on your bike like I have on my Suzuki Bandit.
Cheers
Don from the UK
Ive had my commando from new, way back in march 1973.

Thanks again guys,
I like that mod to bring the lever closer, but the first thing is to find the correct ratio. This is the vital part. That may sort out the lever position.
Thanks Jim, an MZ? Surly one of them can't be expensive? 27:1 sounds high, but you are running it and you are happy...is that with a stock disc? I have the Grimeca caliper and a 12@ RgM floating disc, bigger dia than the original, which will help slightly...
I suppose I could consider sleeving the Kettle bore down...
Oh by the way, I found out this weekend that it doesn't seem to work at all in the wet....I have them Kevlar pads....any suggestions
Stuart.

bigstu said:

Oh by the way, I found out this weekend that it doesn't seem to work at all in the wet....I have them Kevlar pads....any suggestions
Stuart.

Click to expand...

Don't ride in the rain? :mrgreen:

Change the pads? :mrgreen:

bigstu said:

Thanks again guys,
I like that mod to bring the lever closer, but the first thing is to find the correct ratio. This is the vital part. That may sort out the lever position.
Thanks Jim, an MZ? Surly one of them can't be expensive? 27:1 sounds high, but you are running it and you are happy...is that with a stock disc? I have the Grimeca caliper and a 12@ RgM floating disc, bigger dia than the original, which will help slightly...
I suppose I could consider sleeving the Kettle bore down...
Oh by the way, I found out this weekend that it doesn't seem to work at all in the wet....I have them Kevlar pads....any suggestions
Stuart.

Click to expand...

As for 27:1, why not trust an expert such as Vintage Brake? The disc I'm running is a 12" floater with a Grimeca 2 piston caliper, same as you. You will have more lever travel with a smaller diameter master.