320mm Front Brake Conversion

[ChasF]

Or ride it within those OE brakes parameters? It ain't a performance bike

I have done for many years but it is strange how your expectations change as the years go by. I’m definitely not a heavy brake user but as I usually ride 2up with luggage, a little extra braking is a good thing. An effective back brake would be nice to - I can get the drum brake to work quite well with a lot of fiddling but it only seems to last for a couple of hundred miles.

 

[barny]

With regard to the "wooden" feel a lot of that is due to the master cylinder to caliper ratio. A smaller dia. master cylinder will improve feel this is partly what happens by changing the the 4 pot caliper, because the area of the 4 pistons is greater than the 2 pot it changes the ratio in the right direction.

 

[Gelandestrasse]

Or ride it within those OE brakes parameters? It ain't a performance bike

Sometimes you inherit poor brake performance when you buy a bike, so much so you need to do something about it. I've had an R100GS where the original disc was replaced with a cast disc and it stopped ok; no need to change (but not OE components). Last year I got my PD with the OE set-up and it was not adequate or comparable with the previous bike. So something needed to change or the bike would be history. With the youngest of these bikes now 3 decades old you can't just say 'ride within OE parameters'


285mm ø original. 320mm ø 'upgrade'. That's a 12% increase in diameter without changing the hydraulics. In pure leverage terms that has to be worthwhile and more importantly, noticeable and useful.


And then there's https://www.ukgser.com/community/threads/r100gs-pd-simple-guidelines-for-better-braking.236148/

 

[tuftywhite]

Another vote for the HE upgrade. R80G/S. Using Lucas? sintered pads as recommended by Arkwright, OE caliper.

From 4 finger pull and hope to two finger easy braking. Already had the fork brace so no problem there.

You may also consider (if you haven't done it already) the Hyperpro fork spring and oil upgrade. That also made a big difference.

 

[mikeyboy]

Worth considering changing the master cylinder.
IIRC the 100/80 GS has a 13mm cylinder,
I always fit a 12 mm
If an 11 or 10 was available that would be happy days.
The 320 disc is a good conversion,,
The 4 pot caliper gives a better hydraulic ratio.
On an otherwise standard bike,
A 320 disc,P4 caliper and a 12 mm master cylinder is as good as it’s ever going to be,
Not much change from £700 but you’ll soon spend that at the dentist if you rear end someone.

 

[Vulpine2]

With regard to the "wooden" feel a lot of that is due to the master cylinder to caliper ratio. A smaller dia. master cylinder will improve feel this is partly what happens by changing the the 4 pot caliper, because the area of the 4 pistons is greater than the 2 pot it changes the ratio in the right direction.

True. From an earlier thread (edited & added to)

Sweet spot is supposed to be a ratio of 27:1 (total piston : master cylinder) which can be achieved by changing either the master cylinder, the caliper or both. The 13mm master cylinder fitted to the r100s gives a surface area of 132.7mm2. So...

Standard P08 caliper with 38mm piston x2 has a combined surface area of 2268mm2, giving a ratio of 17.09:1
P4 caliper has 32 and 34 mm pistons, 2 of each. 3425mm2, a ratio of 25.81:1
Harrison Mini6 has (unsuprisingly) 6 27mm pistons. 3436mm2 and 25.89:1
No wonder the standard set up feel so poor. The P4 or the Harrsion caliper are pretty much spot on.

A 12mm piston (as fitted to ST and r65?) would improve ratio with the P08 slightly - but only to 20.06:1. 11mm would be better at 23.88, whilst 10mm would be 28.90

Think I got the maths right but do note but it's been a few decades since school...

 

[timeandtide]

Another vote for the HE upgrade. R80G/S. Using Lucas? sintered pads as recommended by Arkwright, OE caliper.

From 4 finger pull and hope to two finger easy braking. Already had the fork brace so no problem there.

You may also consider (if you haven't done it already) the Hyperpro fork spring and oil upgrade. That also made a big difference.

Exactly my setup Tufty!

 

[Gelandestrasse]

True. From an earlier thread (edited & added to)

Sweet spot is supposed to be a ratio of 27:1 (total piston : master cylinder) which can be achieved by changing either the master cylinder, the caliper or both. The 13mm master cylinder fitted to the r100s gives a surface area of 132.7mm2. So...

Standard P08 caliper with 38mm piston x2 has a combined surface area of 2268mm2, giving a ratio of 17.09:1
P4 caliper has 32 and 34 mm pistons, 2 of each. 3425mm2, a ratio of 25.81:1
Harrison Mini6 has (unsuprisingly) 6 27mm pistons. 3436mm2 and 25.89:1
No wonder the standard set up feel so poor. The P4 or the Harrsion caliper are pretty much spot on.

A 12mm piston (as fitted to ST and r65?) would improve ratio with the P08 slightly - but only to 20.06:1. 11mm would be better at 23.88, whilst 10mm would be 28.90

Think I got the maths right but do note but it's been a few decades since school...

And exactly how does disc diameter fit into the maths?

 

[wilbjr21]

Exactly my setup Tufty!

Me too.

 

[Posh Pete]

Not had that problem on mine which is the HE 320 disc with the standard caliper
Greatly improved over the decidedly wooden normal setup.

Wot e said. Innit!

 

[batman1]

The 320 is was a very worthwhile addition to my G/S.
Fork twisting is just nonsense.

 

[robertotoole]

Many years ago a good BMW dealer upgraded my brakes with 4 piston Brembos under warranty, because he said the originals were too dangerous for someone with children to look after! I also got a new gearox and numerous other things from him.

 

[APHezzer]

The 320 is was a very worthwhile addition to my G/S.
Fork twisting is just nonsense.

I have also just ordered the 320 mm kit .. wow that is a clean bike .. nice work!