trail_monkey

This involves fork length but I figure an experienced frame builder will still have the knowledge to help. I built my rigid 650B hardtail 4 years ago using a Salsa cromto grande grande steel fork with a 45 mm rake and an axle to crown of 468mm. Since I PRIMARILY ride gravel,then some double track and rare but occasional singletrack I asked my wife for a new fork last year for Christmas. I wanted one with threaded mounts for my cages. I decided on a Salsa firestarter fork with 45 mm rake and an axle to crown of 485 mm. I called Soma to verify this was safe to use on my Bside frame and they said it was. I have been unhappy with the way this bike handles at low speed since adding this fork. It has too much wheel flop and if I am bike packing and have any weight on the front bars it is worse. At anything 15 mph or higher it is pretty stable. I used an angle app on my phone and after ensuring my garage floor was reading 0 degrees with my phone I used it on my headtube and got a reading of 68-69 degrees. I plugged my figures into a trail calculator on the yojimg website and got a trail reading of 92 for 68 HTA and 89 if it was 69 HTA. Last night I threw my old cromoto grande fork back on and took new measurements. My HTA changed to 70 with a 17 mm drop in axle to crown and my new trail reading on the calculator is 82. I took it out on the street to compare handling and although there was still some low speed wheel flop (this bike always had some) it was NOTICEABLY BETTER than it was with the firestarter fork.

I am contemplating selling both forks and buying something like a Surly ECR fork which is non suspension corrected. If I get the standard ECR 29 fork it has the same axle to crown (468mm) as my cromoto grande fork but it has a rake of 47 instead of 45. This fork plugs in a trail reading of 79 on the calculator. But I could also go with the ECR 27.5 fork from Surly which has an axle to crown of 447 mm and a rake of 43. This will give me a trail reading of 70 assumming a HTA angle change of another 2 degrees. But I am thinking that much of a drop in front end height might put my BB too low and I won't have any clearance if I get off road. Can an experienced frame builder recommend one of those forks and also tell me about how many degrees of HTA change you get with a given drop in fork length ( I want to believe that 17 mm netted me a change of around 2 degrees from my readings). Also how many mm of BB drop will you get with a given change in fork length?

trail_monkey

After doing more research I think I am going to stick with the ECR 29er fork. Now to see if I can sell my 2 Salsa forks. Didn't mean to post a bunch of jibberish.

unterhausen

it was a bit long-winded, but certainly not gibberish. Moving too far away from the original a-c has to be done with care. I wouldn't make the head angle steeper in most cases. Bike manufacturers seem to be wedded to 45mm rake for some reason. My All City has horrible flop, replacing the fork with one with more rake is on the list

fietsbob

VBQ readers are all over in a 'more rake, lower trail', fanclub for 650b French randonneur type bikes..

Nessism

I though more rake caused worse flop? One of my early home built frames use a similarly home built fork with 52mm of offset and that bike flopped quite noticeably.

trail_monkey

Nessism

Two bikes with the same amount of trail: 1) has a steep head angle and short offset and 2) has a slack head angle and more offset. In my experience bike 2 will have more wheel flop. I'm not exactly sure why.

unterhausen

is that really true? I'm not sure flop is all of the story, moving the tire contact patch further from the steering axis seems to make parking a bike more difficult, but not necessarily because of flop.

Not really questioning you, but I am interested in your examples. I know BITD with bikes with a lot of rake, the wheel actually flopping was annoying. It's not quite the technical definition of flop, which is the amount the top of the fork rises as you go from minimum height to center.