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Suspended to Rigid: How Important is Sag?

Old 07-31-19, 05:00 AM
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J.Higgins 
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Suspended to Rigid: How Important is Sag?

So I bought this Marin 17" San Anselmo frame for my wife. She wants a bike with wider and taller tires for rail trails and unpaved roads.

I'm going to build this frame as a 1x, and with a rigid fork. Since I've never done a suspended-to-rigid conversion before, I've done some research about it, but now I've come up somewhat confused. My major question is, when choosing the correct axle-to-crown length on a rigid fork, how important is factoring sag of the original suspended fork? Is it THAT important?

What about bikes that have lockable forks, and people ride them locked? Is the geometry change THAT big of a deal? From everything I've read, the sag should be factored at 15-20%. Has anyone run across this dilemma?

These frames where apparently held as replacements for warranty issues and now they are up for sale cheap, so I bought one to give it a go. I had to do a bunch of searching for what sort of fork the original bike came with, and SR SUNTOUR NEX SUSPENSION FORK: 700C, 1-1/8" THREADLESS STEERER, 63MM TRAVEL, LINEAR PULL, BLACK is what I found. The axle-to-crown length on this fork is 477mm. If I subtract 15%, that makes it 405mm. I could really use some advice from someone who has been down this road before. Thanks for reading, and I'm looking forward to your comments!


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Old 07-31-19, 05:32 AM
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Originally Posted by NoControl View Post
and SR SUNTOUR NEX SUSPENSION FORK: 700C, 1-1/8" THREADLESS STEERER, 63MM TRAVEL, LINEAR PULL, BLACK is what I found. The axle-to-crown length on this fork is 477mm. If I subtract 15%, that makes it 405mm. I could really use some advice from someone who has been down this road before. Thanks for reading, and I'm looking forward to your comments!
Subtract the % of the 63mm of fork travel, not off the A/C length.

Example:
A suspension fork with 500mm A/C measurement and 100mm travel.
If you ride with 20% sag, the axle to crown measurement will be 480mm
If you replaced the suspension fork with a suspension corrected rigid fork, it should have a A/C measurement of around 480mm (it doesn't have to be exactly that A/C measurement).

Mountain bike forks are often ridden with sag of 20 to 30%


Example:
29" fork with 100mm travel and A/C measurement of 510mm

https://manitoumtb.com/product/machete-pro/?cat_id=23

25% sag equals 485mm A/C measurement.

Here's a suspension corrected fork with 485mm A/C measurement to replace a 29" fork with 100mm travel

https://www.somafab.com/archives/pro...scv-canti-fork


I'd say you would need a fork with about with a A/C measurement of around 460mm.
.

Last edited by cobba; 07-31-19 at 06:00 AM.
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Old 07-31-19, 06:03 AM
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Originally Posted by cobba View Post
Subtract the % of the 63mm of fork travel, not off the A/C length.

Example:
A suspension fork with 500mm A/C measurement and 100mm travel.
If you ride with 20% sag, the axle to crown measurement will be 480mm
If you replaced the suspension fork with a suspension corrected rigid fork, it should have a A/C measurement of around 480mm (it doesn't have to be exactly that A/C measurement).

Mountain bike forks are often ridden with sag of 20 to 30%


Example:
29" fork with 100mm travel and A/C measurement of 510mm

https://manitoumtb.com/product/machete-pro/?cat_id=23

25% sag equals 485mm A/C measurement.

Here's a suspension corrected fork with 485mm A/C measurement to replace a 29" fork with 100mm travel

https://www.somafab.com/archives/pro...scv-canti-fork


I'd say you would need a fork with about with a A/C measurement of around 460mm.
.
I came up with 464mm, so I think that you are correct. A few millimeters either way should work fine.
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Old 07-31-19, 06:16 AM
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Rim brake or disc brake?

Might be hard finding a rim brake fork with that A/C measurement.
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Old 07-31-19, 06:28 AM
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Originally Posted by cobba View Post
Rim brake or disc brake?

Might be hard finding a rim brake fork with that A/C measurement.
Disc, thank God.
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Old 07-31-19, 06:37 AM
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Originally Posted by NoControl View Post
I came up with 464mm, so I think that you are correct. A few millimeters either way should work fine.
Here's one at 468: https://www.jensonusa.com/Salsa-Cro-Moto-Grande-29-Fork

The QR version is priced at $130.

FWIW, my math agrees w/your math.

Also, IMHO, go taller but never shorter. I've done several of these conversions and no one has ever complained about an extra centimeter or two of axle-to-crown. Most older frames benefit from being slacked out a bit anyway.

Salsa and Surly both have a wide range of forks in various axle-to-crown numbers. I used one of their other fork models on a rigid, 650b build last fall.
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Old 07-31-19, 06:42 AM
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468: Bikeman Salsa Cro Moto Grande 29" Fork, Disc Only Black
465: Bikeman Kona P2 29er Disc Fork, Matte Black
465: https://www.universalcycles.com/shop...7&category=703
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Old 07-31-19, 06:45 AM
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Originally Posted by JonathanGennick View Post
Here's one at 468: https://www.jensonusa.com/Salsa-Cro-Moto-Grande-29-Fork

The QR version is priced at $130.

FWIW, my math agrees w/your math.

Also, IMHO, go taller but never shorter. I've done several of these conversions and no one has ever complained about an extra centimeter or two of axle-to-crown. Most older frames benefit from being slacked out a bit anyway.

Salsa and Surly both have a wide range of forks in various axle-to-crown numbers. I used one of their other fork models on a rigid, 650b build last fall.

Thanks!

The Salsa Firestarter fork is 483. Do you think that's okay, Jon? She'll be riding this bike with the Jones Loop 1.5, and as bolt-upright as she can possibly get (don't ask).
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Old 07-31-19, 06:48 AM
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Originally Posted by JonathanGennick View Post
FWIW, my math agrees w/your math.
I went off 25% sag just rounded it up, a couple mm's here or there won't make much difference.
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Old 07-31-19, 07:38 AM
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Originally Posted by NoControl View Post
The Salsa Firestarter fork is 483. Do you think that's okay, Jon? She'll be riding this bike with the Jones Loop 1.5, and as bolt-upright as she can possibly get (don't ask).
Please don't hold me to it, because you can't really know until you put all the parts together. Even something like the Jones bar on that frame is an unknown until you bolt the parts together and test ride.

FWIW, I once ran a Salsa Fargo v1 fork on a Priority Eight bicycle. That represented close to 40 mm over the stock axle-to-crown. The bike rode fine. Going no hands was actually somewhat easier for me.

Forks are expensive. When I build a bike, I usually have it in mind that I might not like all my first part choices, that I might need to buy something twice.
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Old 07-31-19, 12:51 PM
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Originally Posted by JonathanGennick View Post
When I build a bike, I usually have it in mind that I might not like all my first part choices, that I might need to buy something twice.
I'm right there with you, brother. I've got boxes of stuff that either didn't work with something, or didn't fit right. Try try again, right?
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