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Fork Rake(6 posts)

Fork RakePansapien2
Dec 2, 2001 8:37 AM
I might have my terminolgy skewed here but; what is fork rake and how does it effect ride characteristics? I have a Litespeed Tuscany with a Look fork. I have noticed the foreward slant and have never really questioned it's purpose(always been more of a mtb'er until recently). If I was to guess I would assume that straight bladed forks offer better cornering at the expense of less high speed stability? Any truth to that?
Rake is the distance between the line formed by thebill
Dec 2, 2001 9:40 AM
axis of the headtube and a parallel line running through the axle. All bike forks have some rake, and the way that the rake is created, either with an angle at the crown, as with a Wound-Up, or the more traditional curve at the bottom, is purely cosmetic. It makes no difference to the rake, which largely determines how far the wheel is going to turn on the end of the lever (think chopped motorcycles -- with a big fat rake, the wheels turn relatively more for a given turn of the bar).
Rake works hand in hand with trail in determining the bike's handling. You need a pencil and paper to get it, but, basically, to get trail, you draw the axis of the headtube down to the ground and you drop a line perpindicular to the ground through the axle. The distance between where the two lines intersect the ground is the trail. The more trail, the bike is generally more stable.
Others know more about this. Check Sheldon Brown's site.
But, in any case, bikes are designed to work with a particular trail, and you change the rake at risk of ruining your bike's handling.
Rake & trail...C-40
Dec 2, 2001 2:00 PM
Rake is the perpendicular between tow parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance the tire contact point is behind the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H)-(rake/sin H). As an example if R = 33.65cm, H=73, and rake is 4.0cm, trail = 33.65/tan73 - 4.0/sin73. This calculates to 6.1cm or 2.4 inches.

Changing the rake from 4.0 to 4.5cm yields a trail of 5.58cm or 2.2 inches.
Changing the head tube angle to 74 degrees, with the 4.0 rake yields a trail of 5.49cm or 2.16 inches. Thus, a .5cm increase in rake will have a similar effect to increasing the head tube angle by slightly less than one degree.
re: Fork Rakejosh_putnam
Dec 2, 2001 11:16 PM

All standard forks have rake, or offset. How they get it doesn't really matter, handling is the same for a given amount of rake, whether the blades are straight, curved, offset at the crown, etc.

I have an article on steering geometry on my framebuilding web site, see http://www.phred.org/~josh/bike/trail.html. The image below is from that article, and shows how to figure out rake and trail.

TerminologyDAC
Dec 3, 2001 4:31 AM
"Rake" is more generally called "offset", at least in engineering circles. (The term Rrake" may suggest an angle in other fields.)
Trail, for a road bike, is almost universally a number greater than 50mm, sometimes even approaching 70mm. Anything less than 50 will result in high speed instability, and anything greater than 70 can result in low speed instability.
For some traditionalists, "less trail" means "less rake(offset)", and, therefore, a greater numerical value of trail. Confused?? Hopefully logic will eventually end this archaic pratice.
Terminologyjosh_putnam
Dec 3, 2001 11:59 AM
As used on bicycles, "rake" was originally both offset and an angle. Back in the days of high-wheeled Ordinaries, before the introduction of curved fork blades, offset was produced by angling the blades forward at the crown. (This retro look has been enjoying a revival in recent years. Any bets on the return of hairpin saddles and spoon brakes?)

The term rake is definitely used differently in newer fields such as motorcycles and automobiles, but I suspect its old, accepted usage will continue in bicycling, much as many builders still speak of silver soldering when technically they mean silver brazing.

I would suggest being conservative in what you write but liberal in what you read -- use the unambiguous term "offset" when writing technically, but don't complain when fork manufacturers stick with "rake" in their own publications, since within the bicycle field everyone knows what they mean.