single speed chain tensioner

[landsurfer]

I am considering converting one of my bikes to a single speed freewheel. How effective are the various chain tensioning devices on sale ?

[barrym]

I'm about to do the same. I'm not convinced a tensioner is required at all if you choose to use a half-link join or complete half link chain. The only advantage I can see with a tensioner is that you could do minor changes in gearing without altering the chain.

Of course I'm probably totally wrong, so I look forward to the replies.

[Si]

You may well be able to set your bike up with half links. ghost rings and magic gears such that it works fine. But as the chain stretches you'll find that it works less well, meaning that you'll probably get through chains faster.

I use one like this: http://cdn.mos.bikeradar.com/images/bik ... 500-70.jpg
although mine's fairly old and so the design has changed a smidge. I find that it works great if you know how to set it up ib the first place: pushing up is better than down as you get better wrap around the sprocket, and when setting it up you need to whack as much tension into it as you can - I do this by pushing on the screw head with a spanner with one hand while tightening the bolt with the other.

I also tried a couple of surley singulators (again, some time ago) but found them to be rubbish - tension spring broke.

A friend cannibalised an old mech - fine if you use a bit of gear cable looped through the tension nut to keep the chain line but not so great if you rely on just setting the A & B screws.

[Brucey]

most tensioners work after a fashion but there are two main sorts;

1) the sort that are fixed, and need to be adjusted

2) the sort that are spring-loaded.

It might seem clear cut which is the best but it isn't as simple as you might think; a fixed tensioner (IME) needs to be much more robustly built than a spring loaded one because whenever you go over a bump the chain bounces and the forces developed are considerable. I built a fixed tensioner for a SS MTB and in hindsight I should have made the bracket x2 as thick; it clearly flexes (quite a lot!) in use, and I expect it to fail in fatigue eventually.

Spring loaded ones absorb shock loads better and should avoid the need to adjust on a regular basis. But they have pivots and springs etc, just like derailleurs, so can have similar issues, too. They vary a fair amount.

Fundamentally I question the need to keep the chain fully tight at all times; if it is otherwise going to come off then I guess you have no choice, but once the chain is a little worn, it needs more slack in it if it is to run smoothly on the chainwheel and sprocket; a tightly tensioned chain cannot sit in the right place (on the sprocket particularly) until it sees a good portion of the full load, and if it moves on the tooth under load, it causes more wear. It is likely to be inefficient, too.

So one route might be to use a fixed guide at the chainwheel (that doesn't touch unless the chain is jumping) and then use a sprung tensioner with a rather weak spring, or a fixed one set slack, so that the chain will run smoothly even when it is a little worn.

If you have VDOs then you need a tensioner or to be able to run a 'magic gear' (perhaps with a little filing of the VDOs). If you run a magic gear you may need to change the chain on a regular basis; such part-worn chains can be 'demoted' to duty on an IGH bike or something. You can work out your magic gear various places eg. here;

http://www.botlanta.org/converters/dale-calc/sprocket.html

cheers

[Si]

What the OP doesn't say is whether it was for road or MTB use. If MTB then I'd not go for a weak spring in the tensioner for obvious reasons. On reasonable roads you'll probably get away with it. But my experience of using tensioners for around 15 years on several bikes suggests that things work better with a fairly strong* spring on the tensioners that I've used. It should also be pointed out that you'll probably find that even with a sprung tensioner you'll still have to adjust it every now and then.

* of course, personal definitions of strong and weak may vary!

[Ljaydee]

image.jpg (43.72 KiB) Viewed 5572 times

I use one of these from Planet X, works fine.

[Redvee]

When I ran a SS MTB I used a Gusset Bachelor tensioner which is like a redundant rear derailleur as pictured above I also ran it initially with a magic gear, but now I ride my Tricross Single I use MKS chain tugs at the dropouts.

[Brucey]

re the strong/weak spring thing; if the rear tensioner is a good one and you can find a decent fixed guide to use at the front, you can run a pretty slack setup and the chain won't come off at all easily. And if you have a full tooth form on the chainring and a decent chain, it won't unship very easily at the front even if the chain is pretty slack and you don't have a guide there; in normal use the top run of the chain is under tension because you are pedalling.

