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Discussion Starter · #1 ·
I'm getting a new (older) Merlin Extralight frame and am seeking a fork.
The Merlin is the 53 size with a 73 degree HT angle. The geometry for the
frame calls for a 45 cm fork rake. I've found a Bontrager XXX lite in a 45 rake with a 385 crown to axle length. When I checked the Bontrager website, they show that the 45 rake modle is available in two different Axle to Crown lengths...370mm and 385mm. The one with the 385mm length is also 80 grams heavier.

I have a Bontrager XXX lite 45 rake fork on my TREK 5900 size 54, and tried to measure the axle to crown on it, but I'm not sure exactly where on the axle they take there measurement from. Is it the top, middle, or bottom of the drop out?

Anyway, I've read several threads about Rake/Trail, and that the length of the fork can have an effect on the accurate computation of the trail. Which one of the two lenghts is better choice for my proposed build?

I also have a line on a 43 rake fork (Ouzo Pro) which would increase my trail. From what I've read, the difference in handling from a 45 to a 43 would not be very significant if I had to go with the 43.

Any thoughts (no flames please) would appreciated.
 

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I think you ought to call the Tuttles at OCC, and get a set of forks about 4 feet long! :D
 

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Discussion Starter · #3 ·
Visitor302 said:
I think you ought to call the Tuttles at OCC, and get a set of forks about 4 feet long! :D

Gee, thanks for the tip. You're a lot of help.

But, I guess you've got nothing better to do.:rolleyes:
 

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Larry Lackapants
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..

I'd choose the better looking one.
Of course I think that a longer axle to crown distance will require longer reach of the brake calipers?

Good luck,
br
 

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Arrogant roadie.....
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FrankDL said:
I..........................

I have a Bontrager XXX lite 45 rake fork on my TREK 5900 size 54, and tried to measure the axle to crown on it, but I'm not sure exactly where on the axle they take there measurement from. Is it the top, middle, or bottom of the drop out?

......................
The dropout is where your rear wheel is connected. The fork holds your front wheel.
 

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Kant phuckin sphell
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I would think fork length would be measured from base of steerer tube to the center of the axle parallel with the steerer tube, but like frame geometry it can be done differently by some manufactures.
 

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the correct answer...

Merlin could probably provide the exact fork length that the frame was built around, but in general, road bikes take forks in the 365-374mm range and the 370mm would be right in the middle. Don't know what type of frame would take a 385mm.

The 385 would make the HTA more slack, which is probably not what you want. The exact amount would depend on the orginal design length.

Fork length is measured from the center of the axle to the top of the crown race seat (base of steering tube as Bull stated), parallel to the steering tube, so it's not easy to do with sufficient accuracy.

A 45mm offset will only produce 56mm of trail which is on the small side, although at least one builder calls this trail "neutral". It should produce some pretty quick high speed handling. Others don't agree with 56mm being neutral, like Colnago and LOOK, who both use larger amounts of trail on this size frame.
 

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Kant phuckin sphell
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Hmm

C-40 said:
Fork length is measured from the center of the axle to the top of the crown race, parallel to the steering tube, so it's not easy to do with sufficient accuracy.

So fork length can change depending on the brand of headset one is using?
I assume that different crown races will very in height.

One should also consider tires and headset stack height when splitting hairs like this.

:)
 

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naranjito
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what do you call the fork tips then? fork tips? or (front) dropouts? I´ve always called both the front and rear dropouts just that - front and rear dropouts... because the wheel will just 'drop out' of either one. I´m not being pedantic - just widening my vocabulary!

foz
 

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C-40 said:
Merlin could probably provide the exact fork length that the frame was built around, but in general, road bikes take forks in the 365-374mm range and the 370mm would be right in the middle. Don't know what type of frame would take a 385mm.

The 385 would make the HTA more slack, which is probably not what you want. The exact amount would depend on the orginal design length.

Fork length is measured from the center of the axle to the top of the crown race, parallel to the steering tube, so it's not easy to do with sufficient accuracy.

A 45mm offset will only produce 56mm of trail which is on the small side, although at least one builder calls this trail "neutral". It should produce some pretty quick high speed handling. Others don't agree with 56mm being neutral, like Colnago and LOOK, who both use larger amounts of trail on this size frame.
Ditto what C-40 says about asking Merlin about what length fork this frame was built for. But as C-40 says, the 370mm fork is the most likely candidate.

Also keep in mind that the purpose of the 385mm fork is to be able to use fatter tires, but that it requires long reach brake calipers. Most higher end road brakes (Dura-Ace, Ultegra, Record, Chorus, etc.) are only available in short reach. There are only a few long reach calipers presently available.
 

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yes...

the bull said:
So fork length can change depending on the brand of headset one is using?
I assume that different crown races will very in height.

One should also consider tires and headset stack height when splitting hairs like this.

:)
It's not splitting hairs. With only a tape measure you can't accurately measure fork length with the fork on the bike.

