Roof-mounted bike rack: fork mount or whole bike?

mark wrote:
Actually I'm still shopping for the bike rack, but that's a very good
point to remember. I would probably remove the bike rack anyway, just to
make trunk access easier (I don't want the extra weight and expense of
the fancier racks that swing out of the way).

What effect, if any, does just the receiver hitch have on the car in a
rear impact?

If the hitch doesn't extend out past the bumper, probably little effect.

On a vehicle with a real frame, the hitch and the bumper are mounted to
the frame, so it probably doesn't matter all that much which gets hit,
except the bumper will at least absorb a little impact, while the hitch
will transfer all the impact to the frame. I wonder about this on my
4Runner, where the rear of the hitch is almost flush with the bumper.

I had a Honda C-RV that got hit in the rear. The hitch tore a hole in
the floor. A whole new floor needed to be welded in. This wasn't even a
serious accident, no injuries, and the damage didn't look that bad until
I looked under the vehicle.

SMS wrote:

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.

The car is also sitting on wheels which cushion it (and the bike) far
more.

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.

No they aren't

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.

Not really. The bike does move sideways some, as much as when riding,
and that moves the headset slightly.

But the headset is not turning and therein lies the problem. From:
http://draco.acs.uci.edu/rbfaq/FAQ/128.html:

I snipped the quote from Jobst, since it is irrelevant. He was talking
about fretting _while riding_, not while the bike is on a roof rack. My
original reply pointed out that modern headsets are much less vulnerable
to fretting damage than the old ones that Jobst was talking about. That
also is mentioned in the FAQ, when he talks about more modern Shimano
headsets.

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

True. But you were not writing, then, from experience. You can't
really say that all this damage happens from carrying a bike on a fork
mount. You are, to be honest, guessing, and extrapolating from
situations that just aren't relevant. Without a rider sitting on top of
the bike, the stresses on the fork are minimal. The only damage that
could result from a fork mount occur when putting the bike up there, or
taking it off, or having it fall off because it was improperly attached.
All this is a risk for any rack.

--

David L. Johnson

The lottery is a tax on those who fail to understand mathematics.