2_i 

I am glad to hear that this is of practical utility. The typical expectation is that the market will weed out poor products, but this does not quite work here. One may ask a broader question why this is the case. Looking elsewhere, if the naive expectation worked consistently, we would have no gambling since it makes the gambler poorer. However, the gambling persists as it fulfills the thrill needs of a gambler. I think that one psychological need that is fulfilled here is of adorning the beloved bike, of spending funds and effort to pimp it up, getting an expensive shiny product, even if the product itself might be pointless. And, yes, among those aftermarket products there may be genuine, professional ones, but regular market forces are challenged here, so far more effort is needed from the consumer to see through the smoke than under other circumstances. Market fragmentation is also a factor here.

Jipe

Did you see that these 20" wheels are ETRTO451 and not ETRTO406 wheels ?

2_i 

An inexpensive tensioner for 2/3/6 Brompton is marketed on Aliexpress under the Litepro brand, but the brand printed on the tensioner itself is actually Week Eight. Its main dimensions are identical to that of the original Brompton tensioner, see the photo below, where the top tensioner is Week Eight and bottom the original. The important difference between the tensioners is that Week Eight lacks indentations to accommodate the Brompton's rear wheel security washer. With this, when you mount it on the rear axle, Week Eight slides into one or other direction and ends up in an accidental position after you secure it with the axle nut.

From other details, the jockey wheels consists out of plastic parts held together by 2 screws (the latter are out of regular steel and will rust). The teeth are on a wheel that rotates separately from the pulley. This principally can impair shifting as the shift may need to work against static friction of the pulley against its axle, rather than kinematic. When I put the tensioner on, though, the shifting was OK at least on the short time scale.

Interesting is the tensioner spring which is softer than in the original Brompton tensioner. For it to work, it needs to be preloaded more than that original one. This results in a potential benefit of the torque provided by the tensioner arm being more independent of the arm's orientation. I plucked that spring and put it into the original tensioner on my Brompton. For now, there seems indeed to be some benefit, but I will need to see how this holds over time. There had been indications in playing around with the tensioners, that the Litepro/Week Eight spring is easy to deform permanently.



Top: Litepro/Week Eight tensioner. Bottom: Original Brompton tensioner.