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Sam W

BSA Bantam D5 head stock/ fork angle

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Hi guys,

Not sure if you will be able to help but I have an disassembled Bantam D5 frame which is partway through a trials conversion. I would like to change the headstock angle but can't any information on how much to change it. I am unsure of the original angle from when the bike is built up with original forks and rear shocks - I would like to increase both the front and rear by the same height, so if I could find the original angle of the forks when resting, I could tweak this by x amount until I have the angle I require, which will remain constant despite the height. Has anybody else done this modification and knows how much to change it by, or does anyone have an original built Bantam D5 that they could measure the headstock angle on for me please? 

Many thanks, Sam

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Hi Sam, it's a bit of a black art setting the head-angle, as you have to achieve so many inches of trail.   The triple clamps can also be either parallel, or with a slight angle, depending upon which make you go for.   Then again the wheel clamps can be either co-axial, or offset towards the front.

I seem to remember figures such as 68/69 degrees for road and racing bikes, 70/71 for trials bikes and 75ish for grasstrack and speedway bikes.    The trials bikes require about 3 inches of trail, which is measured from the axis of the steering head to the contact patch of the tyre (directly under the axle.)

Best of luck with cutting and re-welding the frame, to alter the angle.  I would just leave it as standard, and hope that it is still rideable, after your mods.

.

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Hi Sam, Have a read of the post in "Trials Australia", its all about bike geometry front and rear. Essentially it ends up as a series of compromises. But it helps you understand the trade offs, and how/what to alter for what effect. Really good for tuning things once riding as well.

Anyway, go to; Trials Australia,

then; Forums,

then; Twinshock and Classic Trials,

then; Steer Clear, Understanding steering

Hope this helps. I had to read it a few times to get my head around it fully, but it was worth the effort.

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Hi Sam, I've just followed my instructions and the web site has changed so they don't work.

Try;

Trials Australia,

Forums,

Twinshock

(At the moment its on page 3)

"Steer Clear, understanding steering" posted by Guy53, on sept 16 2016

 

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Sam

talk to a friend of mine Ossy at Audit cnc here in the North East of England

He does the mods but also makes the yokes and loads of other stuff for Pre 65 trials etc

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The Head angle thing is only half the problem.   If you intend to use longer rear shocks, there is a whole new can of worms to deal with.   Even just lengthening them by half an inch is likely to create 2 inches more chain slack.

I tried lengthening the swinging arm, and laying the shocks down on one bike I had, to increase the amount of rear wheel travel, but the results were a bit disapointing as the suspension seemed to collapse on big jumps.

.

 

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On 5/10/2020 at 10:41 PM, phiggs said:

Hi Sam, it's a bit of a black art setting the head-angle, as you have to achieve so many inches of trail.   The triple clamps can also be either parallel, or with a slight angle, depending upon which make you go for.   Then again the wheel clamps can be either co-axial, or offset towards the front.

I seem to remember figures such as 68/69 degrees for road and racing bikes, 70/71 for trials bikes and 75ish for grasstrack and speedway bikes.    The trials bikes require about 3 inches of trail, which is measured from the axis of the steering head to the contact patch of the tyre (directly under the axle.)

Best of luck with cutting and re-welding the frame, to alter the angle.  I would just leave it as standard, and hope that it is still rideable, after your mods.

.

Thanks for the response and after reading everyones responses and yours, i have decided to follow your advice and leave it as standard, atleast for the time being until i know how it rides once complete - if its too bad there will still be time to change it then! Thanks again, Sam

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On 5/11/2020 at 5:17 AM, still trying said:

Hi Sam, I've just followed my instructions and the web site has changed so they don't work.

Try;

Trials Australia,

Forums,

Twinshock

(At the moment its on page 3)

"Steer Clear, understanding steering" posted by Guy53, on sept 16 2016

 

Thanks for the page and i managed to find exactly what you were referring to, and it was a big help - theres a lot more to it than i originally realised so thanks for the heads up :)

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16 hours ago, steviet said:

Sam

talk to a friend of mine Ossy at Audit cnc here in the North East of England

He does the mods but also makes the yokes and loads of other stuff for Pre 65 trials etc

Thank you for the response but based off the other replies ive realised i may be getting myself into more than i understand just yet - thank you again though and ill re asses this once the bike is complete and i have a full view on what im working with!

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15 hours ago, phiggs said:

The Head angle thing is only half the problem.   If you intend to use longer rear shocks, there is a whole new can of worms to deal with.   Even just lengthening them by half an inch is likely to create 2 inches more chain slack.

I tried lengthening the swinging arm, and laying the shocks down on one bike I had, to increase the amount of rear wheel travel, but the results were a bit disapointing as the suspension seemed to collapse on big jumps.

.

 

Thanks for the heads up and that is another mod that im looking into doing in the future - did you notice any difference with more "normal" riding as ive seen a lot of pre 65 trials bikes have longer rear shocks and as i wont plan on jumping at all, do you have any idea if this shall be a problem for me? Thanks again, Sam

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