# Framebuilders - Add some rake to my fork?

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View Full Version : Add some rake to my fork?

02-02-11, 07:55 PM
I have a Trek 610 fork and I want to add about 1.5 to 2 cm of rake to it. Any frame builder here willing to discuss this and take it on?

NoReg
02-02-11, 08:14 PM
Ann Arbor is kinda cool, don't they have a bike shop with frame tools there? Also, depending on the details, there are forks out there that come bent, and might cost less than getting it done.

unterhausen
02-02-11, 09:03 PM
how old is it? Any pictures?

Dave Kirk
02-02-11, 11:45 PM
Keep in mind that when you change the rack of the fork it will effectively get shorter and the head tube will drop and the brake reach will change.

Dave

tuz
02-03-11, 09:52 AM
You could calculate the precise change in length (and consequent HT angle change) but you'd need to know the original fork length & rake, as well as the radius and location of the bend. I made a rough calculation using triangles and, for 20mm of rake increase you'd loose about 10 mm of fork length (and tire clearance or brake reach). This document (http://www.phred.org/%7Ealex/bikes/Fork%20Re-raking%20Summary.pdf) by Matchak tends to agree. By loosing 10mm the HT/ST angles would increase by approx 0.6 degrees, and the BB would drop 5 mm.

Dave Kirk
02-03-11, 10:15 AM
You could calculate the precise change in length (and consequent HT angle change) but you'd need to know the original fork length & rake, as well as the radius and location of the bend. I made a rough calculation using triangles and, for 20mm of rake increase you'd loose about 10 mm of fork length (and tire clearance or brake reach). This document (http://www.phred.org/%7Ealex/bikes/Fork%20Re-raking%20Summary.pdf) by Matchak tends to agree. By loosing 10mm the HT/ST angles would increase by approx 0.6 degrees, and the BB would drop 5 mm.

that sounds about right - it ends up being a significant number.

Dave

ClemY
02-03-11, 01:08 PM
15 to 20 mm sounds a little big to me. I had a couple of forks bent to decrease trail. They were bent 12 to 13mm. The head angle increased about 0.4 degrees. Trail decreased to about 46 mm and they handle much better with a touring load.

02-04-11, 05:58 AM
The original fork matches the frame and is a 1984 model. Trek seems to have said it's 531 dB main tubes, and 531 forks and stays. Many people doubt these statements by Trek, but I'd expect it's 4020 or 531. I have no idea if Reynolds actually butted any fork blades.

This fork was already reshaped once to add trail. This builder is not interested in adding offset back in, because he says it will reduce the stability of the bike. I think now that the first change was a very bad idea, and want to go in the other direction. The current high-trail design is more stable with no front load and skinny tires, but it's not good at all with a 10 to 15# front load. Design with 72 to 73 degrees head angle with 55 to 60 mm offset has been a well accepted front geometry for bikes carrying moderate front loads, such as the Boxdog Pelican, Velo-Orange production Randonneur, a custom design I have some knowledge of, and several of the current and vintage bikes reviewed by Jan Heine over the years. I think my target is reasonable.

As-new it had an offset of 52 mm per the Vintage Trek site, and I'd like to get it near 60, for a trail in the 40 to 45 degree range. After the first re-shape it has about 47 mm offset. So I'd want to restore the original offset (which was good for OEM 27x1 1/8 Panaracer tires without much fender clearance), then increase offset by another 5 to 8 mm. The effective fork length should shorten by at least 2 mm (see the illustrated 44 to 60 mm adjustment example in Matchak's paper), and a more likely value of 5 to 10 mm. The correct length change depends on the exact shape after bending, so I don't believe we can predict it exactly. I'm not that picky about the beauty of the new bend, but I do want the job done neatly

After this bend I would expect to use 700x25's with fenders. I currently use 700x32 Paselas, and with fenders they are a tight fit. The frame's lateral clearances really aren't set up for this much tire and fender. With the new setup I'd be using wheels with 4 or 5 mm less radius than the 32 Paselas. They measure 348 mm radius, and my 700x28 UltraGatorskins measure more like 340. I'm willing to go with smaller wheels to enjoy better front bag handling. Don't take this to mean I'm planning on 650s, however!

If as ClemY suggests 12 to 13 mm adjustments are as much as can be expected, that would get the new rake in line with the '80 through '83 Trek 610 forks, which were offset 55 mm, not 52. Even this change would be beneficial to me, from a rando-bag point of view, though short of the target.

Have I scared you all away?

ClemY
02-04-11, 06:32 AM
The original fork matches the frame and is a 1984 model. Trek seems to have said it's 531 dB main tubes, and 531 forks and stays. Many people doubt these statements by Trek, but I'd expect it's 4020 or 531. I have no idea if Reynolds actually butted any fork blades.

This fork was already reshaped once to add trail. This builder is not interested in adding offset back in, because he says it will reduce the stability of the bike. I think now that the first change was a very bad idea, and want to go in the other direction. The current high-trail design is more stable with no front load and skinny tires, but it's not good at all with a 10 to 15# front load. Design with 72 to 73 degrees head angle with 55 to 60 mm offset has been a well accepted front geometry for bikes carrying moderate front loads, such as the Boxdog Pelican, Velo-Orange production Randonneur, a custom design I have some knowledge of, and several of the current and vintage bikes reviewed by Jan Heine over the years. I think my target is reasonable.

