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Old 04-22-14, 11:18 AM   #1
rms13
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Will fork from current bike work with a new frame?

I am currently riding a Motobecane Super Strada from Bikesdirect:

Save up to 60% of new Shimano SRAM Apex 20 Speed Road Bikes | 2012 Motobecane Super Strada Road Bikes Sale | Save up to 60% off your next new Road Bike

I am considering buying a Soma Smoothie frame and transferring everything over. I would like to reuse the carbon fork from the Motobecane at least for now to keep the cost down. I cut the fork so there are no spacers so I don't have much room for error. Comparing the geo of the 54 cm frames it looks like the headtube length on the current bike is 10mm longer then the Smoothie 15 cm vs 140 mm:

Smoothie | SOMA Fabrications

http://www.bikesdirect.com/products/...mot_al_geo.jpg

I assume that means I should be fine on that but I know that rake should be considered as well. The Moto doesn't list rake on the geometry. Is there any way to be sure if the fork will work with the new frame?
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Old 04-22-14, 11:34 AM   #2
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There are four considerations for switching over a fork:
1. Is the steerer diameter the same?
2. Is the steerer long enough to fit the new frame?
3. Is front dropout axle center to crown race seat distance identical or close enough?
4. is the rake close enough to the same?

1 You have to measure. I expect both are 1-1/8" but be sure to check.
2. It seems you do have enough steerer length.
3. Ask Soma what the recommended AtoC dimension for their frame and measure yours.
4. Same thing. Ask Soma. Most road forks have 42 to 45 mm of rake with 43 mm being the most common.
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Old 04-22-14, 12:39 PM   #3
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The Motobecane chart does list the rake of your fork, but they refer to it as offset. 4.5cm
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Old 04-22-14, 12:43 PM   #4
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Originally Posted by HillRider View Post
There are four considerations for switching over a fork:
1. Is the steerer diameter the same?
2. Is the steerer long enough to fit the new frame?
3. Is front dropout axle center to crown race seat distance identical or close enough?
4. is the rake close enough to the same?

1 You have to measure. I expect both are 1-1/8" but be sure to check.
2. It seems you do have enough steerer length.
3. Ask Soma what the recommended AtoC dimension for their frame and measure yours.
4. Same thing. Ask Soma. Most road forks have 42 to 45 mm of rake with 43 mm being the most common.
Well discussed. Good summary of the situation.
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Old 04-22-14, 02:57 PM   #5
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Thanks for the advice. I looked on Bike Island website where they sell Bikesdirect parts. I found the fork that came on my bike and it is 43mm rake so I should be good to go
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Old 04-22-14, 03:20 PM   #6
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Thanks for the advice. I looked on Bike Island website where they sell Bikesdirect parts. I found the fork that came on my bike and it is 43mm rake so I should be good to go
Assuming the other dimensions are also right.
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Old 04-22-14, 04:47 PM   #7
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Assuming the other dimensions are also right.
hmm...axle to crown on my fork is 355mm and the matching one Soma sells is 364mm. Will that be too much of a difference?
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Old 04-22-14, 05:04 PM   #8
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hmm...axle to crown on my fork is 355mm and the matching one Soma sells is 364mm. Will that be too much of a difference?
It shouldn't be noticeable. The increased length will raise the front of your bike and slightly decrease the effective headtube angle. It will slightly reduce the responsiveness of the bike and slightly increase straight line stability. Neither change should be significant.
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Old 04-23-14, 09:29 AM   #9
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Actually, it's the other way around- re-using the old fork will cause the front end to be 0.9 cm lower than Soma recommends, increasing the head tube angle by a fraction of a degree and making the bike imperceptibly quicker in the turns. And increasing toe overlap by a tiny fraction and making the top tube slope just a tiny bit less.

The key is "imperceptibly". I doubt anyone could tell.
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Old 04-23-14, 08:07 PM   #10
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Actually, it's the other way around- re-using the old fork will cause the front end to be 0.9 cm lower than Soma recommends, increasing the head tube angle by a fraction of a degree and making the bike imperceptibly quicker in the turns. And increasing toe overlap by a tiny fraction and making the top tube slope just a tiny bit less.

The key is "imperceptibly". I doubt anyone could tell.
Whoops, thanks for the correction. I had the exchange backwards. At least we agree the change will be a non-issue.
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Old 04-24-14, 09:51 AM   #11
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Thanks for the advice. I looked on Bike Island website where they sell Bikesdirect parts. I found the fork that came on my bike and it is 43mm rake so I should be good to go
One word of advice: measure. My MB Fantom Cross Pro was supposed to have 45 mm fork rake, but it measured 50 mm. When I pulled the fork, sure enough the sticker on the steer tube clearly said it was a 50 mm rake fork. I don't know if it's a case of the manufacturer just substituting parts to fill orders, or websites not being kept up to date as production specs change. Either way, don't put a lot of faith in the specs on the websites.
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Old 04-24-14, 10:07 AM   #12
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i might be interested if the 9mm discrepancy would affect brake reach and/or cause problems. i suppose it could if the bike is already close to brake reach limits. IDK.
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