2_i 

Since the topic gets scattered around different, sometimes nominally unrelated, threads, I thought about starting one where the ideas could be focused. Myself, after equipping my Brompton with a triple crank, I suffered with some consequences of an enhanced chain slack. Others might suffer with the consequences for a standard rear derailleur on a folder, for which the tensioner could lack the capability to absorb full slack. Chain catcher was used early on on the Birdy, then superseded by additional BB mounted tensioner. Jipe suggested its use on Brompton, but there is no space there for attaching the tensioner to the Brompton's BB shell. hihi2u2 plans to use Mr.TiParts hook in order to control chain on Brompton with a standard rear derailleur.

For me, the extra slack on Brompton, due to the triple crank, resulted in problems in 3 areas:

1. The tensioner wheel running close to the ground for low gears used in winter would clog up with snow resulting in pretty routine chain drop. Mud clogging the wheel could lead to similar outcomes.

2. Shifting could be impaired due to low tension when smaller rings were employed. An eerie sensation sometimes occurred, that might me compared to that during skating, where the bike responded with a delay to the pedals being pressed, that I attribute to waves moving on a loose chain.

3. Chain+tensioner arm might go a bit haywire when the bike was folded on the smallest ring.

An obvious question is whether these problems might be solved with one solution or some separate ones. In the end, I went with the latter. I solved 1. with the tensioner chain keeper. I solved the shifting and possibly the skating (to be still determined) in 2. by increasing tension in the tensioner spring. I harvested the spring that gave a higher tension from a cheap aftermarket tensioner for Brompton. In retrospect, the enhanced tension could be achieved by shortening the original Brompton spring by 1/4-3/4 of a turn, and pretensioning then the spring by this much more when putting the tensioner together. A full turn, in my experience, would be too much.

For 3. I ran an alu bar down from a clamp around the Brompton's seat tube and made a hook out of a bolt at the end, see below. This seemed a simpler solution for me than Mr.TiParts, given the already tightly packed area on the drive side of my BB.

Jipe

The issue for mounting the Birdy tensionner on the Brompton is the fact that the Birdy frame has only a round bottom bracket box while the Brompton has a square box added behind the round bottom bracket box that contain the rear triangle axle. This "box" is almost as wide as the bottom bracket box and make the mounting of the Birdy tensionner around the bottom bracket box impossible.

Your solution to add a ring around the seat tube to attach the tensionner on the frame is very clever.

I think your solution could be improved with a spring loaded parallelogram tensionner like the one of the Birdy instead of a fixed rm tensionner.

2_i 

I think that the spring mechanism for the Birdy's hook plays twofold role of protecting the arm on which the hook sits, when it ventures underneath the chainring, and of repositioning the hook during folding. However, in my case the hook is tucked to the side of the large chainring and is there protected from being randomly hit as it is just difficult to get there before encountering something else. With this, I think the hook's arm can do fine without the spring protection. As to the reason for the particular hook position, my slack problem for the folded bike appears when the chain is on the smallest ring and the hook needs to act then. If I placed the hook like for the folded Birdy, forward from the rings, it would hardly play any role in picking up the slack for the chain on the small ring. Interestingly, if one were to worry about the large ring situation only, like for the standard rear derailleur, the arm could still be mounted off the seat tube on Brompton, just reorienting the clamp around the seat tube.

Fentuz

What is the teeth difference? with a large teeth difference (usually over 16T), the chain gets very "slacky" which was an issue with MTB. Would you be able to fit a MTB single speed tensioner? they have a stronger spring that allow a greater load/tension. Or could modify the brompton plastic tensioner and increase the coil preload? This might damage the plastic housing so it may require a aftermarket metallic tensionner.

Alternatively, you could fit a old fashion MTB tensioner guide. I had to put one on. Either buy one or make one out of a bent metal sheet bracket and a jockey wheel/pulley from a derailleur.

2_i 

Thanks for the suggestions. The smallest ring is 20T and the largest 50T, so the extra slack to absorb, compared to a regular Brompton, is 30T. I already dealt with such a large difference on a full-size bike and first I indeed used a solution reminding one in the photo you posted. Thereafter I went to a 3-pulley derailleur and that resolved the situation completely, but that latter solution cannot be ported onto Brompton. Mounting of the guide under the chainstay is not practical either, as the chainstay gets turned upside down in folding and something attached to the original bottom of the chainstay will interfere with the hook that grabs the chainstay and holds the folded bike together. However, the Birdy solution actually plays a similar role as that chain guide, but for the folded bike only.

I did increase the tension in the tensioner spring, by >~ 50% compared to original. I hope the tensioner will take it. I can afford some decrease in lifetime of the components as I am on the edge by design there anyway. As to the metal aftermarket tensioners, I have no confidence in exposed rigid alu arm dangling by the ground - it is asking for trouble both from reasoning and my unfortunate experience.

Winfried

Out of curiosity, what advantage does this kind of housing offer compared to the usual stuff?

2_i 

This noodle housing is lighter and can work over tighter angles without much loss in the transferred cable tension. In that, it is very appropriate for folders. Moreover, you hardly use any cable ends and it is very reusable. If you need to increase cable length you reuse the links, losing only inexpensive cable lining. You can combine colors for color coding of different cables. Come to think of it, 5 bikes in the household got completely converted to this housing and 2 more are in line for the change. There is a trick for lowering costs in buying inexpensive nearly no-name cable sets to get the links and combining them with Jagwire liner. I learned about that housing here, from Raxel I believe.

Winfried

Here's the conversation :

https://www.bikeforums.net/folding-b...g-folders.html

2_i 

With the option of increasing spring tension in Brompton tensioner discussed in another thread, I thought I would post here the data for the springs available in the market that are usable in the Brompton tensioner. The torsional spring constants, in my measurements for these springs, from the stiffest to softest, are as follows:

1. 1.20 Nm/turn, 3Sixty
2. 0.95 Nm/turn, Original Brompton
3. 0.63 Nm/turn, Litepro/Week Eight

The 3Sixty springs are available individually as separate parts, while the two other only together with the tensioners. Fortunately, the Litepro tensioners are relatively inexpensive. The neutral position of the tensioner arm is at 3 o'clock, so at its tensioned position close to 3 o'clock in riding, it has moved by one turn and thus torque it provides is close to the numerical value of the spring constant. The Litepro/Week Eight spring provides the interesting option of pretensioning it by an extra turn, so the maximum torque it provides is about 1.26 Nm, in the vicinity of the maximum for 3Sixty, but with far lesser drop in the torque for the folded bike, because of that pretensioning.

2_i 

Usually we highlight our successes, but failures can be instructive too. Below is the Litepro spring, No. 3 above, pulled out from the tensioner few months after that spring was put there pretensioned by one turn. The spring changed shape over time permanently in response to the tension.

Schwinnsta

0.63 Nm = 5.6 inch pounds. Check out McMaster-Carr torsion springs. Determine torque you need and inside diameter. You won't have to over wind spring, and it should last. Unfortunately, you might have to buy in packs of 6 and pay UPS shipping. I may be a little out of my wheelhouse here, but just a suggestion.

2_i 

At this point I just went back to the original Brompton spring, just shortened by a quarter turn, I think, for the extra pretensioning. Thanks for the suggestion, though, I may look at these in the future.