As-new it had an offset of 52 mm per the Vintage Trek site, and I'd like to get it near 60, for a trail in the 40 to 45 degree range. After the first re-shape it has about 47 mm offset. So I'd want to restore the original offset (which was good for OEM 27x1 1/8 Panaracer tires without much fender clearance), then increase offset by another 5 to 8 mm. The effective fork length should shorten by at least 2 mm (see the illustrated 44 to 60 mm adjustment example in Matchak's paper), and a more likely value of 5 to 10 mm. The correct length change depends on the exact shape after bending, so I don't believe we can predict it exactly. I'm not that picky about the beauty of the new bend, but I do want the job done neatly

After this bend I would expect to use 700x25's with fenders. I currently use 700x32 Paselas, and with fenders they are a tight fit. The frame's lateral clearances really aren't set up for this much tire and fender. With the new setup I'd be using wheels with 4 or 5 mm less radius than the 32 Paselas. They measure 348 mm radius, and my 700x28 UltraGatorskins measure more like 340. I'm willing to go with smaller wheels to enjoy better front bag handling. Don't take this to mean I'm planning on 650s, however!

If as ClemY suggests 12 to 13 mm adjustments are as much as can be expected, that would get the new rake in line with the '80 through '83 Trek 610 forks, which were offset 55 mm, not 52. Even this change would be beneficial to me, from a rando-bag point of view, though short of the target.

Have I scared you all away?

You have now included some significant details that were omitted previously. I had gotten the impression you were making a touring bike, the type the French builders would call a camping bike. Perhaps that is because I am preparing for a cross country bike trip and thinking about these things. They seem to work very well with a distributed load and (±) 46mm of trail. I have had a couple of forks bent to duplicate that and the bikes work very well.

What you are now describing is a French Randonneur bike: one carrying a small front rack and handlebar bag. That is a different animal. I haven’t bent a fork to duplicate this because I didn’t have to. I have a 40 yr. old Jack Taylor Marathon. It came with about 63.5mm of offset. I measured the head angle with a digital protractor at 73.6 degrees. The tires I have on it measure 344mm in radius as near as I can measure things, which gives a trail figure of 35mm, which is about perfect for a randonneur bike with a small front load. I currently have a Nitto M12 front rack and an Acorn Boxy Rando bag on it and the handling is super.

Here are a few helpful formulas for calculating trail, or fork offset if you have a desired trail figure in mind. The last one is to calculate the change in head angle with shortening the fork by bending it.

Trail = (R*cosH – F)/sinH

F = R*cosH – {Trail * sinH**

How Fork Length Changes Head Angle

∆ H = arcsin [(old length - new length)/(wheel base)]

Where:
F is fork offset
T is trail

02-04-11, 09:18 AM
At this point I think there has been good discussion, and
I thank all of you for that. But I am very interested to
Find a builder who would be interested in taking this job
and modifying my fork.

If you or any of you are interested, please PM me with
email so we can get into more detail. I really do want to do
this.

Live Wire
02-04-11, 09:58 AM
At this point I think there has been good discussion, and
I thank all of you for that. But I am very interested to
Find a builder who would be interested in taking this job
and modifying my fork.

If you or any of you are interested, please PM me with
email so we can get into more detail. I really do want to do
this.

Just read the whole thread and let's see if I have this right:
-we have a 26 yr old fork that has been ridden quite a few miles..often times with an extra load
-at some point in the past, the blades were bent backward to reduce the rake/offset
-now you want to bend the blades the other way again and go past the original offset

See the main problem here? I think a builder would be crazy to put his rep (and your teeth) on the line and try this.
New fork time.

02-04-11, 10:13 AM
I think your interpretation of my situation is far too negative and dismissive. My bike has not been ridden many miles, and only about 30 with any load other than me. It IS a 1984 fork and frame. I bought it new.

Are you trying to clarify something in my problem description, or add some insight based on specific knowledge and experience?

Do you speak for the bonafide builders here?

ClemY
02-04-11, 10:44 AM
At this point I think there has been good discussion, and
I thank all of you for that. But I am very interested to
Find a builder who would be interested in taking this job
and modifying my fork.

If you or any of you are interested, please PM me with
email so we can get into more detail. I really do want to do
this.

I live in Southern Maryland and found a guy in Baltimore who is a part time builder. He did an alignment of my crash damaged frame and bent a couple of forks for me. I just used the Internet to find him. You might want to start by talking to the good bike shops in the area and see who they might know.

Live Wire
02-04-11, 11:12 AM
I think your interpretation of my situation is far too negative and dismissive. My bike has not been ridden many miles, and only about 30 with any load other than me. It IS a 1984 fork and frame. I bought it new.

Are you trying to clarify something in my problem description, or add some insight based on specific knowledge and experience?

Do you speak for the bonafide builders here?

