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Old 02-02-13, 03:22 PM   #1
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Rebuilding the rear triangle with a lower BB height.

I'm considering having a framebuilder replace the rear triangle on my vintage bike. I have a 1978 Trek 930 built with Columbus SP. The frame details can be found on this link;

http://www.vintage-trek.com/images/trek/Trek2pg3.gif

I would like to lengthen the chainstays from 415mm to 425mm and ovalize the chainstays to improve tire clearance. I would also like to shorten the seatstays by about 1.2mm to provide a lower BB height.

I would also like to drop the BB height from 7.2mm to 8.0. My goal is to improve stability and reduce standover height by a small amount.

I realize that I will be tilting the entire front triangle counterclockwise (when viewing the drive side) by a small amount. Will this provide any unintended consequences or changes in handling or stability?
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Old 02-03-13, 08:54 AM   #2
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As you probably know, the head tube and seat tube angles will slacken. This will also cause your fork trail to increase. My guess is that the change in trail will be more noticeable than the change in angles when riding. The steering will be slower or more stable depending on how you look at it. Will it be truly noticeable? I don't know. Maybe somebody else can provide the actual amount of change in numbers and an opinion about what you will feel.
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Old 02-03-13, 11:09 AM   #3
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If it were me, I'd just sell the frame and buy something closer to those specifications.
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Old 02-03-13, 11:39 AM   #4
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As you probably know, the head tube and seat tube angles will slacken. This will also cause your fork trail to increase. My guess is that the change in trail will be more noticeable than the change in angles when riding. The steering will be slower or more stable depending on how you look at it. Will it be truly noticeable? I don't know. Maybe somebody else can provide the actual amount of change in numbers and an opinion about what you will feel.
I'm glad to hear this evaluation, it's confirmation of the potential outcome I am expecting.


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If it were me, I'd just sell the frame and buy something closer to those specifications.
If only such a frame was ever made. Putting a new rear triangle on this frame is a fraction the cost of having a custom frame made.
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Old 02-03-13, 12:46 PM   #5
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I'm considering having a framebuilder replace the rear triangle on my vintage bike. I have a 1978 Trek 930 built with Columbus SP. The frame details can be found on this link;

http://www.vintage-trek.com/images/trek/Trek2pg3.gif

I would like to lengthen the chainstays from 415mm to 425mm and ovalize the chainstays to improve tire clearance. I would also like to shorten the seatstays by about 1.2mm to provide a lower BB height.

I would also like to drop the BB height from 7.2mm to 8.0. My goal is to improve stability and reduce standover height by a small amount.

I realize that I will be tilting the entire front triangle counterclockwise (when viewing the drive side) by a small amount. Will this provide any unintended consequences or changes in handling or stability?
Not trying to be picky, but looking at the linked geo tables and drawing, all of the dimensions are in centimeters.

Which model do you have? The BB drop on all but the 49 cm frame size 530/730/930 is already 7.7 cm, while the 510/710/910 with the 7.2 cm BB drop already has a 445 mm chainstay length.


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Old 02-03-13, 01:20 PM   #6
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Not trying to be picky...
Not to be picky, but your I find nothing of merit in your question.
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Old 02-03-13, 01:54 PM   #7
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Not to be picky, but your I find nothing of merit in your question.
OK, I'm trying to be helpful here. You want to increase the BB drop from 72 mm to 80 mm, but if the frame is a 530, 730, or 930, unless it's a 49 cm fame size it already has a 77 mm BB drop, so you'd only be gaining 3 mm by going to 80 mm. Since you say you want to both increase the BB drop from 72 mm to 80 mm and increase the chainstay length from 415 mm to 425 mm, that would indicate it's a 530, 730, or 930 since the 510, 710, and 910 all have 445 mm chainstays.

I was simply trying to figure out what you really need to do to get what you want, and since the information you provided was inconsistent with everything but a 49 cm 530, 730, or 930, I asked what model you have.

There was no reason to get snarky. I was trying to help.
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Old 02-03-13, 02:05 PM   #8
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OK, I'm trying to be helpful here. You want to increase the BB drop from 72 mm to 80 mm, but if the frame is a 530, 730, or 930, unless it's a 49 cm fame size it already has a 77 mm BB drop, so you'd only be gaining 3 mm by going to 80 mm. Since you say you want to both increase the BB drop to 80 mm and increase the chainstay length from 415 mm to 425 mm, that would indicate it's a 530, 730, or 930 since the 510, 710, and 910 all have 445 mm chainstays.

