An Experiment In Front End Geometry: Fork #1

Now moving on to the first fork I used on the Blackbuck other than the baseline On One Carbon fork, which is the Blackbuck fork that is offered along with the frame from OS Bikes.

Blackbuck frame/fork

The Blackbuck fork is a radical departure from conventional thinking. Paired up with the Blackbuck frame you get these numbers:

Head Angle: 74 degrees
Axle to Crown: 440mm
Offset: 51mm
Wheelbase: 42 7/8ths
Bottom Bracket Drop Range (EBB) 74mm-85mm
Approximate Trail*: 55mm

Changes Made: The necessary changes made to accomodate my saddle to bottom bracket relationship and my saddle to bar relationship were as follows:
-Moved saddle backwards approximately 20mm.
-Used a total of 50mm of spacers and cut the steer tube longer than normal
-Changed to a 20mm shorter stem

Handling Characteristics: By the numbers, a lot of folks would say this should be scary! In reality, it was an eye opener for me. I was not thrown by the handling at all. It was a bit quicker than it had been, but not twitchy, nervous, or anywhere close to unrideable. Quite the contrary. I found it to be a bit challenging in slow speed manuevers, and descending steeps made me feel as though I really needed to get back over the saddle. The front end felt lower and pinned to the trail. Curves were carved with no feeling of washing out. Initiating a turn was was a subtle act. Strong arming this set up was not a wise thing to do. Lines were switched telepathically. In fact, it was a bit nervous in longer descends and I would venture a guess that it would be a mentally taxing thing to descend at speed for very long with this set up.

Two things that bear mentioning about this fork and frame combination. #1: It rides fantastically smooth with this fork. Something about the frame comes alive with the Blackbuck fork on board. The whole bike feels even more like a steel bike than it did before. Almost an amplification of that “steel feel”, if I can put it that way. #2: It struck me as I was riding it that those with fine motor skills would probably get along just fine with this combination. You really needed to be still on the bike. Weight shifts and hand pressures were minimal but would yeild big dividends in turns, climbs, and manuevers. The only thing that was harder to do was to get the front end up. I think the lower front end and steeper head angle were to blame here.

Close up of the fork

A Note On The Forks Ride: The Blackbuck fork is a pretty smooth steel fork. Even at 440mm axle to crown, it doesn’t rattle your teeth out too badly. Hey! It is still a rigid fork though, so don’t get any ideas! My only beef is that it seemed to flex a tad laterally. Enough so that I could get the rotor to rub my pads at times. A bolt on hub or one of those fancy new locking skewers may take a bit of that away. The big thing for me was the combination of this fork and the frame. Together they make a great pairing in terms of ride feel.

Final Thoughts: This should have been an unrideable combination, if you listen to conventional wisdom. In fact, it is a very fun, albeit quite quick, handling package. As an example, I loaned it out to a friend as we rode the trails one day. He really enjoyed the bike and noted the quick handling, but got along just fine with it. We are pretty adaptable as mountain bikers. Just looking at numbers and making judgements isn’t a good way to analyze a bikes ride. This Blackbuck fork was a testament to that. It was a fun combination, and I’ll be revisiting it someday after this test is complete.

Next: I go to the other end of the spectrum with a Bontrager Race X Lite Switchblade fork.

*The trail charts I used all gave slightly different answers and of course, your tire selection will also affect the trail figure slightly. Take my trail figures with a grain of salt. Your mileage may vary!


No Responses to “An Experiment In Front End Geometry: Fork #1”

  1. Dave Says:

    How is the head angle of the frame changing from 72 (baseline) to 74 (Fork #1)??? That’s a fixed angle once the frame is welded. I can see trail (rake) changing when the fork changes, but the frame’s head angle is a constant, not a variable.

  2. Guitar Ted Says:

    Dave: That’s easy. I shortened the fork length by 30mm, which makes the frame “pivot” downwards toward the ground on the front end from the rear axle. Think of it this way: Take your bike and measure the head angle. Now, let the air out of your front tire and measure it again. You’ll see a steeper head angle than before. That make sense?

  3. Mike Says:

    Back in 1989 Joe Breeze built a bike with (no not 29″ wheels) a 78 degree head angle and a no-offset fork that resulted in a similar amount of trail to his “standard” bike. The report said it took a little getting used to, but rode just fine. It went on to further say getting back on his standard bike felt weird after getting used to the 78 degree h/a bike. I honestly believe that we, as humans, can adapt to just about anything and when we get muddled up in the details we can’t see the bike for what it is – fun. But this is a fun exercise seeing that what can’t be done, can be done. Looking good GT.

