jhnc

I have a Brompton customised by Kinetics and a chubby trailer. If I fold the bike and lay it on its side, the parts on top are only supported in two places:

• at one end, by the frame-hinge
• in the middle (near the fork hook), by the front hydraulic disc brake caliper pressing against the chainring guard

The chubby doesn't provide any support/protection against stuff being placed on top of the bag when the brompton is inside.

How is the middle supported in a normal Brompton? How much weight can it support?

Schwinnsta

Can you secure it right side up with the luggage block supporting the bag?

jhnc

Thanks, I may not have been clear. The "bag" is the hold of the trailer. See for example: youtu.be/Fp6StexKoEQ

Now imagine a luggage handler decides to stack things on top of the trailer (containing the bike).

What I really want to know is: How is the middle supported in a normal Brompton? I have never seen one folded up-close, and my googlefu seems unable to find images taken at a suitable angle.

12boy

Folded Brompton are usually right side up, the rear triangle is trapped by the seat post, when folded, the handlebar stem hooks into a catch on the fork and the fork attaches to the folded rear chainstay.

jhnc

Yes, the handlebar stem snaps into a catch at the fork crown. However the fork hook (at the hub end) does not attach to the folded rear chainstay, it merely rests on it. If a normal brompton is similar to my custom one, this hook provides no support against anything pushing in towards the crank from the handlebar side. On mine, the hydraulic disc brake caliper gets pushed against the chainguard and all load is taken there. Perhaps on a normal brompton, the fork blade near the hub axle gets pushed against the chainstay?

2_i 

From reading about Chubby I recall the greatest complaint being the vulnerability of the cloth when the sharp edges of the bike push against it. I think the solution might be extra padding increasing the bulkiness of the bag. The bike itself is actually quite sturdy. The vulnerable pieces are clamp screws that can be bent when they stick out - you should be OK when you screw them tightly in. I ponder about safety of the Chubby's alu frame when in airline luggage. When I traveled with backpacks I had an alu frame bent by airline.

jhnc

Thank you. This may be the case for a normal Brompton. However on this custom one, the rear hydraulic caliper and rotor are vulnerable, as is the Rohloff external gearbox. And because there is no central support, the front hydraulic disc brake caliper and the chainring/crank will get stressed if much pressure is applied there. How is the middle supported on a normal Brompton?

2_i 

The crankset is largely hidden behind the front wheel. As long as the stresses are spread out there is no concern or need for anything central. On my Brompton I have a triple crank that obviously sticks a bit more than standard but there had never been any problems in travel in either a soft bag or the tight B&W case that presumably puts more stress than a soft bag will in airline circumstances. I cannot comment on the brake caliper, without understanding details, except for an off the cuff that there is a need to choose one's battles.

jhnc

Sure, lets assume the stresses are spread out. To where are they spread? What are the contact points?

Until I hit ten posts, I can't add images. Let's say we lay the folded bike on its side and look at it from the frametube/saddle edge so that it looks like a "<", with the left of the "<" being the frame hinge and the top of the "<" being the handlebar side.

The only way that the bike can stay "<" and not become "-" is if there is a contact point (or several) somewhere along the inside the two parts of the "<".

I am asking what that contact point is, for a normal Brompton.

I would have guessed that it is the fork hook near the hub but that is on the wrong side: the hook prevents the handlebar side being pulled outward but it doesn't stop the handlebar side being pushed inward. My next guess would be that the fork blade eventually contacts the chainstay.


The Kinetics fork has a mount for a disc brake caliper on the left arm at the back. This ends up on the inside of the fold. Pushing in from the handlebar side, the contact point for the two halves of the bike is the part of the caliper that butts against the chainring guard. Pressure applied to the handlebar side is going to end up largely being absorbed at this contact point and by nothing else. Most likely, I'd expect a wobbly chainring to result which would be annoying enough, but neither the caliper nor its mount is suposed to take large forces along this axis so I'd worry there could be much worse outcomes. I can work around the problem by providing suitable support. My question is how a normal Brompton avoids the necessity to do this.

2_i 

There is a variety of contact points there that involve the wheel, spokes touching the tensioner, tire eventually touching the crank, fender stay getting in-between - it is all dispersed.

I upload a photo of the triple crank bike folded, illustrating the dispersion of soft contact.

jhnc

Thanks 2_i, . Sounds like the normal version does have much more support.