If you don't have either of these things and you ride on bumpy ground then yes, you will need a tighter spring or something.

It is worth noting that rear chain tensioners vary a lot in how well they cope with chain slack or bouncing. The one pictured in a post above is typical in that the pulley doesn't have very large teeth and there are no side guides of any kind. IME the chain is liable to unship off a guide like this if the spring is weak or (with a fixed tensioner) the chain is at all slack. If you can find a tensioner that has side guides, all the better.

This is a fixed one I built myself (along with a funny hub, too) ;



you can see the adjustment in the curved slot where a derailleur would mount, and the red coloured side guides to the tensioner pulley. You can run this setup quite slack and the chain doesn't unship at the rear, where with a 'bare' pulley this would happen often. I made the side guides from parts of an old DH front chain guide, BTW.

This is the DIY-singlespeed bike it all fitted to;


cheers

[Paulatic]

Think I fitted mine, after lots of experiments, unconventionally. This way it's quiet running. When under a lot of tension holding the chain down I found the noise intrusive.

Yes I know you shouldn't run a tensioner with a back pedal brake. Over three years use and I've not had a problem.

[Brucey]

Yes I know you shouldn't run a tensioner with a back pedal brake. Over three years use and I've not had a problem...

does the chain top run go completely slack when you use the brake? Does it take much force to do this?

I guess there isn't much slack in that system even if the bottom run is pulled straight; I've often wondered if a small amount of slack would be OK with a coaster brake but I've never tried it myself.

cheers

[breakwellmz]

My effort.
Made out of a cheep spanner and jockey wheel, M8 bolt and nyloc nut for attachment.It`s done up tight enough not to move in use, but can be rotated for wheel removal.

[bobc]

just for interest = there are several pictures above with a fairly even split of tensioners pushing up & those pulling down. The pushup type would appear to give a better wrap on the wheel sprocket, so everything else being equal should be better, but then the chain is flexed in 2 directions rather than just one....
And another thing, when I were a lad - jockey wheels had ball bearings, now they're all plain. I suppose the jockey is comparable in size to the wheel sprocket & power waste is torque x rad/s & won't be much... Can you get jockeys with good bearings?

[Brucey]

for a given amount of chain deflection, I think that the 'pulley above chain' (ie push down) scheme can offer a lower load on the pulley bearing, because the weight of the slack run of the chain is supported differently. Certainly when you go over a bump (with a fixed tensioner) the chain loads can get rather high; with a spring loaded tensioner, it just deflects instead, and the load is more or less governed by the spring tension at all times.

I think considerations of this sort lay behind the design of some older tensioners like the 'osgear' one for example.

Tacx pulleys come with deep-groove cartridge bearings, and so do aftermarket ones made in aluminium (see e-bay) as well as shimano lower pulleys on better quality rear mechs.

Back in the day both Huret and Campagnolo (amongst others) used adjustable ball-bearing pulleys. These effectively used a four-point contact in the bearing, which requires incredibly precise machining if it is to work without scuffing. I always had the feeling that these bearings would 'run in' given time, and normally ran better if they were set very slightly slack. A downside to this design is that the groove in the pulley will fill up with water given half a chance; considerable vigilance (and a spray of lube after every wet ride) is normally required to avoid corrosion.

cheers

[Tigerbiten]

A friend cannibalised an old mech - fine if you use a bit of gear cable looped through the tension nut to keep the chain line but not so great if you rely on just setting the A & B screws.

I use a Sora short cage mech as a chain tensioner as I have twin chainrings on my bent trike.
The low limit screws was the right length to line the chain up with the sprocket but the high limit screw was to short to lock the mech in place.
It worked fine like that but I did end up swapping the high limit screws for a longer one to lock it in place.

[Brucey]

I think you can use a length of old spoke to 'set' a rear mech, if the limit screws won't do it.

cheers