No offense intended RF, just making a point.

Specific knowledge, experience, and common sense tells me that bending metal past it's yield point repeatedly is (trying to be positive here) perhaps not the best for longevity.

You might find someone to do this and if you do I hope it works for you. Speaking for myself (am I "not bonafide"? ;) does anyone else read that and hear the kid from "O Brother, where art thou?"*) the benefit vs risk factor would make me stay away.

Good luck.

Scooper
02-04-11, 12:16 PM
I have no idea if Reynolds actually butted any fork blades.

Reynolds produced 531 tapered fork blades in both "taper gauge" and plain gauge versions as shown in the catalog pages below. Taper gauge was 18 gauge (1.2mm) at the crown, tapering to 20 gauge (0.9mm) at the dropout tip. The plain gauge blades were either 17 gauge (1.35mm) or 18 gauge (1.2mm) from the crown to the tip. Gauges are Imperial Gauge.

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Tubing/Page_16.jpg

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Tubing/Page_18.jpg

02-04-11, 01:49 PM
LW, what I meant was, that some writers here are well-known builders or will at least refer to the frames they have built, or the business they have building frames. None or them have raised the issue you did. Not that fatigue of steel is not real when bent repeatedly past its elastic zone, but I would presume others did not immediately see it as a problem. I would assume someone on this forum who says "I build frames" is a framebuilder. I would not know about anyone who doesn't.

I don't get your reference to "O brother where art thou?", but maybe it's just not my religion.

Live Wire
02-04-11, 04:47 PM
LW, what I meant was, that some writers here are well-known builders or will at least refer to the frames they have built, or the business they have building frames. None or them have raised the issue you did. Not that fatigue of steel is not real when bent repeatedly past its elastic zone, but I would presume others did not immediately see it as a problem. I would assume someone on this forum who says "I build frames" is a framebuilder. I would not know about anyone who doesn't.

.

That might be because you didn't initially mention how old the fork is and the fact that this would be it's third time being raked.

Sorry that I don't have any forks that I've bent back and forth three times to refer you, but like I said, the risk is yours and I hope it works out for you.

02-04-11, 05:27 PM
Second time, my friend, unless you're saying they were cold-set at Trek - I suppose they may have been.

So cold-setting a rear end or aligning dropouts should be analogous - how many cycles do you think those operations can be repeated?

And I did mention it when asked. My original intent in this thread was to find perhaps three builders who are experienced in re-bending forks and speak with them privately to share and hash out all the relevant details, then select my fork guy. That's what I meant by "discuss." I should have said "discuss privately."

02-04-11, 05:31 PM
Scooper, thanks a million! I'll take a pair of those Tapered No. 4's.

02-05-11, 07:28 AM
ClemY, last night I polled our half-dozenlocal shops. None are equipped or have experience. One recommended a metal fabricator whose son builds MTB frames.

unterhausen
02-05-11, 10:51 AM
I have to say I wouldn't recommend bending it a third time either. LiveWire is better known in the framebuilding community than I am. I have heard of people that will re-rake forks, and I'm sorry I don't remember who.

Raking is inherently a cold setting process. When I was at Trek they would bend the forks after they made them. Then they would check the alignment and tweak them into alignment. Tnat was a couple of years before your frame was built, but I doubt they were doing it any differently then.

02-05-11, 11:12 AM
Apologies to LW, but until I noticed his buiness website on his posts, I thought he was like me a bikie with a little knowledge but not expertise.

I think it matters where the stress-affected area is. If the bends are all concentrated in one small zone, I can see the problem. That's the issue with seatstays breaking at brake bridges or pulling out brake bridges. But that might not be the case for all fork bends. I'm going to see the local guy about it but I can't for a few weeks.

unterhausen
02-05-11, 11:44 AM
that's why I wanted to see pics. If there was un-raked area, then you could bend that instead of the already raked area. It would be hard to get a smooth bend that way though. Since fork blades are tapered, we bend them by wrapping them around a form. An already bent blade isn't going to wrap too well.

NoReg
02-05-11, 12:46 PM
I couldn't bring myself to read this thread all over again, but if we are talking about adding a few mm of rake to a 47mm offset fork, such as is found on the majority of touring bikes with low rider mounts happily carrying 40-60 pounds on the front end in order to carry 15 pounds, then I think there is an ailment known as "too many issues of Bike Quarterly" in play here. Sounds perfect as is. My last touring bike had both low riders and a porteur rack on it, carried on a 45 mm fork, narry a problem. Of course my next bike is going to have a 70 HT angle, and a well offset fork, just to solve the kind of problem I wasn't having.

I'm not sure the extra 8mm of wig waggling of the fork will really hurt anything, but I think it's a fools errant, most likely anyway. Of course a lot of custom detail probably fall into that category so ya gotta do what you gotta do. Just slide it under a good solid door and heave. :) My observation over the years is that the "real" frame builders are a pretty conservative lot in the good sense of the word, but it can be a tough crowd to get support from.

fietsbob
02-05-11, 12:53 PM
http://www.bikeschool.com/ resources

has a number of frame-builders
other than the people reading this list.