Wheelbase vs BB height vs the ratio of the two.- Mtbr.com
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  1. #1
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    Wheelbase vs BB height vs the ratio of the two.

    I have some comments and questions about the subject.

    So a longer wheelbase bike will be more stable going up and down hills/rocks/ledges. They will also not turn as tight. Is this turning effect change its severity at different speeds of travel? Will a longer wheelbase reduce the front to back suspension rock when climbing? Imagine a bike that is real high and short, when climbing it will rock back and fourth like a sea-saw. So I would assume a longer wheelbase will rock less when climbing. But I am very unsure.

    A higher BB height will give you more clearance obviously. But am I correct in saying that it will also make the bike tipsier all around. That is in both cornering and technical ascents/descents.

    Most bikes out there seem to have a ratio. The higher the BB height the longer the wheelbase. Are there common ratio numbers out there? Maybe its different for the different types of bikes XC, AM, FR, DH. I do know there are some bikes out there whose ratio is a little out of the norm, whether good or bad.

    What I would love to do is build a LCG bike (low center of gravity). It would be a long travel bike with a long wheelbase but a low BB height. Maybe even a steeper head angle. If my assumptions are correct this is how the bike should ride.

    On the flats the slower handling from the long travel and extended wheelbase will be counter acted by the lower BB and steeper head angle. The result would be balance.

    On the ascents the extended wheelbase, steeper head angle, and lower BB should add positives to the negatives that the longer travel might bring. The result would be that the bike would climb well.

    On the descents the steeper head angle would hurt but should be more than made up for by the extended wheelbase and lower BB.

    Maybe call the bike a hybrid. A 6” bike with the BB height of a 4” bike. This would work for me. I ride without a big ring and rarely smack my crank. Of course I am sure there are bikes out there like this. Just like there are 4” bikes out there with the BB height of a 6” bike.

    Any thoughts?

  2. #2
    conjoinicorned
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    sounds like you described any one of the new "all mountain" bikes, except for the low BB (for the most part). when a suspension (front or rear) cycles, the BB will get closer to the ground, so with long travel there is a definate limit to how low you can go with the BB. the height of the BB itself means little as far as tippiness IMHO, it is entirely the CG, which can be made lower by placing the linkage/shock etc. very low on the frame (ex. giant).
    head angle is a tough one, from my experience a steep angle is always a huge negative on the descent, regardless of BB or wheelbase. head angle tends to change the steering input/feel where the wheelbase changes the overall handling, if that makes any sense. my 2 balfas have super short wheelbase, which is nice on twitchy singletrack/stunts but suffers a touch on speedy downhills. the shorter travel belair has a significantly lower BB, but by the time the 2step settles into sag they are pretty much equal in feel.

    the industry seems to be hovering at 69' ish HA for "trail" use (5-6" travel), and a BB height between 13-14" ish. as far as wheelbase, if your bike is rocking back and forth on the climbs get a new fork, new shock, new technique, or new bike. i've certainly never ridden any bike that did so.

    edit: this is meant to be a subjective opinionated post!
    what would rainbow unicorn do?

  3. #3
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by RoundRockTJ
    ...a higher BB height ... But am I correct in saying that it will also make the bike tipsier all around. That is in both cornering and technical ascents/descents.
    I prefer a tall BB. Do not find it to be tippy or unstable. I find it to make the bike more maneuverable and easier to balance.



    What I would love to do is build a LCG bike (low center of gravity). It would be a long travel bike with a long wheelbase but a low BB height. Maybe even a steeper head angle. If my assumptions are correct this is how the bike should ride.

    On the flats the slower handling from the long travel and extended wheelbase will be counter acted by the lower BB and steeper head angle. The result would be balance.

    On the ascents the extended wheelbase, steeper head angle, and lower BB should add positives to the negatives that the longer travel might bring. The result would be that the bike would climb well.

    On the descents the steeper head angle would hurt but should be more than made up for by the extended wheelbase and lower BB.
    Long travel, low BBs and steep head angles just do not go together. One of the main reasons for higher BBs on sussy bikes is to reduce the chance of bottoming the pedals/chainrings on the ground (and rocks, logs). If you had a 6" travel bike with a 12" BB the center of the BB would be just 6" above level ground at full compression. The radius of a 32-36t chainring is about 3". A vertical 175 mm crank arm - not counting the pedal - would be more than an inch below ground level. Do not forget to add in tire compression.

    Then with a long wheelbase increases the chance of high centering over humps and ledges as the BB drops even lower.

    Steep head angles and long travel: Rough rule of thumb - head angle changes 1 degree for every inch of fork length. If the static HA is 71 degrees would you really want it to go to 77 degrees at full compression?

    I bought a Kona Coiler DL this year - long wheelbase, 6" travel f&r, 14+" BB and a 66 degree head angle. It handles switchbacks - up & down - better than any bike I have ridden in 22 years of mtbing. Other than the weight it is the match for my custom ti hardtail on the trails.
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  4. #4
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    High CG is easier to balance.

    You're also ignoring the whole pedal strike problem.

    Ron

  5. #5
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    test

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    Please test on the recycle board

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