I was simply trying to figure out what you really need to do to get what you want, and since the information you provided was inconsistent with everything but a 49 cm 530, 730, or 930, I asked what model you have.

There was no reason to get snarky. I was trying to help.
The information provided is consistent for my frame. Are you a frame builder?
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Old 02-03-13, 02:13 PM   #9
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The information provided is consistent for my frame. Are you a frame builder?
Yes, but as an amateur. I've only built three.

The 930 specs state that a mix of SL and SP is used, with SP only used for 24" and 25 1/2" frames. Since you said the frame was made with SP, but didn't mention the frame size, it would appear to be a 24" or 25 1/2" frame so your BB drop is 77 mm, not 72 mm?
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Old 02-03-13, 03:21 PM   #10
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Mine measures 72mm, the catalog may not be 100% accurate do to a number of reasons.
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Old 02-03-13, 06:18 PM   #11
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Mine measures 72mm, the catalog may not be 100% accurate do to a number of reasons.
Bingo- The printed specs are for marketing purposes only. "Subject to change without notice" is the usual, and needed clause.

I do agree with John T about the practibility of this project. I dissagree with the claim that replacing/rebuilding the rear end is a fraction of the cost of a new frame. Well it is a fraction, but how small of the new frame is the debate. At $100+ per tube before paint I can see a $800+ (don't forget the drop out cost and any braze ons wanted) bill to get something that is still a comprise. Andy.
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Old 02-03-13, 07:49 PM   #12
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I wouldn't expect any significant variation from frame to frame on a Trek of that vintage. The jigs were set up for production and there were no adjustments
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Old 02-04-13, 12:05 AM   #13
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increasing drop, G, changes the angles of all the other tubes meeting the BB shell ,,

start from scratch with a drawing on paper, and have the frame built for you.
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Old 02-04-13, 07:31 AM   #14
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I agree that I should stick with the original BB height.

However, rebuilding the frame with a new rear triangle has merit. The stiffness of the frame and the ride and handling are close to ideal for me. The frame needs a repaint and having a vintage frame for some events is becoming a requirement.

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I wouldn't expect any significant variation from frame to frame on a Trek of that vintage. The jigs were set up for production and there were no adjustments
The issue is if the frame and the catalog are a match. I'll have the frame builder check the dimensions before proceeding.
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Old 02-04-13, 08:03 AM   #15
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Might consider just changing the wheels to a smaller diameter and slide in some 26"x1" tires. Would seem to do the same change in geometer and bb height without making major changes to the frame.

Second thought for you; When the bottom bracket is low, the pedal strikes during any serious turning is high. You might need to source a pair of 165mm arms also.

Hope that helps.
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Old 02-04-13, 08:22 AM   #16
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In order to compute the angle change, you need to know the front-centre. I'm confused as to what are your frame dims, but it should be around 600 mm. Dropping the BB 8 mm would then slacken the angles by 0.8 deg. It's significant but not excessive IMO.

BTW, I'm not sure how you can significantly increase clearance. Lengthening the stays will help, but there is a limit to which you can indent the stays (they likely already have a round-oval-round profile?)

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Old 02-04-13, 08:40 AM   #17
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In order to compute the angle change, you need to know the front-centre. I'm confused as to what are your frame dims, but it should be around 600 mm? Dropping the BB 8 mm would then slacken the angles by 0.8 deg. It's significant but not excessive IMO.

BTW, I'm not sure how you can significantly increase clearance. Lengthening the stays will help, but there is a limit to which you can indent the stays (they likely already have a round-oval-round profile?)
The frame is the 25 1/2" TX900. Yes, increasing the clearance by lengthening the chainstays only helps a small amount. The frame has enough clearance for a 27x1.0 tire at the chainstays now. I'm using a 700x28 tire, but would like to use a 700x30 Grand Bois tire, these measure closer to 700x32. I've been able to fit a 700x32 Vittoria Hyper tire on the front with good clearance, these measure 33mm wide. The 700x32 Vittoria Hyper tire clears the brakes and seatstays, but rubs the chainstays.

The original chainstays are round, not indented, and are 1mm minimum wall thickness Columbus SP, tire clearance is 32mm with zero safety factor. The plan would be to ovalize a NOS Columbus SP chainstay tube before assembly. The 22.5mm tube would become a 20x25 oval at the point of the tire sidewall. The added length of the chainstay in combination with the ovalized tube will provide about 6mm of extra clearance.

