Folding Bikes - Chain tensioners (Brompton, Merc, Rohloff mods) and crankers

I would like to hear from other Merc, Brompton and "chain tensioner bikes" riders if it was just me, or if you really crank on your bike there is a high chance of your chain to come off?

I am tempted to buy a Brompton and work on twinking it, but my experience with my Merc and my gf's Brommie really makes me think twice. Maybe it's my background in racing, maybe I am just plain stupid, but is the problem really on the rider?

Thank you.

14R (yes, I am already considering a new folder, you know, that significant other thing...)

I'm no Greg Lemond, so I may not put out (certainly won't be) the power that a young fellow like you can, but my chain derailing incidents have usually happened when running over rough surfaces. I had an incident while racing my g/fs 12 year old son in the park. reluctant to drop the small lead I had over him by slowing down when encountering a couple and their dog on the tarmac path, I ran wide of them over the grass and the chain leaped off on the bottom run and came off the rear jockeys and sprocket. I think it was lashing about on the lower run and the tensioner didn't have the measure of the whipping disorderly chain. Very inconvenient, and the kid won by miles before I got it back on again.

I have the aluminium tensioner now after two of the plastic ones broke in very similar circumstances like crossing potholes and traffic calming road humps while pedaling hard - God I hate those things. The good news is that the aluminium tensioner doesn't crack and fly apart if the chain comes adift as it goes over the fixed jockey wheel and into the sproket. The clearance there is very small and plastic just fails as you crank hard on the pedal and force the jammed chain through the too small space.

That's exactly what pushed me away from the otherwise nice design of Mercs and Brommies. Now Evil, even with the lack of broken pieces, do you still have your chain coming off frequently with the metal piece or is the problem fixed for good?

I've never had the chain fall off my B., but I've had the chain skip around the cog when trying to crank hard away after waiting at a red light. This happened twice on two different occasions; I'm guessing it was the tensioner. I hate that feeling!

I used to ride in between traffic in a VERY BUSY city, and I am planning a trip back there. I believe the Curve has a wheelbase that is very short for the irregular asfalt, but I am affraid the Brompton will skip tension or the chain will fall off when I really needed. For the kind of aggressive riding that I am planning, none of these options are acceptable and can result in serious injury or death.

While I'm riding here it's ok:



No room for technical failure here:








No room for 20" wheeled bike on this trip either, so I am with limited options...

That's exactly what pushed me away from the otherwise nice design of Mercs and Brommies. Now Evil, even with the lack of broken pieces, do you still have your chain coming off frequently with the metal piece or is the problem fixed for good?
I suppose the alu chain tensioner went on about 350 miles ago, and the chain has derailed once in that time in the circumstances that I described up above (rolling fast and pedalling hard over grass). I can't say its gone for good, but lots of bikes will occasionally derail a chain, even if it's just from poor adjustment. I saw a woman experience just exactly that last week on the high street with an ordinary derailier bike. Her hands looked pretty grim before she got moving again.

If that's you in the avatar 14R, you look like a powerful young guy in his prime. Your 'aggressive riding style' as you put it will likely stress any transmission parts and may bring out any weaknesses of design. I can't say for certain that my 'old timer' meanderings on the Merc will represent what you will find when you hammer away from traffic lights and so on. I mean - don't get me wrong, I'm 56 and far from disabled, but though I keep up with city traffic easily and get away quicker than many cars for the first few yards, I'm not thrashing the bike like you might.

In my opinion, the derailings are caused by the chain flapping about on the bottom run when negotiating rough road surfaces and bumps. It's obvious why it happens, isn't it? The problem has to be with the lower run of chain getting out of line when it whips due to road shocks. Since I discovered that, I tend to pause my pedaling if I run into a rough patch so as not to create a problem. It depends how fast I'm going. If its a rough surface and I'm going steadily, I can keep on pedaling because the whipping of the chain will be slight or non-existent. At fifteen miles an hour, it might be all over the place. With a small wheeled unsuspended bike, that's not a bad idea anyway. It amazes me that these small wheels don't go out of true more than they do. They certainly get some hard knocks on some surfaces, but it's all a matter of the speed you hit stuff at.

Fitting a Rohloff to my old Bird & my GoBike created huge forces on the Brommie tensioner so I had quite a few disintegrate on me to the point where I carried a spare, filled the plastic webbing with plastic metal and the problem was solved, a better alternative is to fit a Rohloff tensioner to a Merc or Brommie this should withstand our testosterone releases! :roflmao: :roflmao: :roflmao: :roflmao:

Does anyone of you know if the alloy (merc) chain tensioner fits on the Brompton with a six speed or only on a three speed?

It fits only the 3 speed, not the 2 and not the 6speed cause that is different for gear shifting.

... a better alternative is to fit a Rohloff tensioner to a Merc or Brommie this should withstand our testosterone releases!

That is possible??? How is the Rohloff tensioner going to hold all that chain when brommi/Merc is folded? Just from looking at the pictures off the Rohloff tensioner I cant figure out how that could function :-/
Somna

It fits only the 3 speed, not the 2 and not the 6speed cause that is different for gear shifting.

Yes the man is right.

The six speed B has a derailleur type arm that moves in and out to change rear cogs. The Merc chain tensioner is a fixed item - robust but immovable except to tension the chain by its spring.

I have noticed that it is very easy to fit the tensioner so that it is not in line with the chain. In fact I have replaced the washers/idler spindle between the tensioner arm and the idler, and between the tensioner base and the idler to fit the tensioner in line with the chain. Make sure that the tensioner arm is not stuck but moves freely – alloy tensioner can be oiled. And that it fits snugly on the axle. I have had a chain jump about once in 1000 km. The case EvilV described “ .. the derailings are caused by the chain flapping about on the bottom run when negotiating rough road surfaces and bumps” is the most probable for chain jump.