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  • Geom. effect on long travel bikes Q?
  • crotchrocket
    Free Member

    is it unusual for v. long suspension bikes to feel ‘odd’ on the flat?

    I tried an intense uzzi, with 180mm Fox fork, rear drop outs on the most forward setting. tyres were minions…
    feels very neutral with 140 or 160mm upfront but different with 180:
    On the flat: once slightly off ‘straight’, the bike seemed to dive left or right.
    Back in the day I had a GSX-R moto which would do the same thing when the front tyre was heavily worn, but this is an MTB and obviously doesn’t wear tyres out of shape in the same way.
    interestingly once pointed down an incline (even stationary) the effect was gone.

    How is this effect explained?
    is it the castor effect?
    or something else I’ve never heard off – bike designers to the forum please!

    brant
    Free Member

    It’s to do with trail and flop. Weight ahead of steering axis.
    Bike stability is incredibly complicated.

    crotchrocket
    Free Member

    *looks at wiki*

    Hmmm 44mm rake, 67deg Head angle, Theta, sin, cos… I’ll get me calculator out and bang my head with it for a bit…

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    jameso
    Full Member

    Weight effect on steering axis and contact patch, ‘flop’ is the variation between the 2 in distance and height.
    Trail length (long trail from a slack HA) and the change in height between contact patch and the area of the wheel where the steering axis intersects the tyre, as the bike leans. Trail is not really a constant, it’s a figure drawn on an upright bike. Your weight on the bike wants them to be at the same level unless totally upright, so as you initiate a turn the weight of bike and rider wants to turn the wheel until the steering axis is at the contact patch, ie the 2 are level / the same. Think of how the front wheel settles when you lean a bike when holding it by the saddle. So yes, trail and flop. Flop’s not an issue if you’re at speed and steering with bodyweight, but it can be annoying when darting through tight corners on the flat.

    brant
    Free Member

    Jameso. Nice work. I noticed for similar levels of high speed stability that 29ers seem to exhibit less flop due to steeper head angles. Does that make sense?

    jameso
    Full Member

    er, exam time ) so assuming trail is similar if it has similar stability and a steeper HA than equivalent 26″. Less flop becuase the 29er wheel radius is larger – change in height between contact and axis is less, felt as less flop, so yes seems to make sense. I visualise this easier than I explain it tho.

    brant
    Free Member

    Yeah. That’s what I thought. Another good reason for 29ers 🙂

    crotchrocket
    Free Member

    “Another good reason for 29ers”?

    As an engineer, surely you mean “another feature of 29ers that might be useful in some circumstances and not in others”?

    it also seems that the flop only occurs when the fork is at it’s full travel. When I’m on it and it’s sagged it by 33%, it doesn’t flop.
    So in their case the flop is exaggerated by the fork & when compressed there is no flop.

    I presume greater flop can be an advantage if you want to preserve manoeuvrability at speed and are steering with bodyweight? i.e. DH. or you want more neutral steering when deep in the travel of a longer fork.
    Oui?

    jameso
    Full Member

    I’d agree with Brant, it’s a positive trait of 29ers. I think it’s always a positive, more stability with less flop is a good thing on any bike imo. Whether the other negatives of big wheels outweigh the other positives is another thread )

    I think you’re correct on getting the flop / trail right at the right point in the travel, spot on. Designing around 25% seated sag never made much sense to me. I’m not sure that more flop than necessary is ever an advantage, just a balance of stability and manouverability that depends on the bike / rider. Longer rake DH forks would be cool to play with..

    qwerty
    Free Member

    During moments of boing the associated angle of plop is interjected by the gnarr of rad and the outcome is smugness.

    crotchrocket
    Free Member

    just for discussion – how do the designers/technocrats feel about this development?

    brant
    Free Member

    I think that the 0mm reach stem is fantastic. It does have it’s issues, and it’s possibly an extreme too far for all bikes, but I love it for lots of reasons.

    jameso
    Full Member

    I had an amoeba(?) zero rise stem on my v1 Chameleon, for a few rides in the late 90s. I wasn’t rad enough for it ) the bike probably wasn’t either.

    brant
    Free Member

    Me too. Jinn Yeh. Ameoba. Jeff Yang. Jumpers for goalposts.

    Makes the bars rather high of course. Can’t see it working too well for 29ers unless you start running -ve drop risers.

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