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Old 10-13-05, 06:00 PM   #1
HDTVKSS
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Gday,

i was wondering if anyone knew of any web sited that explained how different geometries affect how a bike handles. i.e long top tube, vs short. head angles VS high bottom bracket height etc. if anyone oculd help it would be greatly appreciated

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Old 10-13-05, 07:09 PM   #2
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I have no website, but MTB Action wrote about this in a past issue, 6 months or so ago. Try looking it up.

All I can recall of the top of my head is head angles less than 70 deg yield to poor steering in tight spots and uphill. I havn't seen bb height vary by more than 1/2 inch, though I can imagine a high center of gravity issue and less stability would come from excessively high bbs. Top tube is fit, pure and simple. Perhaps some of the board's brainiacs can elaborate....
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Old 10-17-05, 09:47 AM   #3
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i also have no source...

high bottom bracket = more clearnance
but higher center of gravity = less cornering ability

long top tube -> long wheelbase = more stability at expense of turning radius (sharp corners difficult)

longer stem = more stability at low speed but more difficulty getting from end up (wheelie)

relaxed seat angle = with seat up high rider more back, so going uphil (bad) more tendency for front end to lift up, but... hmm, what's the advantage of a relaxed seat angle? my guess would be option to have shorter wheelbase for same seat-stem distance, so a more "quick-turning" bike??

relaxed steering angle: more backward front-rear position = less likely to go over bars (e.g. downhill), longer wheelbase (more stability at speed in a straight line), but less precise steering --- here is really complex b/c the stem length as well as the fork rake (how much the front wheel axis is offset from the fork centerlin) also play a major role...
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Old 10-17-05, 11:44 AM   #4
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Sounds good...
Quote:
Originally Posted by nathank
i also have no source...

high bottom bracket = more clearnance
but higher center of gravity = less cornering ability

long top tube -> long wheelbase = more stability at expense of turning radius (sharp corners difficult)

longer stem = more stability at low speed but more difficulty getting from end up (wheelie)

relaxed seat angle = with seat up high rider more back, so going uphil (bad) more tendency for front end to lift up, but... hmm, what's the advantage of a relaxed seat angle? my guess would be option to have shorter wheelbase for same seat-stem distance, so a more "quick-turning" bike??...
On freeride or DH bikes, having a slack seat angle keeps the rider's weight farther back as the seat height increases. If a DH bike ran say, 74-degree seat angle, a high seat position might put the rider too far forward. That said, you can get back over the rear wheel by using a layback seatpost.

Quote:
Originally Posted by nathank
relaxed steering angle: more backward front-rear position = less likely to go over bars (e.g. downhill), longer wheelbase (more stability at speed in a straight line), but less precise steering --- here is really complex b/c the stem length as well as the fork rake (how much the front wheel axis is offset from the fork centerlin) also play a major role...
Fork rake on most forks is 40-45 mm these days - maybe the industry has standardized a bit?

Overall, there has been a shift to shorter stems, medium top tubes and slacker frame angles. This seems to reflect the trend that going down is more fun than going up, as none of these developments makes climbing easier, IMHO.
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Old 10-17-05, 12:19 PM   #5
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Quote:
Originally Posted by benzene
I havn't seen bb height vary by more than 1/2 inch, though I can imagine a high center of gravity issue and less stability would come from excessively high bbs.
Actually bb heights vary by quite a bit as well. The bb on my full suspension is almost 2" higher than the bb on my hardtail.
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Old 10-18-05, 03:15 AM   #6
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Quote:
Originally Posted by pinkrobe
Sounds good...
On freeride or DH bikes, having a slack seat angle keeps the rider's weight farther back as the seat height increases. If a DH bike ran say, 74-degree seat angle, a high seat position might put the rider too far forward. That said, you can get back over the rear wheel by using a layback seatpost.
hmm.. yeah i guess so. i didn't think of it as i ALWAYS have my seat ALL the way down on my Freerider when doing anything downhill so the angle is less important -- but i think you are right.

Quote:
Originally Posted by pinkrobe
Overall, there has been a shift to shorter stems, medium top tubes and slacker frame angles. This seems to reflect the trend that going down is more fun than going up, as none of these developments makes climbing easier, IMHO.
yes. very true.
in the early 90's mountain bikes were really optimized for maximum climbing setup. almost all the trends now are for better downhill/technical performance often at the expense of uphil capability -- my 38 lb Freerider with a short stem, short wheelbase, high fork and monster-heavy tires is a BEAR going uphill, but for me it is worth it -- we just had a streak of great weather here and i've pedalled mine up 23000ft in the last 10 days and man are my legs beat. but wow were the extreme technical downhills fun!!

but yeah, mountain bikes these days tends to have:
more weight
wider handlebar (usually riser bar)
no bar-ends
shorter stem
longer fork (more travel) so higher headset/bar
shorter wheelbase (shorter top-tube)
higher bottom bracket (my Freeride has a 355mm BB = 14" whereas XC and older mountain bikes are more around 320-325 = 12.5 to 13")
wider/heavier tires
slacker head angle (Downhill 65-67, Freeride 66-69, Enduro 67-70, XC 71-72, rac 72-73)
--> more upright, more leaning-back rider position
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Old 10-18-05, 10:24 AM   #7
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Quote:
Originally Posted by LowCel
Actually bb heights vary by quite a bit as well. The bb on my full suspension is almost 2" higher than the bb on my hardtail.
I am assuming you measure this when no one is one it. Becuase my bighit is high, with proper sag, it is 'normal' when the bike compresses the bb is freaking REALLY low
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Old 10-18-05, 11:26 AM   #8
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With me on the bike the BB height on my Truth is still at least 1" higher than the bb on my Enlightenment. That was the hardest thing to get used to when I started riding the hardtail again. For a while I was clipping my pedals on everything, it also makes log crossings quite a bit more interesting.

Keep in mind mine is set to about 20% sag. Also, the Enlightenment has an 80 mm fork and the Truth has a 100mm fork.
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