I'm planning on attending the L'Eroica which requires a vintage bike. Since the route includes dozens of miles of Strada Bianca (gravel road), a wider tire is very good to have.
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Old 02-04-13, 09:53 AM   #18
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Perhaps if you can find a Hetchins wannabe to do the work, they can lengthen the chainstays and drop the BB without altering the geometry.
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Old 02-04-13, 12:31 PM   #19
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Perhaps if you can find a Hetchins wannabe to do the work, they can lengthen the chainstays and drop the BB without altering the geometry.
Not a bad idea, but if the OP doesn't shorten the fork, that means an altered geometry. there is no problem lengthening the chainstays if a bb drop is desired. Even if it wasn't, the bb shell is easily blacksmithed into suitable angles.
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Old 02-04-13, 12:53 PM   #20
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In order to compute the angle change, you need to know the front-centre. I'm confused as to what are your frame dims, but it should be around 600 mm. Dropping the BB 8 mm would then slacken the angles by 0.8 deg. It's significant but not excessive IMO.

BTW, I'm not sure how you can significantly increase clearance. Lengthening the stays will help, but there is a limit to which you can indent the stays (they likely already have a round-oval-round profile?)
Tuz points out an approximate 0.8 degree slacking of HT and ST angles, not a lot.
With 55mm fork rake and 73 deg HT, trail is at 46mm, a bit short (for my taste) but nice for light front loads. The same rake at a 72.2 degree HT gives 51 mm of trail, still in a very good general range.

To recover the original 46 mm trail, increase the fork rake to 60 mm.
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Old 02-04-13, 01:00 PM   #21
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Not a bad idea, but if the OP doesn't shorten the fork, that means an altered geometry. there is no problem lengthening the chainstays if a bb drop is desired. Even if it wasn't, the bb shell is easily blacksmithed into suitable angles.
D'oh, that's right -- you simply have to work from both ends to drop the BB without altering the geometry.

Hmm, another thought from left field: how much BB drop could you get by using longer dropouts (and putting the rear wheel further back) instead of replacing/modding the stays? Is anything like the Campy 1010 dropout still available?
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Old 02-04-13, 01:08 PM   #22
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His bike no doubt has Campagnolo 1010 dropouts. There are some modern dropouts that are longer, for example the Paragon sliders would really add some length. Probably raise the bb shell though.
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Old 02-04-13, 03:26 PM   #23
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Just a followup thought; Might want to see if someone has a 910 but is wanting a 930 to have something a bit more sporty?

Both are outstanding silver brazed bikes with the 910 having longer the stays and long rake... the effect would be fairly close to what you are striving for and it would be a torch free effort!

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Old 02-11-13, 03:19 PM   #24
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The frame is the 25 1/2" TX900. Yes, increasing the clearance by lengthening the chainstays only helps a small amount. The frame has enough clearance for a 27x1.0 tire at the chainstays now. I'm using a 700x28 tire, but would like to use a 700x30 Grand Bois tire, these measure closer to 700x32. I've been able to fit a 700x32 Vittoria Hyper tire on the front with good clearance, these measure 33mm wide. The 700x32 Vittoria Hyper tire clears the brakes and seatstays, but rubs the chainstays.

The original chainstays are round, not indented, and are 1mm minimum wall thickness Columbus SP, tire clearance is 32mm with zero safety factor. The plan would be to ovalize a NOS Columbus SP chainstay tube before assembly. The 22.5mm tube would become a 20x25 oval at the point of the tire sidewall. The added length of the chainstay in combination with the ovalized tube will provide about 6mm of extra clearance.

I'm planning on attending the L'Eroica which requires a vintage bike. Since the route includes dozens of miles of Strada Bianca (gravel road), a wider tire is very good to have.
OP.... rec measure your CS's asyou may already have longest made which would be about 44.5cm
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Old 02-11-13, 08:06 PM   #25
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OP.... rec measure your CS's asyou may already have longest made which would be about 44.5cm
He says it's a 930 which should have 41.5cm chainstays according to the geo tables in post #5. I can understand why he wants longer chainstays.

His 930 should have a BB drop of 77mm, so going to 80mm will only give him 3mm (a little over .1") more drop. That's what I don't understand.
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