2_i 

When debris gets in-between Brompton's chain and tensioner pulley, the chain routinely liberates itself. For me this has been a problem in winter, with the tensioner orienting itself low due to a front derailleur, and snow inducing the chain drop. It seems that I managed to solve the problem by building up a chain keeper around the derailleur pulley, extending the bolt on which the pulley mounts and adding a short alu bar with another bolt + stainless tube such as for the pulley axle. I vigorously rode into piled up last remnants of snow around our area and the chain stayed put. I also experimented letting cloth work itself in-between the chain and pulley and the chain stayed put, unlike without the keeper.

Fentuz

nice job!

biker101

Could it also be that the tension is on the low side when you use the small chainwheel? I was able to increase the tension of the chain tensioner by putting more tension on the spring.

2_i 

As I now have a quantitative assessment of the tension, it drops by just 25% for the tensioner arm in the 6 o'clock position, compared to the 3 o'clock, this for no modifications of any sort done to the tensioner. Certainly more tension would be helping with the snow, but any reasonably increased tension would presumably get overwhelmed by sufficiently sticky, dense snow packing the tensioner wheel. In any case, I now have both that chain keeper and increased tension compared to the original. So far, shifting seems to be more often smoother with the increased tension, than deteriorated - no obvious negatives yet of the increase.

I wonder how did you increase the tension in your case?

Estuche

that's an interesting idea,

biker101

You can increase the tension in the following way: carefully loosen the central screw until you can wind up the spring by one full turn by rotating the tensioner arm over the molding, and then retighten the screw.

2_i 

Are you riding with that increase in the tension? Is the tensioner holding up fine? In my count, the modification you discuss increases the tension at the 3 o'clock position by extra 100% over the original. I was scared that this kind of increase could destroy the plastic tensioner. I did what you describe, but starting with a weaker spring than the original one, so I ended up with the tension at the 3 o'clock higher by only 30% over the original.

biker101

I have added one turn to the standard Brompton tensioner, and so far it has worked well (for two years now).