Repacing a fork
 

I am in the initial planning stages for a new fork. I'm currently riding a bike with a 1" threaded fork & headset and a quill stem. I want to achieve two things:

1) Switch to threadless so I can more easily experiment with stem length and handlebars
2) Reduce the trail on the bike from 74 to something lower.

I have found a Dimension fork that is very reasonaby priced, will accept my cantilevers and will reduce the trail to 63. Based on my research the following will need to be replaced:

1) Fork
2) Stem
3) Headset
4) Possibly shims so a 1" steerer tube can utilized 1 1/8" stems.
5) Spacers

If I haven't missed anything major, I'm thinking I'll have an LBS switch out the headsets and then do the rest myself.

Is the process as simple as it seems? Have I missed anything major here?

Regardless of whether I replace the fork, I will be buying a new stem, either another quill or a threadless via adapter. I figure if I can replace the fork for not too much money, I'll just go that route.

you pretty much have everything covered, except you will probably want to experiment with the height of your stem, so leave it uncut and use a lot of spacers until you find out where you want to cut your steerer.

If you are doing it for the sole purpose of using threadless stems, I would reconsider since you probably don't have much to gain over just getting a different stem. If you are so dissatisfied with your current trail, then that's up to you to change, but I don't know if it's going to be a worthy investment.

If you want to experiment with a quill-threadless adapter, I've got a couple in my stash. PM me if you are interested and we can work something out.

Thanks for the response. The trail is a pretty big deal for me. I'm not unhappy with the ride of my current bike but making it a little "sportier" would be nice. I'm really trying to dial in my fit so I'm going tall with lot's of spacers and probably a rise stem.

I guess I'll just have an LBS replace my headset and then go from there.

One other thing to consider is your front brake and it compatibility with the new fork. You might need a different reach brake caliper for the pads to contact the rim, if you make a major fork change. Also, it's possible you'll need a longer or shorter bolt/nut combo for the brake.

You might also want to look at the axle to crown dimension of the new fork versus your old fork if you can get those measurements.

I'm using cantilevers so does that negate the first issue? Also, the axle to crown dimension is listed for the new fork and I'll have to figure out how to meausure it on the old one. A difference there has been the one nagging concern that I had in the back of my mind.

ETA: I'm not sure where to measure from but at the tip of the dropout to the top of the bottom ring of the headset is about 1cm different from the new fork.

http://img.photobucket.com/albums/v6...n1968/fork.jpg

The crown-to-axle dimension is measured from the center of the dropout slot (actually the center of an installed axle) to the crown race seat, i.e. the shelf the crown race sits on. It will be shorter than the measurement locations you show.

Looks like the new fork will be about 3 cm longer. Is this a major issue?

With canti brakes, you don't have to worry about fork swap issues. Should be no problem.

I don't have a good answer for how the changes in geometry will work out. You're shortening your wheelbase a bit, and raising your front end. I think decreasing rake, if all other variables are kept constant, actually makes a bike feel more solid and stable at speed. I'd welcome an educated analysis on this.

I recently broke a fork on my road bike, and I'm an uptight racer, and have been agonizing over whether a 3 mm difference in rake and a 2 mm difference in axle to crown has screwed everything up. I talked to my original frame manufacturer, the repair people at Calfee, and Enve, and they were all like "Eh, you can play around a bit . . . "

So, good luck.

I'm also curious about how much difference it will make and that's a huge part of my motivation. I'm really intrigued by frame geometry and how it affects ride. I had a Trek FX and of about the same size as my current bike with the same seat tube and head tube angles. The FX had longer chainstays and I don't know about other aspects of the geometry but the bikes rode completely different from one another. I think trail had a significant impact on that difference and this will let me test that idea. With the new fork, the trail on this bike will be about what the FX had.

Functionally - no.

But for someone who's intent on a very specific rake reduction, I think it might be.
Particularly if you're not happy with the new feel, it'd be real hard to tie the change down to the reduced rake or the increased height.

I'd use a quill-to-threadless adapter instead. It'll let you investigate stems and bars, while you keep looking for a fork that'll be closer to what you want.

In my experience riders who try to adjust fit merely by raising the handlebars end up unsatisfied. You do not say what else you have done, but it's important to first address seat height and fore-aft position and then seat-to-handlebar distance before height. Otherwise you are chainging weight distribution more than you are changing reach.

