# Framebuilders - Measuring trail

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View Full Version : Measuring trail

Mike T.
12-12-05, 07:54 PM
Does anyone have a good method for measuring trail on an existing bike?

Getting the point on the ground perpendicular to the hub axle is easy (plumb line) but extending the steering axis is a bit tougher. I usually eyeball a straightedge down the centre of the headtube for this one. There has got to be a more accurate way. Hmmmm, maybe if I made a little jig that would hold the straightedge at 1/2 the diameter of the headtube............... :rolleyes:

fore
12-13-05, 09:49 PM
Trail = (R * cos(H * pi / 180) - K) / sin(H * pi / 180)

K = fork rake

Bing
12-14-05, 06:37 AM

Bing
12-14-05, 06:58 AM
what are you doing up so early?

Mike T.
12-14-05, 07:11 AM
Trail = (R * cos(H * pi / 180) - K) / sin(H * pi / 180)

You are assuming a couple of things here Fore.
1) I have the ability to do something with that formula stuff.
2) I know the head angle.

I have an angle finder and a 3' level (to find a level piece of floor) but the graduations on the angle finder are so fine as to make its reading inaccurate at best. I would think that even a mildly incorrect head angle reading injected into your formula would produce a very inaccurate answer.

I do think and actual "on the floor" measurement would be the way to go. Thanks for your formula (or whatever that is!) anyway.

cooker
12-14-05, 09:16 AM
I did a rough estimate this way: I took a digital photo of my bike ...I leaned it against a wall with the front wheel straight, and I stood some distance from the bike and lined the camera up on what I thought was the front wheel axle line. Then opened the photo in MS-Paint, and drew lines on it down the head tube, to the ground, and vertical from the axle to the ground. I measured the trail distance between the two ground points using Paint's pixels as units, and the diameter of the rim in the photo and calculated their ratio. Then I multiplied that by 27" since it's 27" wheel. It ended up being about 1.5 " or about 4 cm, but it may have been out by about 1 cm

Mike T.
12-14-05, 09:53 PM

Hey that works great !! Thanks. Even my mathematically challenged brain can use it. At school they tried to tell me pie are square and I knew that was bull**** as they are all round. :(

Thylacine
12-14-05, 10:07 PM
Except for those square pies, they're definitely square. ;)

Mike T.
12-14-05, 10:19 PM
Except for those square pies, they're definitely square. ;)

Momma always made round pies. I ain't never seen a pie square.

Kogswell
12-17-05, 08:03 AM
Does anyone have a good method for measuring trail on an existing bike?

Getting the point on the ground perpendicular to the hub axle is easy (plumb line) but extending the steering axis is a bit tougher. I usually eyeball a straightedge down the centre of the headtube for this one. There has got to be a more accurate way. Hmmmm, maybe if I made a little jig that would hold the straightedge at 1/2 the diameter of the headtube............... :rolleyes:

tire diameter:

hold a ruler up to your front wheel and measure from the ground to the center of the front axle - then multiply by two for the tire diameter

get a \$0.49 plastic protractor - they have a hole at the origin - put a string in the hole and tie a weight to it - makes a dandy angle meter

fork offset:

tricky - use a ruler and eyeball it

calculation: