David L. Johnson wrote:
SMS wrote:
Bob Weissman wrote:
I've been using a trunk-mounted strap-on bike rack on my car, but
it doesn't exactly inspire confidence that my beloved bike won't
fall off at freeway speeds.

I managed to damage an aluminum frame on a trunk mount, because of how
the hooks grabbed the frame. I would think a carbon bike would be even
more subject to such damage.

I'm starting to look at roof-mounted racks and see there are basically
two kinds: the kind that hold onto the fork and require you to
remove the front wheel, and the kind that allow you to leave the
front wheel on.

Which is better? Obviously the fork mounts require more work, so
they must have some other advantage? Is there a problem with the
whole-bike mount which would cause me to regret choosing it?

(The bike is a 2007 Trek Pilot 5.0 and the car is a 2000 BMW 528i
sedan, if that makes any difference.)

The newer mounts that let you leave the wheel on will work with that
bicycle, i.e.
"http://www.thuleracks.com/thule/product.asp?dept_id=8&sku=594". I
have upright mounts that cannot be used with a carbon frame as they
grip the down tube, and would deform it.

I would distrust the mounts that clamp the middle of the downtube. But
the one you point out would place just as much force on the fork and
headset as a fork mount.

I don't think this is the case. The bicycle is mainly sitting on the
wheels which cushion it, the upright mount is just holding it up.

I think you are understating the advantages of a fork mount. The fork
ends are _designed_ to be clamped. Any fork strong enough to safely
support you while riding over a rough road are certainly strong enough
to hold just the bike on top of the car.

The forces are completely different.

When the vehicle is in motion shock from the road is transmitted up to
the roof rack. When a bicycle is clamped by the fork, into a fork mount,
the fork and headset are held rigidly in place. As the vehicle goes down
the road the headset will be constantly absorbing small shocks. This
would not be a problem if the headset were also turning, because the
lubricant inside would be distributed by the rolling of the bearings.
But the headset is not turning and therein lies the problem. From:
http://draco.acs.uci.edu/rbfaq/FAQ/128.html: "The damage occurs when
these small motions occur when there are no steering motions to
replenish lubricant while the bearing balls fret in place. Fretting
breaks down the lubricant film on which the balls normally roll and
without which they weld to the races and tear out tiny particles." This
was written by Jobst Brandt, a well known and well respected engineer
and author of The Bicycle Wheel.

You seem now to be talking about a mount that grabs the downtube.

Yes, my mistake. The new Thule mount is holding the wheel.

Anyway, I should have prefaced my comments with a statement that users
of fork mount racks will disagree with just about everything I wrote.