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help with changing to a new rake/crown-axle measurement for my custom frame

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help with changing to a new rake/crown-axle measurement for my custom frame

Old 08-25-12, 03:02 PM
  #1  
dspaulson
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help with changing to a new rake/crown-axle measurement for my custom frame

I have a custom frame that was designed around a form (Easton EC90SLX 43mm rake 360mm AC measurement) and I'm looking to swap over to an ENVE 2.0 with a AC measurement of 367 and various rake options. I'm leaning toward 43 or 40. I understand the implications of changing rake when all things are equal, but given the AC crown measurement is 7mm longer on the ENVE, how will this change the dynamic? Seems like it will slacken out the bike if I go with the same rake, so it might make it even more appealing to go with a 40mm rake to balance out the extra length. Am I thinking about this correctly? Also, will 7mm of additional fork length make a noticeable front height difference with an equivalent rake?
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Old 08-25-12, 06:08 PM
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What I suggest is either plot it on graph paper, or find a primitive 2D CAD program. Basically you will elevating the head tube by 7mm which will fractionally slacken the HTA and slow the steering down. So directionally to compensate you would seek less fork rake to reduce castor and speed the steering back up.
If you are adept at trig, you can calculate this...or plot it and analyze the fork rake you seek with a taller fork...net wash maybe only a deg of rake reduction but you would need to calculate it.
It also depends directionally which way you want to take the steering of the bike. For example I ride a Roubaix which natively has a slack hta and more fork rake and I love the rock solid handling. Your preference maybe for a quicker handling bike.
Good luck with your custom.
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Old 08-25-12, 07:33 PM
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Any way you can discuss with the framebuilder??
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Old 08-25-12, 09:40 PM
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You want more rake to offset the increased trail caused by lifting the headtube (slackening the HTA) . ENVE fork with 45mm rake will likely have similar geometry to the Easton fork with 43 mm rake. With the Enve fork you will be lengtening the wheelbase also. You can use https://www.bikecad.ca/ to plug in all the numbers and see the actual differences.
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Old 08-26-12, 04:57 AM
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Originally Posted by vettracer View Post
You want more rake to offset the increased trail caused by lifting the headtube (slackening the HTA) . ENVE fork with 45mm rake will likely have similar geometry to the Easton fork with 43 mm rake. With the Enve fork you will be lengtening the wheelbase also. You can use https://www.bikecad.ca/ to plug in all the numbers and see the actual differences.
Perhaps counter intuitive to me at least and thanks for setting the record straight. I would think that slackening the head tube with a longer fork would increase wheel base and naturally slow steering and reducing fork rake would restore handling. But you may indeed be correct.
Also, thanks for the link for bike cad.
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Old 08-26-12, 07:00 AM
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The increase in wheelbase is minimal, compared to the change in the HTA. Here's a link to an explanation.

https://www.sheldonbrown.com/rinard/forklengths.htm

If you put the change of HTA into the formula for trail, you need 3mm more fork offset. The trail formula is R/tanH - (rake/sinH).
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Old 08-26-12, 09:27 AM
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Thanks for all the guidance guys. I was actually able to track down the original CAD, and it turns out there is a discrepancy in the measurements. The frame was actually designed around a 367mm AC fork, but according to what I'm seeing, the Easton is 360. So interestingly enough, I've been riding a bike that's "out of spec".

All that said, I'm going to use the advice everyone provided to sketch out the differences that I can anticipate.
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