Utility bike project - need help with the forks please!

JEASTWOO Posts: 24
edited February 2012 in The workshop
I took my utility bike out for it's first trial today, having just finished spot welding the main frame togther. Purpose of this was to see whether it felt 'straight' and to check the steering. To check the steering I've taken the bike to shallow hill and given myself a push. Bear in mind it has no brakes or pedals yet and it has just snowed in England so I wasn't going fast, however I couldn't get it to go 'no handed', it wouldn't centre it's self at all. I tired loosening the head set in case that was too tight but no change, I gave it at least 3 attemps.. no go.

I measured the the trail by dropping a plumb line from the axle and marking the floor, then using a meter rule to find the point at which the steering axis intersects the floor, effectly measuring the caster (I think this is called the "trail") . I have 65mm of trail. I also measured the fork themselves, the drop outs are 44mm head of the steering axis (as accurately as I can measure), believe this is called the 'rake'.

So help required;
* Any one know what typical values would be? I can't change the rake but I could change the trail by altering the head tube angle.
* Do longer bikes like tandems and utility bikes need more or less rake/trail, or do they need to be going faster before they'll ballance themselves and go 'no handed'. Should I put a brake on and take it to a steeper hill!?

I've included a pic of my good friend Hennie holding the bike.

Any help gratefful received, thanks James Eastwood


  • nicklouse
    nicklouse Posts: 50,675
    What is the head angle?

    It looks very very slack.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Have you read or got 'Bicycle Design' by Mike Burrows? Very good and interesting reading. Maybe try running your forks backwards.
    FCN16 - 1970 BSA Wayfarer

    FCN4 - Fixie Inc
  • Thanks for replies. I have a caster angle of 70 degrees, and a Rake of 45mm.

    The forks and head tube that came off the original Trek frame has a Rake of 41mm, and a similar drop out to crown height. I was hoping to find some thing more radically different.

    I read the Wikipedia articles on the "Bicycle Fork" and "Bicycle Geometry", which is interesting but doesn't give any actual recommendations. So I'm still after some good 'known' values of Trail, to compare to.
  • nicklouse
    nicklouse Posts: 50,675
    not what i asked. what is the head angle? as if by caster angle you mean head angle it looks nothing like 70 degrees.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Caster angle is commonly used to describe any steering axis angle to the horizontal, but I can refer to it as head angle no problem. I think you're visualy looking at the fork blade angle which it's self is further inclined from the head angle. I measured it with a spirit level and protractor, and repeated the measurement several times, I'd guess I'm within +/- 1/4 degree. I also checked the floor was level, before any wise cracks ask how steep was the hill I was standing on!

    I've found a point of reference, the Surley Big Dummy has a similar wheel base with a 42mm Rake and a 72 deg head.

  • Just in case there's anyone still awake whilst I discuss the merits of steering angles etc..

    I put a brake and gears on the bike last night and took it for a faster (& seriously cold -4C !) test run. Good news is that it will go 'no handed' it just needs to be going quite a bit faster before it gets stable. It also goes straight which means that every thing is still in line having taken it off the jig.

    There are some useful geometry formulaes on wikipedia under 'cycle forks' and 'cycle geometry' which can calculate Trail based on knowing Rake and head angle, and also introduce a new factor called 'wheel flop'. Using these I have calculated that I need to reduce my head angle from 70 to 71.5 degrees to create the same Trail as the Surely Big dummy, so this is the plan to undo the spot welds move my front axle jig point rearwards by about 9mm and re-weld. I've checked that the front wheel won't come too close to the pedaling feet, looks like I can still use a mudguard.

    I'll report back for anyone that hasn't dozed off!
  • nicklouse
    nicklouse Posts: 50,675
    right we are talking the same language. no i was not löooking at the fork blades. the head angle does look V slack and not in the 70s.

    are using rake as the Offset measurement?

    what is the AC length?

    Or what soft ware did you design it in? can you provide an iges file I can have a look at?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Nick, thanks.

    Having found the geometric formulae for Trail and 'wheel flop factor' from wikipedia I set up a spread sheet to compare some values, see attachement. I compared the Surley Big Dummy, as my project is the same length, and the Dummy geometry is available on line. Surley have used a head angle of 72 to get a trail of 63mm. I then modified my geometry from 70 to 72 deg which reduced the trail from 72mm to 60mm. I rode this with pedals and it felt better and would go 'no-handed' at a crusing speed (18mph-ish). Good result.

    I looked again at the Surley bikes and what is clear is that their Head angle/Trail/Rake combination on the Big Dummy has simply been carried over from their other range of bikes, and is not specifiic to a longer bike. For a given cornering radius a longer bike needs more steering (handle bar) angle, this would suggest that on a longer bike a little less steering stability is what is required, to counter the stability the length gives.

    With this in mind I've then shortened the head angle again this time from 72 to 73.5 degree giving a Trail of 51mm, which is about a far as I can go any way before risking clearance issues with the feet and the front wheel if a mud guard is fitted. The result was even better, it will now go 'no handed' from about 12-15mph.

    In answer to your question I used the http://www.bikecad.ca program, although this was more to see what the overal bike would look like, with different amounts of 'stretch' , to gauge the handle bar position and what size stem I 'll need.

    Plan is now to continue with the welding, and make up the rear carrier, should I post the progress?
  • Mr Plum
    Mr Plum Posts: 1,097
    JEASTWOO wrote:
    ...should I post the progress?

    For what it's worth I'm finding your posts on this project really interesting, I'd certainly appreciate any further 'progress update' type posts.
    FCN 2 to 8