Different headset crown race dimensions could affect the HTA. It only takes a 7-8mm difference to change the HTA by .5 degree and the trail by 3mm. The wrong fork length could double or cancel out an intended change to the bike's trail, or create an unintended change in trail. With fork offsets usually only ranging from 38-50mm and most only within 40-45mm, a few millimeters can be important. Consider that a fast handling trail might be 54mm and a slow (Colnago) trail might be 67mm.

Tire radius is also part of the trail calculation, but since trail = R/tanH - offset/sinH, tire radius doesn't have a great effect. The tangent of a typical 73 degree STA is 3.27, so changes in the radius are reduced by about one third. Cross bikes with significantly larger diameter tires would certainly need to take this into account, when the frame is designed.
 

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Discussion Starter · #13 ·
Thank you all (well most) for your input and suggestions regarding my question. I very much appreciate it

This forum is a virtual treasure chest of knowledge which I turn to quite often. You guys/gals are a great bunch!:)
 

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Splitting hairs: Perceptability of minor differences in head angle and trail

C-40 said:
It's not splitting hairs. With only a tape measure you can't accurately measure fork length with the fork on the bike..
I don't know about that. You can measure a fork's length to within a few millimeters of accuracy with the fork on the bike, which is generally close enough.

C-40 said:
Different headset crown race dimensions could affect the HTA. It only takes a 7-8mm difference to change the HTA by .5 degree and the trail by 3mm. The wrong fork length could double or cancel out an intended change to the bike's trail, or create an unintended change in trail. With fork offsets usually only ranging from 38-50mm and most only within 40-45mm, a few millimeters can be important. Consider that a fast handling trail might be 54mm and a slow (Colnago) trail might be 67mm.
A 7-8mm difference in fork height is fairly large, and can easily be measured even with the fork on the bike. But even so, bikes are only built with angles to within about 0.5 degrees to begin with, and a difference in trail of 3mm can't be detected by most cyclists anyway. Here is a post by Gary Helfrich, one of the founders of Merlin bicycles, on a test he did on varying head angle to see if riders could tell the difference:

http://groups.google.com/group/rec.bicycles.tech/msg/e0d23833d2a6898?q=72+73+74+head+group:rec.bicycles.*&start=20&hl=en&rnum=22
 

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I ride a 5500 Trek and a C-50 Colnago, nearly opposite ends of the spectrum when it comes to trail. After upgrading my wife's bike I had an extra Ouzo Pro fork, 40mm rake, otherwise identical to the 43mm rake Ouzo Pro in my Trek. After switching to the 40mm rake fork the increase in trail was immediately obvious. The handling was definitely different but after a couple of rides I was adjusted and the bike felt normal again. The Colnago has a lot more trail and I like that.

Racing style Trek road bikes have relatively steep head tubes, with a 45mm rake fork that bike would feel twitchy to me, I'd want move toward more trail.

If you don't want to tackle the trig here is an online rake & trail calculator.

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

Al
 

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Discussion Starter · #16 ·
...Took your advice and asked the man.

I took a shot and tried calling the designer of my frame, Tom Kellog at Spectrum Cycles a few hours ago. To my surprise, he answered the phone. I described my frame to him and told me that I should get the 45 rake. I asked him my fork length question and he said that manufacturers actually measure the fork from a point under the carbon to the axle center of the dropout and because of that most consumers would not be able to accurately measure it anyway. I had a real good chat with him. He's a real friendly and helpful guy with a ton of info.

Oh yeah, Dave, I e-mailed Easton and Real Designs and told them that they were incorrectly refering to the fork tips as "dropouts" in their description of their forks. They said that they have always used the term dropout as the slot at the end of the fork blade, but since hearing that Dave Stohler says that is not correct they will immediately update their advertizing data to be correct and will pass the info on to Look and Reynolds too!:p

Thanks again to all for your comments and info.
 

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you're so special...

You've posted about your amazing mechanical abilities before. I'm truly impressed with your opinion of your abilities. After working in a precison machining and manufacturing environment for over 20 years, let's just say that I think the average guy probably won't be so lucky. Obviously you're special.
 

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??????

C-40 said:
You've posted about your amazing mechanical abilities before. I'm truly impressed with your opinion of your abilities. After working in a precison machining and manufacturing environment for over 20 years, let's just say that I think the average guy probably won't be so lucky. Obviously you're special.
To measure fork length, remove the front wheel, place a round object of similar diameter to an axle in the drop outs (like a pencil or pen), measure from the bottom of the crown race (i.e. the fork/headset junction) to the substitute "axle", and you've got the fork length to within a couple of millimeters. What's so special?
 

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Kant phuckin sphell
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I think you missed my point...
What I am trying to ask, is why would a fork company measure there fork length with a unknown fork crown race?

Like I posted before, Base of steerer tube, not as you posted top of crown race.

You know allot, but to come here and say, "the correct answer" is just plain......
 

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Well,,, I just thought scince we were talking fork lenth, (or maybe I was just drunk) I just figgured you might want to set a chopper road bike.... Imagine showing up for a crit, or a century on a way cool chopper....
One day, I'm going to build one!
 
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