a really long question about steering geometry

"Dave Wilson" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

My buddy and I have been happily riding our WalMart dual suspension mountain bikes on the local
single tracks for the past couple months. They work well enough. But I'm 6'4" so I feel a little
cramped. I looked on eBay and I won a 150mm stem and put it on my bike to replace the really short
stem on there. I rode it up and down the street and it felt fine.

I took it out on trail today and everything worked great until the first downhill where I touched
the front brake and went right over the handlebars. I did it four or five more times after that,
but that was the only one I wasn't really expecting. Plus the front was was trying to tuck in
everytime I turned the handlebars. It would slide sideways really easily. And finally the steering
was really touchy. I was constantly overcorrecting.

I'm trying to understand the dynamics of how a longer stem turned my normal bike into an
evil-handling thing that tried to kill me. I can see how moving my weight forward caused me to go
over the handlebars much easier.

An inch or two further forward of the hand position doesn't necessarily equate to shifting the cg
forward by the same amount unless it's putting you on the nose of the saddle. In my experience with
longer stems, it's more a case of getting used to the change in setup and position rather than any
steering affects. I can't see why moving back slightly on the seat for hard front wheel braking
wouldn't put you right back where you were with a shorter stem.

And that might explain why the front wheel would slide

Quoted message said:

easily. But I thought the longer stem would make the steering slower, since the bar ends would
have to travel through a larger-diameter circle. If anyone can explain this, or point me to a
source that does, I would appreciate it.

Yes, the mass moment of inertia of the handlebars etc. is increased but this is small compared to
the input force capability of a normal rider. I think this would be cancelled by the greater
distance from the steering axis requiring less input force to make a steering correction and if you
analyze the geometry, 2 inches further forward is only ~ 1/4 inch increase in moment arm at the ends
of the handlebar. I can't see this making a big difference.

Quoted message said:

Anyway, I put the old stem right back on tonight. Who wants a 150mm Control Tech stem ? I can't
figure out what use it could be to anyone.

I use a 140mm on my training bike to maintain the same geometry as the rest of the fleet.

Phil Holman

"Phil Holman" <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

Yes, the mass moment of inertia of the handlebars etc. is increased but this is small compared to
the input force capability of a normal rider. I think this would be cancelled by the greater
distance from the steering axis requiring less input force to make a steering correction and if
you analyze the geometry, 2 inches further forward is only ~ 1/4 inch increase in moment arm at
the ends of the handlebar. I can't see this making a big difference.

It was actually 5 inches difference, since the new stem was 6" and the original was a stubby 1".
But I guess most of the problem stemmed (hah!) from me not being ready for the change in cg and
riding position.

thanks for the reply, Dave

Dave Wilson said:

It was actually 5 inches difference, since the new stem was 6" and the original was a stubby 1".
But I guess most of the problem stemmed (hah!) from me not being ready for the change in cg and
riding position.

That's quite likely. But, though I know nothing of your bike, it's likely that a cheapo dept store
suspension bike will have a non-robust fork that compresses very sharply under braking (brake dive).
That could enhance the problem. It's faintly possible that the geometry of the bike is designed to
work better with the 1" stem, but I wouldn't expect too much effort goes into the design of bikes at
that price point. Anyway, 150mm stems were common in cross-country (not downhill) use, though I
think stem fashion has been getting a little shorter lately.

Dave Wilson said:

I took it out on trail today and everything worked great until the first downhill where I touched
the front brake and went right over the handlebars. I did it four or five more times after that,
but that was

The touchy brakes could have been caused by the brake cable being stretched or kinked due to the new
handlebar position.

Quoted message said:

the only one I wasn't really expecting. Plus the front was was trying to tuck in everytime I
turned the handlebars. It would slide sideways really easily. And finally the steering was really
touchy. I was constantly overcorrecting.

Did you make any other changes? It sounds like a tire-pressure change could have caused
those issues.

Quoted message said:

I'm trying to understand the dynamics of how a longer stem turned my normal bike into an
evil-handling thing that tried to kill me. I can

It shouldn't be that extreme, unless you were extremely accustomed to riding that bike at that speed
on that trail in that direction with the ground being exactly that wet (no more or less, however wet
or dry it was).

In other words, under very tightly controlled conditions, you'd have strong awareness of the stem
change in the handling; but in most situations, you would not, and if you could, it would likely be
masked by some minor condition change.

Quoted message said:

see how moving my weight forward caused me to go over the handlebars much easier. And that might
explain why the front wheel would slide

Nope, moving your weight forward gives your front wheel more traction.

Quoted message said:

easily. But I thought the longer stem would make the steering slower, since the bar ends would
have to travel through a larger-diameter

Academically, you might expect it to be noticably slower. In the real world, you will barely notice.
Further, it can be offset by the extra leverage...

Quoted message said:

Anyway, I put the old stem right back on tonight. Who wants a 150mm Control Tech stem ? I can't
figure out what use it could be to anyone.

You might consider a set-back seatpost.

Quoted message said:

thanks, Dave Wilson West Palm Beach email is actually: dlwilson roundAthing davewilson period cc

--
Rick Onanian