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Fork Sizing recommendation(14 posts)

Fork Sizing recommendationLight Pink
Dec 19, 2002 8:13 AM
I would like to replace the fork on my bike but Im not sure what sizing would be best. I have an older model Kestrel Fork that is 372mm axel to crown with a 43.5mm rake the frame is a custom Waterford (~60 cm C to T) built for that fork. I am considering either a Ouzo Pro (370 mm axel to crown available in 40,43, or 45mm rake) or an Alpha Q (374mm axel to crown available in a 40, or 44mm rake). None of these are the exact same as what I have and I don't want to adversely effect the handling of the bike. I am somewhat afraid of the shorter Reynolds fork quickening my steering and I'm not sure what changing rakes will do to the bike. It looks like most bikes the size of mine have 40 mm rakes these days. All of these dimensional variation are so tiny that I'm not sure it even makes that big of a difference. I was looking for suggestions on which for which sizing might work out best for my situation.

Any help would be appreciated.
re: Fork Sizing recommendationabelson
Dec 19, 2002 8:24 AM
Try contacting Waterford. They are very helpful and would probably be able answer your questions or suggest a fork that will work well
I tried WaterfordLight Pink
Dec 19, 2002 8:39 AM
I tried Waterford and they weren't too helpfull.
Partial answerChen2
Dec 19, 2002 8:32 AM
If you were to decrease the rake from 43.5 to 40 it will "slow" the stearing a bit because the trail will increase. Increasing rake decreases trail and will make the turn quicker, how much quicker I have no idea. Take a look at these:

http://www.phred.org/~josh/bike/trail.html

http://www.kreuzotter.de/english/elenk.htm

~Al
Partial answerShaunus
Dec 19, 2002 6:03 PM
This is incorrect. The Josh Putnam article is incorrect as well because he has the tire contact patch TRAILING the axis when in fact the axis trails the contact patch. Greater rake will slowdown your steering. eg. A Harley Chopper will have very slow steering yet a Honda sportsbike will have very fast steering. The less the trail the sharper the steering. If you move to a fork with a rake of 40, you will notice that the steering is a lot faster, more akin to a Pro crit bike.
you're a bit confused...C-40
Dec 20, 2002 5:32 AM
The Putnam picture is entirely correct. This same type of drawing can be seen on many frame manufacturers web sites. A line through the stering axis will be ahead of the tire's contact point with the ground. Trail is the horizontal distance that the tire's contact point is behind the intersection of the steering axis and the ground.

Putnam's formula for trail is accurate but not in it's most simple form. Trail = (R/tanH) - (rake/sinH) where R is the tire radius, and H is the head tube angle. The first half of this formula (R/tanH)is the amount of trail if the fork was entirely straight, with no rake. The second half of the formula is the effect of rake on the amount of trail. Since this half of the formula is negative, all rake reduces trail. The more rake, the greater the reduction in trail. As you correctly noted, less trail makes for quicker steering and less high speed stability.
Partial answermerlint3
Dec 19, 2002 10:55 PM
I believe Chen2 and the info links are correct. The 4.0 fork will give you more stability, whilst a 4.3 and greater will be more responsive, i.e. less rake. If you can find an old supergo catalog, they have a chart next to the forks with different head >'s that let you see where a a given fork will fall on a stability vs. responsive chart.
Partial answermerlint3
Dec 19, 2002 11:03 PM
oops I meant to say whilst a 4.3 and greater will be more responsive, i.e. less trail. (or greater rake)
Isn’t trail somewhat irrelevant at speed?TFerguson
Dec 20, 2002 8:47 AM
At slow speeds where turning depends on turning the front wheel in the direction you want to go, the castor effect very much affects your stability. But at cycling speeds where counter steering and lean are what turns the bike, I would think that wheelbase would be, by far, the dominant factor. If true, less rake would be quicker turning at speed. This would hold true for the Harley/Honda and cruiser/race bike examples. I agree that the castor effect is still there or you would not be able to ride with no hands and that it affects the initial counter steer to drop the bike into a lean, but I would argue that wheelbase is dominant once you are slicing through that turn.
TF
trail effect is speed dependent...C-40
Dec 20, 2002 9:32 AM
Take a look at these references on the subject. Not all agree 100%, but at least there's a general consensus.

http://www.dclxvi.org/chunk/tech/trail/
http://www.kreuzotter.de/english/elenk.htm
http://www.johnforester.com/Articles/BicycleEng/dahon.htm
http://www.juliuss.clara.net/bicycle_handling.html
http://www.spectrum-cycles.com/612.htm

The changes to the amount of trail have opposite effects at high and low speeds. A high trail bike (like Colnago) will be rock steady on a high speed descent (resistant to a change in direction), but easy to steer at low speed. A low trail frame will perform just the opposite. Easier to steer at high speed, but not as easy at low speed. Too little trail could result in high speed shimmy.

Spectrum believes that 56mm is the "neutral" amount of trail, but the discusson doesn't mention changing the trail to correspond with changes to wheelbase. Most manufacturers do reduce the amount of trail as the frame becomes larger and the whellbase longer.

Most Colnago frames, except for the largest sizes, have considerably more than 56mm of trail.
not enough info....C-40
Dec 19, 2002 8:47 AM
You didn't mention the head tube angle. The amount of trail can't be calculated without the HTA.

As others noted, a little more rake will speed up the steering and a little less will slow it down a bit. None of them would "adversely" effect the handling.

The change in the fork length will make an imperceptable difference. The change in head tube angle will only be 0.1 degree for a 2mm difference in length.
HTA = 73.5Light Pink
Dec 19, 2002 10:03 AM
HTA = 73.5C-40
Dec 19, 2002 10:31 AM
I'd say that your options are wide open. If you like the steering now and don't want to alter it, either fork with a 43 or 44mm rake would be fine.

If you'e ever had problems with high speed stability or want to enhance the tendency to hold a straight line, then the 40mm rake would be appropriate.
43, 44, &45, no noticable change, 40 probably noticeable (nm)Kerry
Dec 19, 2002 5:29 PM