Viewing 13 posts - 1 through 13 (of 13 total)
  • Fork length and offset and their effect on handling. Advice please.
  • I’m still looking at options for a tandem build using a frame with a 73 degree head angle.
    The frame is suitable for 26″ OR 700c wheels. I haven’t got any forks for it yet.
    The intended 26″ forks are 402mm A to C with 45mm offset.
    I don’t know the dimensions for the 700c forks.

    I’m thinking of using a 20mm front axle, which limits the choice of rigid forks.
    Identiti do a 425mm or 465mm 20mm axle rigid fork with 36.7mm offset.

    If I was to use one of these, how would it affect the geometry if used with a 700c wheel ?

    Slightly longer, less offset and a bigger wheel. What would the combined effect be ?

    ratherbeintobago
    Full Member

    Will this trail calculator help any?

    More trail = more stability but less willingness to change direction.

    cynic-al
    Free Member

    Why 20mm? Suss fork compatability?

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    Thanks, ratherbeintobago, that’s a useful calculator.

    Al, no I want a rigid fork and want it to be er… rigid.
    Reading anecdotes on various forums, 20mm is almost universally regarded as noticeably more rigid than 5mm or even 9mm QR, so it sounds like a good idea for a tandem.

    Putting the numbers in that calculator, I’d end up with a trail of 68mm with the 425mm fork or 74mm with the 465mm fork.
    That sounds quite a lot. Would it make the tandem go past the point of “more stable” to “unmanouverable” ?

    scotroutes
    Full Member

    I thought you’d get a better choice of 15mm axle forks these days.

    RobHilton
    Free Member

    Reading anecdotes on various forums, 20mm is almost universally regarded as noticeably more rigid than 5mm or even 9mm QR

    Are they discussing sus forks or rigid?

    From: http://waltworks.blogspot.co.uk/2011/01/trying-new-things.html

    Question: What’s the benefit of a 15mm on a rigid fork?

    Walt said…

    The benefit is that for many people, their only front wheel is a 15mm one (and they’re not all convertable) so if they want to swap from a rigid fork to a suspension fork, this is the easiest way to do it without buying a second wheel.

    In terms of stiffness/performance, very few people need a through axle on a rigid fork, IMO. But it doesn’t hurt anything.

    There’s not much choice of 15mm rigid forks either, unless you go for carbon, which I don’t want to on a tandem.
    Salsa do one, and the offset looks a bit more normal than the Identiti 20mm fork.

    …In terms of stiffness/performance, very few people need a through axle on a rigid fork…

    Very few people use a 200mm disc brake on a tandem with an all up weight of 180+kg 😉

    nbt
    Full Member

    OUr all up weight is closer to 150, but we do have 203mm hydraulics on rigid forks with 9mm QR axle

    jameso
    Full Member

    In terms of stiffness/performance, very few people need a through axle on a rigid fork, IMO. But it doesn’t hurt anything.

    And he knows a thing or two about forks.
    If you want a rigid fore-aft fork for rigidity under braking you’ll get that from a stiffer-built jump/trials forks (qr or bolted) but there’s not so much twisting/shear force on a road bike or tandem fork and that’s what a bolted axle affects. The mismatch in dimensions sounds like a bad idea though. 60-65mm, even 75mm of trail can be fine, I’d be more worried about adding 60mm of fork length and whether the added stress on the head tube would be a risk.
    I think you’d gain more wheel strength and stiffness from a 135mm Surly Enabler fork, too long for the bike but the wider front hub is great ime.

    I was allowing for the weight of the bike and a packed lunch in the panniers as well.

    Like so many bike choices, there’s no scientific evidence to base a decision on, just anecdotes.
    There’s a good case for disc brakes and QRs being a bad choice, with the weight and braking force of a tandem pushing the clamping force way beyond its safe design limit.
    Yet, other people say they have no problems.
    9mm QR looks a lot stronger than normal 5mm QR, but some people say it makes no difference and you need 15mm or 20mm.

    It’s hard to know what to do, but as I’ve got to buy a fork anyway, I might as well go for the theoretically stronger option.

    Jameso, of the various forks I’ve looked at, the 425mm Identiti is about the shortest we could get away with. I reckon with a 700cx32mm tyre, there would be about 5mm clearance under the crown.

    There’s not much info on tandem gemetry on the web, and even less on tandem fork geometry.
    It would be interesting to hear first hand accounts of what 65mm of trail is like to ride.

    jameso
    Full Member

    Depends where your weight is, bar/stem, etc. I’ve not ridden a tandem so I have no idea how the wheelbase and weight affects it in reality so I’m just speculating. The longer the trail and smaller the wheel, the more flop you get, where the weight of bike+rider wants the wheel to rest at the point where steering axis and wheel contact patch are in the same place (I find this hard to explain sorry but wheel flop is google-able) ie that’s why bikes rest naturally with the wheel off to one side.
    A bike with less weight on the front can have more trail than normal and still feel ok to ride (DH bike), so your tandem may be the opposite – a lot of weight on the steering plus I’d guess a road bike bar+stem. All that says a longer trail is going to feel heavy and want to turn in / flop into corners? 65mm trail is fine on a 700c road bike (is that trail with the right wheel OD?), 55-65 is an average or normal range, I prefer 60+. But fatter tyres and the weight may make that feel steady or not as light as you’d like.

    jairaj
    Full Member

    Here are a couple of good articles, first explaining fork rake and trail and the second explaining the relationship of fork offset and wheel size:

    Rake-and-Trail

    Wheel-Diameter-VS-Fork-Offset

Viewing 13 posts - 1 through 13 (of 13 total)

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