Balance-Geometry Question

Quoted post said:

Originally posted by x
Two bikes: "A" and "B". Same wheels, same tires, same
220# rider.

A B
---------------------------------------
Eff Top Tube 26.5 26.5

Chainstays 17 17.25 Spindle-Front 28.75 27.625 Wheelbase
(tape) 46 45 Wheelbase (calc) 45.75 44.875

Spindle height 13.5 13.5

BarsToGround 45.5 44.25 SaddleToGround 46.25 45.75
---------------------------------------

Based on the diff between calculated and measured wheelbase,
I consider all measurements TB +- 1/8". Wheelbase and spindle-to-
front axle are especially problematic because just a little
turn of the bars can change the measurement.

Having said all that, bike "A"'s front is really, really
light. It wants to float on the slightest hill and can
wheelie with almost no effort at all.

Bike "B"'s front end seems more-or-less normal. Definately
not floaty on the climbs and it takes a conscious effort
to wheelie.

But it seems to me (who knows nothing...) like those
measurements are essentially identical - especially
considering the room for error when one person is doing the
measuring.

But the bikes' balances are definately different.

Bike "A" is a full-sus. Bike "B" is hardtail.

Howcome such a diff?

--
PeteCresswell

Dear Peter,

While subtle differences in geometry might
affect things, I suspect the rear suspension
is what makes bike A pop its front wheel up
so easily.

When 220 pounds of rider rocks back onto
the rear suspension, the comparatively feather
light front end of the bicycle springs up off its
previously compressed suspension--presto!

Bicycles being so light, it doesn't take much
to produce impressive effects. If bike A has
a lock-out for the rear suspension, you could
test my insane theory by locking the rear and
seeing if the front end suddenly becomes
sluggish.

In motorcycle trials riding, rocking-horse
working of the front and rear suspension is
a common trick for leaping up ridiculous
sheer faces. Without the rear suspension,
it doesn't work.

Carl Fogel

(Pete Cresswell) said:

Two bikes: "A" and "B". Same wheels, same tires, same
220# rider.

A B
---------------------------------------
Eff Top Tube 26.5 26.5

Chainstays 17 17.25 Spindle-Front 28.75 27.625 Wheelbase
(tape) 46 45 Wheelbase (calc) 45.75 44.875

Spindle height 13.5 13.5

BarsToGround 45.5 44.25 SaddleToGround 46.25 45.75
---------------------------------------

<snip>

Quoted message said:

Bike "A" is a full-sus. Bike "B" is hardtail.

Howcome such a diff?

The bars being higher on the FS bike will also have some
effect. One thing that puzzles me is that the saddle height
is 0.5" (1.2cm) higher on the FS bike, but the spindle
height is the same... that would also indicate a higher
center of gravity on the boinger (but a different fit).

But I suspect the biggest difference might be in the saddle
position. Hang a plumb line from the nose of the saddle and
measure to the BB spindle on both. I'll bet you find that
the boinger's saddle is farther back. It might be a good
idea to do this measurement sitting on the bike - errr, have
someone ELSE do this measurement while YOU are sitting on
the bike, since the geometry of both will change
dramatically when weighted.

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of
the $695 ti frame

RE/

Quoted message said:

The bars being higher on the FS bike will also have some
effect. One thing that puzzles me is that the saddle height
is 0.5" (1.2cm) higher on the FS bike, but the spindle
height is the same... that would also indicate a higher
center of gravity on the boinger (but a different fit).

I would attribute this partially to measurement error. I
find it quite a bit more difficult that I thought to get
consistant, repeatable measurements. The saddle is a good
example: it's a 3-dimensional shape with really no flat
spots at all - so choosing the point to measure to is a
judgement call that will differ from saddle-to-saddle.

Quoted message said:

But I suspect the biggest difference might be in the saddle
position. Hang a plumb line from the nose of the saddle and
measure to the BB spindle on both. I'll bet you find that
the boinger's saddle is farther back. It might be a good
idea to do this measurement sitting on the bike - errr,
have someone ELSE do this measurement while YOU are sitting
on the bike, since the geometry of both will change
dramatically when weighted.

Believe-it-or-not, the setback seems identical - and also
seems TB the easiest to get repeat measurements on. I just
hang the bob off of the rear of the saddle and let the line
lay between the chainstays and the wheel. As long as it
isn't snagged on anything and the bob is plenty heavy it
seems to stay true from measurement-to-measurement.

Right now I'm leaning toward Carl's theory about the double-
squish effect.

But I would also agree - in spades - that measurements taken
solo aren't the most reliable....so maybe I've messed up big-
time somewhere on the chainstay/wheelbase figures.

--
PeteCresswell

(Pete Cresswell) said:

Two bikes: "A" and "B". Same wheels, same tires, same
220# rider.

A B
---------------------------------------
Eff Top Tube 26.5 26.5

Chainstays 17 17.25 Spindle-Front 28.75 27.625 Wheelbase
(tape) 46 45 Wheelbase (calc) 45.75 44.875

Spindle height 13.5 13.5

BarsToGround 45.5 44.25 SaddleToGround 46.25 45.75
---------------------------------------

Based on the diff between calculated and measured
wheelbase, I consider all measurements TB +- 1/8".
Wheelbase and spindle-to-front axle are especially
problematic because just a little turn of the bars can
change the measurement.

Having said all that, bike "A"'s front is really, really
light. It wants to float on the slightest hill and can
wheelie with almost no effort at all.

Bike "B"'s front end seems more-or-less normal. Definately
not floaty on the climbs and it takes a conscious effort
to wheelie.

But it seems to me (who knows nothing...) like those
measurements are essentially identical - especially
considering the room for error when one person is doing
the measuring.

Identical? I think the front-center and wheelbase
measurements are significantly different.

Quoted message said:

But the bikes' balances are definately different.

Bike "A" is a full-sus. Bike "B" is hardtail.

Howcome such a diff?

Bike A has a shorter chainstay and a longer front-center
measurement. In the absence of any suspension effect, that
bike should have its weight distributed aft compared to Bike
B. That would make it a bit easier to wheelie. Perhaps the
suspension adds to that effect.

Given that the top tubes are the same but the front-center
distance is much longer for the hardtail, I'd guess that the
head tube angle on the hardtail is much slacker than the full-
sus. That's kind of unusual, isn't it?

--
Dave dvt at psu dot edu