Some of my fellow Greeves Series competitors will know that for years my Hawkstone had been plagued by the dreaded departing of the chain from the rear sprocket, which has enabled me to rack up more DNFs in classic scrambles than I care to remember.
The bike had a standard (for want of a better word) "guide" fitted when I got the frame from John Fryatt several years ago, presumably the one that came with the bike from the factory (see attached pic) and I have always made sure the chain was correctly aligned.
I had also fitted a spring-loaded jockey wheel from a modern motocross bike by fabricating a bracket to mount this to the rear of the left-hand engine plate. This made little difference however and the chain kept coming off.
What I had ascertained was the chain was always coming off when landing from jumps and when riding over bumpy, rutted ground at speed.
Initially I thought this was possibly because I was adjusting the chain tension too tight, so when the rear shocks were on near enough full compression, the chain was being pulled off the rear sprocket, as this is when the chain is under the greatest tension.
To try and prove myself wrong, I took the rear shocks off and lowered the rear of the bike down so the swinging arm was in the same position as when the shocks were under full compression. The chain was a bit tight - I had been leaving about 1.5" movement, so I worked out I needed between 2 and 2.5".
So there I go turning up at Maylandsea a few weeks ago. It was wet in the morning, but as the track dried, so speeds increased and...well...you can guess the rest.
With the proposition of a 450mile round trip to make the next round of the Greeves Series at Sproxton, another solution was needed. I then began thinking that perhaps it was just the pure "violence" of the chain being shaken around on heavy impacts that was aiding it from slipping off the bottom of the rear sprocket.
Looking at modern motocross bikes, nearly all of the them have a guide made up of an initial hole followed by a groove and run for the chain to pass through aimed at the bottom of the sprocket. I guess it's logical to deduce that the hole and run of a modern guide effectly takes the vibration out of the chain and settles it ready for mating with the rear sprocket.
I went to Jim Aim's shop in Sible Hedingham and his son Ian got a whole bunch of guides out for me to look at. I found one to suit the width of the 428 chain I run, from a KTM SX 65cc kiddies MX bike costing around £15. You can get them on ebay:
I did away with standard chain guide and made up two brackets out of 3.5mm steel with aluminium spacers to align the guide correctly in-front of the sprocket. One bracket utilised the bolt holes in the swinging used by the old guide, whilst the other used a small bracket on the inside of the swinging arm that (I believe) would have been once used for a rear chain guard fixing.
The result was fantastic. A whole day's racing, hitting all sorts of bumpy ground and jumps without a problem. Was I, finally, about to complete all Greeves Championship races in a meeting without issue? A rear puncture (my first in my 12 years of classic scrambles) meant I missed the start of the second championship race...but I managed to finish second from last.
So, my next outing will be with Greeves & Mortimer at the 100miler (I missing Stebbing for a close friend's wedding). Look forward to it.
The bike had a standard (for want of a better word) "guide" fitted when I got the frame from John Fryatt several years ago, presumably the one that came with the bike from the factory (see attached pic) and I have always made sure the chain was correctly aligned.
I had also fitted a spring-loaded jockey wheel from a modern motocross bike by fabricating a bracket to mount this to the rear of the left-hand engine plate. This made little difference however and the chain kept coming off.
What I had ascertained was the chain was always coming off when landing from jumps and when riding over bumpy, rutted ground at speed.
Initially I thought this was possibly because I was adjusting the chain tension too tight, so when the rear shocks were on near enough full compression, the chain was being pulled off the rear sprocket, as this is when the chain is under the greatest tension.
To try and prove myself wrong, I took the rear shocks off and lowered the rear of the bike down so the swinging arm was in the same position as when the shocks were under full compression. The chain was a bit tight - I had been leaving about 1.5" movement, so I worked out I needed between 2 and 2.5".
So there I go turning up at Maylandsea a few weeks ago. It was wet in the morning, but as the track dried, so speeds increased and...well...you can guess the rest.
With the proposition of a 450mile round trip to make the next round of the Greeves Series at Sproxton, another solution was needed. I then began thinking that perhaps it was just the pure "violence" of the chain being shaken around on heavy impacts that was aiding it from slipping off the bottom of the rear sprocket.
Looking at modern motocross bikes, nearly all of the them have a guide made up of an initial hole followed by a groove and run for the chain to pass through aimed at the bottom of the sprocket. I guess it's logical to deduce that the hole and run of a modern guide effectly takes the vibration out of the chain and settles it ready for mating with the rear sprocket.
I went to Jim Aim's shop in Sible Hedingham and his son Ian got a whole bunch of guides out for me to look at. I found one to suit the width of the 428 chain I run, from a KTM SX 65cc kiddies MX bike costing around £15. You can get them on ebay:
- Part number: 46007066000
- Ebay: http://www.ebay.co.uk/sch/i.html?_fr...7066000&_rdc=1
I did away with standard chain guide and made up two brackets out of 3.5mm steel with aluminium spacers to align the guide correctly in-front of the sprocket. One bracket utilised the bolt holes in the swinging used by the old guide, whilst the other used a small bracket on the inside of the swinging arm that (I believe) would have been once used for a rear chain guard fixing.
The result was fantastic. A whole day's racing, hitting all sorts of bumpy ground and jumps without a problem. Was I, finally, about to complete all Greeves Championship races in a meeting without issue? A rear puncture (my first in my 12 years of classic scrambles) meant I missed the start of the second championship race...but I managed to finish second from last.
So, my next outing will be with Greeves & Mortimer at the 100miler (I missing Stebbing for a close friend's wedding). Look forward to it.
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