jimboss wrote:What concerns are there with hanging bikes from the top tube?
Some bicycles manuals, for example, my Cannondale Six Carbon 5, specifically warn that
On this type the bicycle hangs on two arms projecting aft from a trunk mounted rack
Not recommended for either carbon or aluminum bikes.
Do not use on bikes with carbon fiber tubes.
The point loading where the arms contact the underside of the tubes may crack the tubes. Additionally the
lower part of the bike is difficult to secure, and it may move, causing chafing and structural wear.
Additionally when two bikes are mounted on these racks the contact points between the bikes may
be cracked or chafed, causing structural damage.
The main risk is the carbon fibre frames typically save weight and material by having top tubes
that thin-out towards their mid-section. They are not designed to support the full weight
of the bike, particularly if its bouncing around with the car suspension.
You can buy an adaptor bar
that will overcome the issue
of the top tube support - using the headstem and seat post for the support.
You will need adaptor bars for most MTB's and ladies step-thru frames.
Bubble-wrap or foam blocks can be used to prevent contact between bikes, and extra straps
can be used to hold the bottom firmly.
I've got both roof racks (Mont Blanc) and towbar mounted racks. If I've got to transport just one bike, it goes in
the back of my Xtrail. Two bikes go on the roof, if more I get the towbar mount out.
Lifting the bikes onto the Xtrail roof rack is a bit of a back-strain, and another down-side is that you can't see
the bikes to check they are secure when driving. You also need to think about the risk of scratching
the car while lifting the bike to/from it.
The main down-side of the towbar mount is the cheap one I've got blocks the rear-door access -
I've never had an issue with road debris, even driving 1000km+. And you will
likely obscure your rear number plate - in NSW this means you've got to buy a bicycle plate.