    I’ll take some more space here to report on a conversation I had a couple of weekends ago with a custom frame builder. He was attending the NAHBS when a show attendee asked him what trail dimension he used on his bike. The builder had no idea. Never thought about trail. Always knew that a certain head angle with a certain fork offset (both within a given range) resulted in a good riding bike. The attendee couldn’t believe that this frame builder who’d been building frames for 30 years didn’t know what the trail was on his frames. The builder told the attendee that he doubted that the two other frame builders next to him at the show knew what the trail was on their bikes. They asked and both said “I don’t know.”

    I don’t know what that story exactly means here, but it gives me a chuckle.

  4. Mike Says:

    Oh, and here’s a link to a scan of that Breeze head angle story written by Charlie Kelly

  5. Garrett Says:

    55mm trail may be unconventional for mountain bikes, but I think that’s right around average for a road bike so it wouldn’t be totally unrideable, just not what you’d expect. Loving the test so far Ted, keep up the good work.

  6. Guitar Ted Says:

    Mike: Great story about the frame builders and thanks for that Breeze link. Very interesting! I always thought it was more about just riding, but it’s more about just riding than I thought. If that makes any sense! πŸ™‚

    Garrett: Yeah, Thanks! I’m having fun with this. (Of course, I get to ride a lot! πŸ˜‰ )

    Makes sense that if road bikes can get away with it, why not a mountain bike? Gary Fisher told me a few years ago, “Now we can all tune our rides”. in referance to being able to get other than 38mm off set suspension forks for 29″ers. I think it’s great and more folks should check it out. Hopefully, this test will inspire some of you folks to do just that. πŸ™‚

  7. Steve Says:

    What are you using to get your angle measurements? Digital (good), gravity (so not good), or geometry (math is hard)

  8. jeremy uk Says:

    I’ve always thought that one inch of drop on the front fork roughly equates to one degree. Given that general rule surely the head angle is nearer 73 degrees?

  9. Guitar Ted Says:

    Steve: I’m using a Craftsman pendulum protractor, which gets me as close as I need for this comparison. I measure three times to check my findings, so they are pretty accurate. If you can feel a few tenths of a degree difference in head angle, well then I applaud your sensitivity. πŸ˜‰

    jeremy uk: Nope. 74 degrees. πŸ™‚

  10. Mike Says:

    …and if you use the same method to measure each time, you will get results that are consistent for a comparison test.

  11. Dave Says:

    Sorry… been away a while. Yes, Ted, that makes sense now.

  12. Dave Says:

    Man… after reading part 2, I think the quicker Blackbuck fork numbers would suit my style better than either of the two carbon offerings. I’d prefer the bike to be quick and my inputs subtle, not ham-handed. Very interesting.

    I’d be more than happy to test a Blackbuck fork on my MC29er. πŸ™‚

  13. Steve Says:

    Why not commission that Walt fork or have a better ( adj. A-C and rake) fork built fore this test? I bet Walt’s fork is just sitting, collecting dust.

    And what aspiring framebuilder wouldn’t want to donate such a fork in exchange for some publicity?

    Any attempt to compare geometry is great, but the ability to change a single dimension at a time would encourage greater understanding.

    (My LBS/IBD told me so)

  14. Guitar Ted Says:

    Steve: My aim in doing this is to help those that might be considering “tuning their rides”, not an exercise in the effects of different geometry tweaks.

    Not everyone can have a fork like Walt Wehner’s test fork, but many of you can and are doing real world fork swaps with commonly available forks and frames. That is why I am using the forks I am posting about. πŸ™‚

  15. Dave Says:

    I do appreciate it, Ted. You know, I thought about this stuff a lot when I was selecting my road frame/fork. I even went to a 43mm rake from stock 40mm rake (steel Schwinn) to quicken the already quick handling.

    I guess I was conditioned from my front-sprung days on the MTB where the rake/offset “was what it was” when you got a suspension fork. But this series of posts got me thinking again that the rigid setup is quite tweakable.

    So a Salsa Cromoto Grande is on its way with its 43mm of rake to replace my stock MC29er’s 38mm rake. πŸ™‚

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