In a message dated 1/16/04 9:34:32 PM Pacific Standard Time,
john.hawrylak@att.net writes:
> Just to illustrate Bob's explanation and using the formulae from Bob's
> reference, I calculated the Trail for 2 Heron bikes:
> the Road model and the Touring model.
> These seem to be representative of current steel, lugged frames and the
> specs are readily available on the web.
>
> Road model, Head angle = 73.5° and Rake = 4.25 cm, Trail > 57.2 mm
> Touring model, Head angle = 72.0° and Rake = 4.5 cm, Trail = 64.1 mm
>
> Although the Tourer has 2.5 mm more Rake, it has 6.9 mm more trail due to
> the slacker head tube angle and the larger change in the cosine of the angle
> versus the sine of the angle.
>
> Note, both Trail values are >55 mm which Bob points out is less prone to
> shimmy, assuming the 2 frames are equally rigid. Hope this illustrates
> Bob's point.
>
> John Hawrylak
> Woodstown NJ
>
Good work, John. Personally, I would have made the fork on the touring model with a little more rake, keeping the trail closer to 60. 64 is a little on the sluggish side for my taste, but certainly acceptable and very stable. At some point you get the "chopper" effect, slow steering at high speed and floppy at low speed. The wheel diameter figures in there too, which means the choice of tire size will affect how your bike handles too. I assume you put values in for the actual diameter of your tires.
Note that trail is also not an absolute, but must be considered for the diameter of tires you are using. So 650 x 25C tires, which are really about 24" in diameter, need trail of about 52 mm to ride similarly to a 700 x 25C (about 26.5" diameter) on a bike with 58 mm of trail. And 700 x 38c tires which really are about 28" (That's why 700C is sometimes called 28" by the way), could benefit from trail of about 61 mm to feel the same.
It's all about how big an obstacle appears to be in relation to the tire diameter. A one-inch bump will not seem as big to a tire that is 28" tall, compared to a tire that is 24" tall. When you hit an obstacle of a certain size, trail is momentarily decreased, and can go to zero or maybe even negative, a highly unstable condition (visualize riding a circus bike with vertical head tube and fork with no rake). The size of the obstacle needed to make trail go to zero can be determined if you draw a circle representing your tire size, and a line down to the ground through your steering axis. The point at which the lines exits the tire, which is in front of the point of contact with the ground, is the place the obstacle would have to hit for trail to momentarily go to zero.
Regards
Bob Freeman
Seattle