I would also point out that a fork with longer crown to dropout distance will reduce the head and seat tube frame angles, move weight distribution toward the rear, and increase trail.

3cm is a huge difference, are you sure it's that much?

That was definately a mismeasurement. I went back and looked at the technical manuals for my bike and the only one that lists a fork length is three years later than mine. Based on the geometry charts, nothing changed over those three years so if it's accurate, I'm looking at a difference of 5mm. I got some mathematical help and found out a difference of 13mm (and a wheelbase length that was 10mm too short) would yield a trail measurement of 68 with this fork. Given that I used some very wrong measurements, I should get an even lower number. It likely won't be 63 like I thought, but probably mid-60's and that's good enough for me to take a chance and try it. I've wasted more money on other experiments.....I think that's part of the fun!

Major? No, but it may undo some of the handling change you're trying to achieve. Adding 3cm to the fork will raise the front end of your bike by that amount. That will make the head angle shallower and lengthen the trail somewhat. It may not be anything significant (I'm not going to plot the difference) , but it's something to consider.

It will also raise the handlebars by about 2cm vs. the seat (as you raise the front end, the seat also goes up with respect to the ground (but not the pedals), so you might factor that into your choice of stem angle.

so do you know how to measure the trail distance dimension?

change blade length, tip to fork crown, alters that.
as it changes the head tube angle,

... then rake spec can compensate.. but 'off the peg' parts are by definition, as is.
:innocent:

Changing wheel base has no effect on steering trail. Trail is a function of head tube angle, rake offset, and wheel size. Changing fork length does change head tube angle. Increasing the length decreases HTA and increases trail. Decreasing rake offset increases trail.
http://yojimg.net/bike/web_tools/trailcalc.php
In bicycle terminology "rake" is fork steering offset. In motorcycle terminology "rake" is the steering axis angle from vertical.
http://en.wikipedia.org/wiki/Bicycle...cycle_geometry

You are right, but he is decreasing 'trail,' not 'rake.' Rake is the distance in front of the steering axis the fork puts the front axle; trail is a derived variable taking into account wheel size, head-tube angle, and fork rake.

I found a trig formula on Sheldon Brown's website that uses the difference in length between the two forks and the wheelbase measurement of the bike to calculate the change in headtube angle. I used an online trail calculator for trail figures. My bike, as is, has a trail measurement of 74. Replacing my 40mm rake fork with one of 50mm yields a trail measurement of 63. However, as was noted in this thread, a longer fork will affect the heatube angle, which throws off the calculations I did. The measurements I used to calculate the change in head tube angle were off in both the difference in length of the forks and the overall wheelbase. Those numbers indicated the longer fork would decrease my headtube angle by -.78 or something like that. That new angle gave a trail measurement of 68. Given that the actual fork length differences was wrong by a factor of two and the wheelbase I used was too short, the change in fork angle should be less than -.78....I think. I figure it will be somewhere between 63 and 68 as a ballpark figure. Whether it makes a discernable change in the bike's handling remains to be seen.

I got the bike back last night and I'm happy with the new fork. With the old fork off the bike I was better able to measure it and it's either exactly the same length as the new fork or 1-2mm longer. This means any change in head angle would be advantageous in getting a lower trail amount.

I had them leave the steerer tube uncut so I could experiment and found that using a 40 degree rise stem, 6cm of spacers put the bars at the same height as my old quill stem + bars with 2-3 inches of rise. More importantly, this same configuration put the fore-aft bar position right where I wanted.

I only rode about a mile to test out the new position and there is a definite change in the handling of the bike. It feels more sporty as I hoped it would and using a trail calculator, I went from 74mm to 63mm or less of trail. Once I get the steerer tube cut it will also look better with the flatter handlebars.

Overall, this fork swap accomplished all the goals I had for it and it was also quite a learning experience. I like the simpleness of the threadless system although it requires you to leave the steerer tube a bit long if you want any vertical adjustment after the fact.

Yeah, threadless is sweet.

I just wish more stems used the cool Zoom-style internal wedges instead of the ugly binder bolts hanging off the back... I guess they're not kosher for carbon steerers though.