Measuring trail

[Jeff]

Is the best way to measure trail? I made a rod that fits in the steering head and a plumb bob. It works out at 7mm, which should be heavy steering, but it is quite light.





Jeff

[steve-e]

Shouldn't the weight be behind? Trailing?

[G JONES]

Is the best way to measure trail? I made a rod that fits in the steering head and a plumb bob. It works out at 7mm, which should be heavy steering, but it is quite light.

It looks about right to me - I'm guessing you are passing the "string" on the plumbob to the front of the spindle & then deducting half the spindle diameter from the measured distance ?
I check mine in a similar way - using a square off the floor - just touching the front of spindle - then allowing for half spindle diameter.
As I understand it - point at which headstock centre touches ground measured back to spindle centre - only concerns I usually have are whether to allow for some sag in the suspension when blocking the bike up...

[Whites]

Hi Jeff,

The centre line of your front wheel is in front of your steering angle, this is actually negative trail, so I would expect the steering to be light.

Regards,
Whites

[Jeff]

It looks about right to me - I'm guessing you are passing the "string" on the plumbob to the front of the spindle & then deducting half the spindle diameter from the measured distance ?
I check mine in a similar way - using a square off the floor - just touching the front of spindle - then allowing for half spindle diameter.
As I understand it - point at which headstock centre touches ground measured back to spindle centre - only concerns I usually have are whether to allow for some sag in the suspension when blocking the bike up...

Yes, Gordon, I deduct 12.7mm from the one inch axle. My floor is not very level, hence the plumb bob not a square. I measured the height with the front wheel on and my weight in approximately the riding position, then packed under the floor to the same height when I removed the front wheel. I did consider making vee-blocks so the axle was loaded, but figured it wouldn't move forward or back much.

Jeff

[Jeff]

Hi Jeff,

The centre line of your front wheel is in front of your steering angle, this is actually negative trail, so I would expect the steering to be light.

Regards,
Whites

It is quite twitchy, and I used to very confident under brakes, but not so lately. This is probably why.

Jeff

[G JONES]

Hi Jeff,
The centre line of your front wheel is in front of your steering angle, this is actually negative trail, so I would expect the steering to be light.
Regards,
Whites

It is quite twitchy, and I used to very confident under brakes, but not so lately. This is probably why.
Jeff

I didn't look that closely at the picture - you said you'd measured at 7mm ? - was looking more at the method - which doesn't much matter - as long as you have the forks pointing straight & you can measure the distance between steering axis & spindle centre - with the spindle / wheel centre needing to be behind - I'd have thought 7mm should be ok (if it was positive trail) - you're looking to have some castor effect on the steering so it self centres...

[666]

Is the best way to measure trail? I made a rod that fits in the steering head and a plumb bob. It works out at 7mm, which should be heavy steering, but it is quite light.





Jeff

Jeff, Using a plumb bob is only ok if you floor is perfectly level. you mention that the bike is very twitchy under braking this can be due trail changes under braking. There are 2 things that will help this. 1 is the angle of the spindle and the link pivot in relation to the line through the steering head. 90d is good, with this set up the trail remains constant under bump and braking. 2 is to have good support under your chest so you don't transfer your body weight onto the h/bars.When i build I build on zero trail with adjustment both ways

[oldbelly]

With the caliper mounts directly fixed to the swinging arm as on the photo braking won't affect the trail much. It looks like it has negative trail (lead ).This will make things lively.

[Bill Lymburn]

It is quite twitchy, and I used to very confident under brakes, but not so lately. This is probably why.

Jeff

If it behaved well under braking previously, suggesting trail was ok then....I would be concerned about what has happened to change that situation....For the trail to alter, something must have bent or moved...that situation needs to be investigated before resetting to positive trail, as it should be....I always used 5/16" or 7.9mm and found it very satisfactory.....

[Jeff]

If it behaved well under braking previously, suggesting trail was ok then....I would be concerned about what has happened to change that situation....For the trail to alter, something must have bent or moved...that situation needs to be investigated before resetting to positive trail, as it should be....I always used 5/16" or 7.9mm and found it very satisfactory.....

I have changed tyre sizes and brands several times since I got the bike due to availability. Whenever I settle on a size they either stop making them or nobody has them in stock.

Jeff

[oldbelly]

Make the saddle of the swingingarm ( the pivot point ) adjustable Jeff. Tony Baker has always done this with his bikes.

[G JONES]

Make the saddle of the swingingarm ( the pivot point ) adjustable Jeff. Tony Baker has always done this with his bikes.

It looks as if there is an adjustable eccentric in the fork end ?

[Team RGM]

Couple of notes:
If trail is positive (Axle in front of pivot) machine will be VERY twitchy in straight.
Your first pictures looks like positive trail.
Trail goes toward negative when turning wheel left or right.
A way to quick check if any trail is present; mark rear of chair on outside corner (carpenter's square) to ground. Any movement when turning handle bars is indication of trail being there.
When I ran a Baker F1. I adjusted front axle pivot to 0 trail. Did hunt a little in straights but made for lightest in turns.

A little veting; Only a crazy American with 45 years driving and building