Subject: Re: Fork Measurements Posted by: Hilary Stone on December 23, 1999 at 01:17:04: In Reply to: Re: Fork Measurements posted by Robert on December 23, 1999 at 00:18:00:

Message: >>I am planning to replace the steel fork on my road bike >>with a carbon fork. >>Can someone please tell me the proper way to measure for >>fork rake and steerer tube length. (this is a threaded >>fork) Also, fork is straight bladed fork if it makes >>a difference. >>Thanks, >>Steve >Steerer tube length is easy, after removing the fork you have measure from the end of the steerer tube to the top of the crown piece (I think that's what it's called), that is the solid piece that connects the three tubes together. Another way to figure out the steerer tube length of the fork you need is to measure the head tube, not counting the headset. Measure the head tube and add 40mm, this is a basic way to get the propper length. If the fork you are buying doesn't come in the exact length you need, get the next longer length. The excess can be cut off, but you can't add. >For fork rake it's a little trickier. This is the distance from a line drawn down the steerer tube and/or main fork tubes in conventional forks, to the center of the dropout. With straight blade forks, this is a really difficult measurement because you need to run an imaginary line from the steerer tube all the way to the dropout. For the most part, road forks run 40 or 45mm of rake. Most aftermarket forks only come in one rake so you don't have a choice. The lower the rake the quicker and twitchier the bike will be. If you are looking at forks and want the quicker one, get the lower rake value. If you want the fork to be more stable, get the higher value. Forks with shorter fork offset as described by Steve above do not make for a less stable frame. Shorter fork offset increases trail which is is the distance between the point at which the tyre contacts the ground and the imaginary point at which a line through the centre of the head tube intersects the ground. It is calculated: Trail (in cm) = wheel radius (cm) /Tan head angle — Fork offset (cm)/Sin head angle For stable handling at speed on time trial or touring bikes a trail figure between 6–7.5cm is desirable, with road bikes trail between 5–6cm produces a quicker responding but still easy to ride bike. So a longer fork offset (fork rake) will make a frame with less stability at speed. If you want want a frame with more stability at speed, one with a shorter fork offset will add the stability you desire. But you do need to check for toeclip overlap - this term is used to describe the situation when the foot (in a clipless pedal) or toeclip touches the front wheel or mudguard with the crank in the far forward position catches the front wheel or mudguard when the bars are turned. It's best to avoid this on a frame designed for city cycling or touring or for a beginner.