Bike Forums > Measuring trail
 Register All Albums Elite Membership Forum Rules Calendar Search Today's Posts Mark Forums Read

 Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

 12-12-05, 07:54 PM #1 Mike T. Upstanding member. Thread Starter     Join Date: May 2005 Location: Ontario Canada Bikes: Argon18 road, Seven mtb, Marinoni track, Masi dirt road bike, Marinoni cyclocross. Posts: 447 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Measuring trail 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...............
 12-13-05, 09:49 PM #2 fore Senior Member   Join Date: Jul 2002 Bikes: Posts: 727 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Trail = (R * cos(H * pi / 180) - K) / sin(H * pi / 180) R = tire radius H = head tube angle K = fork rake
 12-14-05, 06:37 AM #3 Bing Senior Member     Join Date: Mar 2005 Bikes: Posts: 78 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) click on the "Download XCL file" link inside this hyperlink to Anvil Bikeworks calculators to get an excel spreadsheet that does the trig for you.
 12-14-05, 06:58 AM #4 Bing Senior Member     Join Date: Mar 2005 Bikes: Posts: 78 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) what are you doing up so early?
12-14-05, 07:11 AM   #5
Mike T.
Upstanding member.

Join Date: May 2005
Bikes: Argon18 road, Seven mtb, Marinoni track, Masi dirt road bike, Marinoni cyclocross.
Posts: 447
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by fore 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.

 12-14-05, 09:16 AM #6 cooker Prefers Cicero     Join Date: Jul 2005 Location: Toronto Bikes: 1984 Trek 520; 1990s Peugeot (Canadian-made) rigid mountain bike; 2007 Bike Friday NWT; misc others Posts: 11,957 Mentioned: 59 Post(s) Tagged: 0 Thread(s) Quoted: 2324 Post(s) 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
12-14-05, 09:53 PM   #7
Mike T.
Upstanding member.

Join Date: May 2005
Bikes: Argon18 road, Seven mtb, Marinoni track, Masi dirt road bike, Marinoni cyclocross.
Posts: 447
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by Bing click on the "Download XCL file" link inside this hyperlink to Anvil Bikeworks calculators to get an excel spreadsheet that does the trig for you.
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.

 12-14-05, 10:07 PM #8 Thylacine Industry Maven     Join Date: Oct 2003 Location: Wherever good bikes are sold Bikes: Thylacines...only Thylacines. Posts: 2,936 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Except for those square pies, they're definitely square.
12-14-05, 10:19 PM   #9
Mike T.
Upstanding member.

Join Date: May 2005
Bikes: Argon18 road, Seven mtb, Marinoni track, Masi dirt road bike, Marinoni cyclocross.
Posts: 447
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by Thylacine Except for those square pies, they're definitely square.
Momma always made round pies. I ain't never seen a pie square.

12-17-05, 08:03 AM   #10
Kogswell
Matthew Grimm / Flunky

Join Date: Jan 2005
Bikes:
Posts: 656
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by Mike T. 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...............
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:

convert everything to millimeters and use our trail calculator - you'll need to have Flash installed:

 12-17-05, 12:55 PM #11 Mike T. Upstanding member. Thread Starter     Join Date: May 2005 Location: Ontario Canada Bikes: Argon18 road, Seven mtb, Marinoni track, Masi dirt road bike, Marinoni cyclocross. Posts: 447 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Thanks Kogs that's a dandy trail calculator! It's within 3mm of what I crudely measured on the ground.