- Sun May 30, 2021 5:59 pm
#96943
This is on my iron barrel 350 trials bike.
In short, apply brake. Excessive braking force applied, rear brake locks, wheel skids, brake plate rotates in swingarm, mayhem ensues.
First time I put it down to me having the brake rod too tightly adjusted and it locking as the suspension compressed. The pivot pin actually sheared and ripped itself out of the swingarm.
I replaced it, made a new brake rod reassembled and went for a test ride. First application of the brake it did the same again. This time it also broke the pivot pin end of the brake shoe and buckled the end plate of the swingarm so badly, I felt it was safest to replace the swingarm.
This time, new swingarm. New brake plate because the other one was buckled, new pivot pin, new brake shoes. Attached the cam bush "finger tight", placed it in the drum, applied the brake firmly, removed the brake place carefully and nipped up the cam bush retaining bolts with a thin spanner, appled the locknuts. I then put the assembly in the bike, loosely fastened the hub spindle and pivot pin nuts, mounted the wheel then applied the brake and nipped up the hub spindle nut, pivot pin nut and wheel nut before releasing the brake and adjusting the brake so it had free play even on full suspension compression.
Went out for a ride today. Covered about 30 miles with the rear brake being fairly poor (as expected with new shoes). Towards the end of the ride I had a wee "incident" (both me and a car approaching a narrow bridge, both of us going too fast). Emergency stop and the rear locked. After this, I noticed the brake was grabbing with minimal application so I rode home carefully without using it.
On inspection, yet another pivot pin is now bent (this is number 3, a fourth one never made it to a road test because the threads stripped as I was tightening the nut). Another set of shoes fractured at the pivot pin end.
Couple of other points.
It's done it with two different drums.
My brake setup has a homemade shorty brake lever which operates from the standard fulcrum. The back part of it where the brake rod attaches is the same length as the original. The pedal section is about 1/4 the length of the original for the trials footpeg positioning so I should be applying significantly less pressure than a standard one.
I do notice (and always have noticed) a degree of sideways flex on the brake plate when the lever is applied.
Thoughts so far as to what might be going on here:
Is the flex in the brake plate causing the shoes to misalign when applied?
Perhaps the floating bush moved when I panic braked?
The grip of the trials tyre when it locks is such that the brake assembly can't cope with the force?
The pivot pins are sub-standard and are bending when a powerful braking force is applied?
This is getting expensive. It's cost me two sets of shoes, a swingarm, four pivot pins and a brake plate so far!
For one thing I'm going to ask my local engineering guy to make me a new pivot pin out of EN8 or better.
I'm considering looking at some way of preventing flex on the brake plate:
1) Welding ribs to the original,
2) Fabricating a new one out of steel plate that's "full thickness" all the way round instead of just reinforced in a strip along the middle.
3) Fitting a Reddich made alloy brake plate, but I'm not sure if they'll fit?
4) Springing for a Somner rear brake plate. I'm not sure my German is good enough to order one though.
Interested in other thoughts...
In short, apply brake. Excessive braking force applied, rear brake locks, wheel skids, brake plate rotates in swingarm, mayhem ensues.
First time I put it down to me having the brake rod too tightly adjusted and it locking as the suspension compressed. The pivot pin actually sheared and ripped itself out of the swingarm.
I replaced it, made a new brake rod reassembled and went for a test ride. First application of the brake it did the same again. This time it also broke the pivot pin end of the brake shoe and buckled the end plate of the swingarm so badly, I felt it was safest to replace the swingarm.
This time, new swingarm. New brake plate because the other one was buckled, new pivot pin, new brake shoes. Attached the cam bush "finger tight", placed it in the drum, applied the brake firmly, removed the brake place carefully and nipped up the cam bush retaining bolts with a thin spanner, appled the locknuts. I then put the assembly in the bike, loosely fastened the hub spindle and pivot pin nuts, mounted the wheel then applied the brake and nipped up the hub spindle nut, pivot pin nut and wheel nut before releasing the brake and adjusting the brake so it had free play even on full suspension compression.
Went out for a ride today. Covered about 30 miles with the rear brake being fairly poor (as expected with new shoes). Towards the end of the ride I had a wee "incident" (both me and a car approaching a narrow bridge, both of us going too fast). Emergency stop and the rear locked. After this, I noticed the brake was grabbing with minimal application so I rode home carefully without using it.
On inspection, yet another pivot pin is now bent (this is number 3, a fourth one never made it to a road test because the threads stripped as I was tightening the nut). Another set of shoes fractured at the pivot pin end.
Couple of other points.
It's done it with two different drums.
My brake setup has a homemade shorty brake lever which operates from the standard fulcrum. The back part of it where the brake rod attaches is the same length as the original. The pedal section is about 1/4 the length of the original for the trials footpeg positioning so I should be applying significantly less pressure than a standard one.
I do notice (and always have noticed) a degree of sideways flex on the brake plate when the lever is applied.
Thoughts so far as to what might be going on here:
Is the flex in the brake plate causing the shoes to misalign when applied?
Perhaps the floating bush moved when I panic braked?
The grip of the trials tyre when it locks is such that the brake assembly can't cope with the force?
The pivot pins are sub-standard and are bending when a powerful braking force is applied?
This is getting expensive. It's cost me two sets of shoes, a swingarm, four pivot pins and a brake plate so far!
For one thing I'm going to ask my local engineering guy to make me a new pivot pin out of EN8 or better.
I'm considering looking at some way of preventing flex on the brake plate:
1) Welding ribs to the original,
2) Fabricating a new one out of steel plate that's "full thickness" all the way round instead of just reinforced in a strip along the middle.
3) Fitting a Reddich made alloy brake plate, but I'm not sure if they'll fit?
4) Springing for a Somner rear brake plate. I'm not sure my German is good enough to order one though.
Interested in other thoughts...