How much fork could a bike fork fork, if a bike fork could fork bikes?
March 1, 2015 10:31 AM   Subscribe

I'm building my dream bike, and I need help understanding forks, specifically fork rake and axle-to-crown distance, and how those variables will change the bike's handling for better or worse.

So far, bicycle-centric forums have been really unhelpful with this question; lots of conflicting opinions floating around about this stuff. I've assembled bikes before from scratch, but I've never deviated in terms of swapping out a fork for something different. Every frame-set I've built up has always had the frame and fork included.

The bike in question is a fairly boring build (that I purchased used) of a Surly Steamroller. I really like this frame as its set up; I don't have any real complaints about the geometry of the bike. Its easily my favorite bike that I've owned and it handles like a dream, and it fits me oh-so-perfectly like no other bike i've owned. I've got a serious crush on it. Something about the geometry about this bike feels really great on me, which is why i'm obsessing a little bit over switching out the forks. Due to a combination of aesthetic choices (faux bois woodgrain rims that don't have brake pads…mmmmmm) and performance desires, I'm looking to swap out the fork for one that accepts a disc brake.

My non-negotiables:
-Disc brake compatible
-Eyelets for running a small-to-medium front porteur-style rack.

My wish list:
-Lugged forks, for aesthetics would be preferable, but not a deal-breaker.

Every other option is up for grabs. So far I've been limiting my search primarily to Surly forks, because it seems like the easiest known way to swap out the fork…but if there's another magical perfect fork out there, that fits my criteria I'd love to know about it. As far as Surly forks go, I'd be swapping out the regular Steamroller fork to either a LHT disc fork, or a Straggler fork

This is where I start running into problems figuring out which one will work best. I know that none of them are identical to the stock fork. But I don't know how these dimensions will alter the ride.

The Steamroller has a 38mm offset/rake, where the Straggler has a 44mm offset/rake, and the LHT has a 45mm offset/rake. Axle to crown distance. Stock fork is 375mm, Straggler is 350mm, and the LHT is 390mm.

To be clear, I know full well what all the measurements are, I just don't know how they'll change the handling of the bike, or otherwise break the feeling of my currently awesome ride.

Will any of these forks work well for my requirements, or am I really looking at the option of getting disc mounts welded to the stock steamroller fork (which I'm open to, but seems like an even more nebulous prospect).
posted by furnace.heart to Travel & Transportation (2 answers total) 2 users marked this as a favorite
Less offset is going to make the bike a bit more twitchy. More is going to feel more stable and then more sluggish in turning.

Hight AtC is going to raise your head tube, which after a point is also going to increase rake.

I'm running a Troll fork with discs on my frankenbike gravel grinder and it can accept 700s, though it was meant for 26". I would pick one, try it, and if you don't like it unload it on eBay or Craigslist and try something else.
posted by spikeleemajortomdickandharryconnickjrmints at 11:18 AM on March 1, 2015

Given the same head angle, more fork offset will decrease trail. (You can figure out how much exactly using this trail calculator).

If you listen to, e.g., Jan Heine/Bicycle Quarterly, lower trail is supposed to be helpful when carrying a front load, which it sounds like you plan to do if you are installing a porteur rack.

My personal experience has been with a Cetma 7-rail porteur rack—a very big, very heavy rack. When I first bought the rack I had a bike with around 60mm of trail (fairly high), and the bike didn't handle very well even with the empty rack mounted. With a significant load on the rack it was almost unrideable.

Since then I've gotten an older Japanese bike with lower trail (I don't know the exact figure), and it's a completely different experience. It rides better with the rack than without it, and I have done camping trips with 20 pounds on the front rack and it's handled great. (On shorter rides I've carried even more weight.)

On preview: spikeleemajortomdickandharryconnickjrmints has forgotten more about bikes than I'll ever know, but my understanding was kind of the opposite: that generally lower trail (meaning higher fork offset, given the same headtube angle) was considered "twitchier" and higher trail more stable (but sluggish in turns).
posted by enn at 11:27 AM on March 1, 2015

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