Fork Length

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11/10/2019 9:17 AM

I get on here sporadically so if this topic was already covered, pardon me.
Fork Length is a geo factor I don't hear covered in a lot of discussions about fork performance. That would be the length from the crown to the center of the front axle.
The standard seems to be 315mm but I've noticed some have 317, 320 and even 324 lengths.
If math works, then theoretically this would both raise your front end (and to a lesser degree your B and also make the HTA more slack. I dont know how much more hta slack 5mm of fork would give you but can imagine it would be noticeable.
Chad Kerleys sig fork has a 320mm length and with a 75° HTA, I believe leaves him with more of a 74.something hta compared to the same bike with a 315mm fork length.
Has anyone studied this and have any info on how this affects bmx performance or why S&M would make a 13mm offset fork and then add more height that slackens the HTA? It even why companies make higher forks?

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11/10/2019 9:40 AM

Most brands do not give us this info, do you know how long are odyssey forks ?
I like slacker head angle with “steep” forks more than the opposite
Let’s say 74.5° + 25 offset would theoretically feel the same as 75° + 28mm but it’s not.
Playing with crown to axle length might balance it out.

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11/10/2019 11:24 AM

Francky wrote:

Most brands do not give us this info, do you know how long are odyssey forks ?
I like slacker head angle with “steep” forks more than the opposite
Let’s say 74.5° + 25 offset would theoretically feel the same as 75° + 28mm but it’s not.
Playing with crown to axle length might balance it out.

After measuring my r32's when height always felt tall compared to my other forks they turned out to be 320mm.
Others I found at 320mm are wethepeople battleship forks, some eclat, S&M widemouth pitchforks (even though some sites advertise as 315) Mutiny Wands, and im sure theres more.
I can say they make a big difference in how a set up feels by just changing to a fork with a taller or lower crown. Also makes dialing in bar height a pain in the a$$ since both fork height an BB height affect that.

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11/10/2019 12:19 PM

Francky wrote:

Most brands do not give us this info, do you know how long are odyssey forks ?
I like slacker head angle with “steep” forks more than the opposite
Let’s say 74.5° + 25 offset would theoretically feel the same as 75° + 28mm but it’s not.
Playing with crown to axle length might balance it out.

Odyssey RSeries forks are 315mm.
The Colony Sweeth Tooth forks are the same weight and Offset as the R25 but are 317mm instead of 315 so theoretically would raise your front end a bit.

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11/10/2019 12:23 PM
Edited Date/Time: 11/10/2019 12:25 PM

Francky wrote:

Most brands do not give us this info, do you know how long are odyssey forks ?
I like slacker head angle with “steep” forks more than the opposite
Let’s say 74.5° + 25 offset would theoretically feel the same as 75° + 28mm but it’s not.
Playing with crown to axle length might balance it out.

Team who cares wrote:

After measuring my r32's when height always felt tall compared to my other forks they turned out to be 320mm.
Others I found at 320mm are wethepeople battleship forks, some eclat, S&M widemouth pitchforks (even though some sites advertise as 315) Mutiny Wands, and im sure theres more.
I can say they make a big difference in how a set up feels by just changing to a fork with a taller or lower crown. Also makes dialing in bar height a pain in the a$$ since both fork height an BB height affect that.

Which do you prefer and why? I dont have enough experience with the different heights to really know. I was on r25s for more than 2 years and the Process Forks now, both with the same height. I'm just wondering if there's a reason they make them different heights or why they'd do it.

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11/10/2019 12:32 PM
Edited Date/Time: 11/10/2019 12:34 PM

Just realized that while we're on it, the height a frame manufacturer uses on their frame jig will also greatly affect how the frame feels/performs when on a fork. If the frame jig has a template that uses a 320mm fork as the reference to the ground and the frame goes on a fork with a 315mm height, then that will theoretically change the functional HTA. Could that be why some companies prefer 320mm forks because their frames are built on templates that use a 320fork as a ground reference.
Can anyone building frames in here confirm or clarify?

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11/10/2019 12:36 PM

I’d look into mtb discussions with different fork offsets. There’s a lot of good information if you really want to get into it

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11/10/2019 2:38 PM

A2c isn’t really thought about in bmx. There’s a few other things no one seems to care about. Yet tt length, cs length etc everyone wants to be exact. We over think geometry

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11/10/2019 3:01 PM

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11/10/2019 3:42 PM

Wow, didn't even know this was a thing.

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11/10/2019 3:45 PM
Edited Date/Time: 11/10/2019 3:49 PM

Coopers25 wrote:

I’d look into mtb discussions with different fork offsets. There’s a lot of good information if you really want to get into it

I mean maybe but I'm asking in a bmx forum because they're questions about bmx products and fork offset isn't really what I'm asking to know about either here. Could there be a bmx frame building forum or page on it?

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11/10/2019 4:10 PM

Would these actually hold up to street use, or is this just a flatland only thing?

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11/10/2019 11:27 PM

I actually bought this up a while ago. If you think about it, shorter offset forks would have a longer a2c, to accommodate the wheel and wide tyres underneath the crown. Which is like standing on a box and crouching. Bmx geometry is basically just following trends without giving true info on how it affects the bike. Short offset is cool rn so bike brands will keep making it shorter. The a2c doesn’t get mentioned because they either don’t want customers to know it slackens the headtube, or they just don’t understand geometry properly.
But it’s supply and demand. While people lap up certain angles and lengths, brands will keep pushing them.
It reminds me of frames when 13” cs and wide tyres first came around. A load of brands made frames to meet those requirements, but you could not get a good chainline

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11/16/2019 7:15 PM

.pegless. wrote:

I actually bought this up a while ago. If you think about it, shorter offset forks would have a longer a2c, to accommodate the wheel and wide tyres underneath the crown. Which is like standing on a box and crouching. Bmx geometry is basically just following trends without giving true info on how it affects the bike. Short offset is cool rn so bike brands will keep making it shorter. The a2c doesn’t get mentioned because they either don’t want customers to know it slackens the headtube, or they just don’t understand geometry properly.
But it’s supply and demand. While people lap up certain angles and lengths, brands will keep pushing them.
It reminds me of frames when 13” cs and wide tyres first came around. A load of brands made frames to meet those requirements, but you could not get a good chainline

Whoa.. that's true about the a2c range changing with offsets. I can see how that would make it more complicated for someone unfamiliar with how it works.. I know I saw one zero offset fork with like 124mm length and couldnt figure why they'd add the height if they wanted it steeper. That actually makes alot of sense. Tx!

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11/20/2019 7:55 AM

This is interesting

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