Re: Fork Measurements

United Bicycle Institute

TECHNICAL FORUM

UBI Main Page

UBI Courses and Schedules

[ Post a Reply ] [ UBI Technical Forum ] [ FAQ ]

Subject: Re: Fork Measurements
Posted by: Robert on December 28, 1999 at 00:45:37:
In Reply to: Re: Fork Measurements posted by Hilary Stone on December 23, 1999 at 01:17:04:

Message:
>Forks with shorter fork offset as described by Steve above do not make for a less stable frame.
When I said a shorter rake makes a bike "quicker and twitchier" I didn't mean less stable, I meant quicker to turn and twitchier relating to your style, you couldn't move about when riding, you would have to stay in good form. Less rake means less lift when the fork is turned, so it turns easier.
>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.
Trail doen't place higher for stability than rake does, they are about equal for practicle purposes. If you had a fork with no rake, you would do a face plant avery time you went around a corner but trail would be the greatest. If you had a fork with rake so high it had negative trail, the bike would also be very unstable. You can't give one or the other (rake/trail) total accoutablility for stability.
>So a longer fork offset (fork rake) will make a frame with less stability at speed.
My mountain bike has a greater rake than my road, and a shorter trail. They are equal as far as stability goes, but my road is much quicker to turn. They have equal stability at high speeds, but my road bike (the one with greater trial) is more susceptible to bad style.
Trail has as much to do with head tube angle as it does with the forks. I don't build frames so I don't know any specific formulas regarding stability and rake/trail, but I am an engineer and I do understand physics and can relate the way my bike's handle to the way they are built. I know enough to say that if you replace a fork with 45mm of rake with one with 35mm of rake, the bike will handle quicker. If you replace a fork with 35mm of rake with one with 45mm of rake, the bike will handle slower. Stability would be the same, and can be more dependant upon a properly built wheel than rake or trail (an off-center front wheel causes shimmy at speeds).

Post a Follow Up To This Message

Follow Up Message:

>>Forks with shorter fork offset as described by Steve above do not make for a less stable frame.

>When I said a shorter rake makes a bike "quicker and twitchier" I didn't mean less stable, I meant quicker to turn and twitchier relating to your style, you couldn't move about when riding, you would have to stay in good form. Less rake means less lift when the fork is turned, so it turns easier.

>>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.

>Trail doen't place higher for stability than rake does, they are about equal for practicle purposes. If you had a fork with no rake, you would do a face plant avery time you went around a corner but trail would be the greatest. If you had a fork with rake so high it had negative trail, the bike would also be very unstable. You can't give one or the other (rake/trail) total accoutablility for stability.

>>So a longer fork offset (fork rake) will make a frame with less stability at speed.

>My mountain bike has a greater rake than my road, and a shorter trail. They are equal as far as stability goes, but my road is much quicker to turn. They have equal stability at high speeds, but my road bike (the one with greater trial) is more susceptible to bad style.

>Trail has as much to do with head tube angle as it does with the forks. I don't build frames so I don't know any specific formulas regarding stability and rake/trail, but I am an engineer and I do understand physics and can relate the way my bike's handle to the way they are built. I know enough to say that if you replace a fork with 45mm of rake with one with 35mm of rake, the bike will handle quicker. If you replace a fork with 35mm of rake with one with 45mm of rake, the bike will handle slower. Stability would be the same, and can be more dependant upon a properly built wheel than rake or trail (an off-center front wheel causes shimmy at speeds).

[ UBI Technical Forum ] [ FAQ ]