Eatadonut

Someone call me stupid. What would be the problem with a screw-adjusted chainstay-mounted tensioner? It seems to me like that would allow you to quickly and easily turn any junker into a fixie (provided you have a wheel) for less than $10 in parts without worrying about chain tension/length or finding a magic ratio. Plus when the rusty chain gets longer, you just tighten it up.

Obviously this is a solution looking for a problem on pricier bikes, but for the dumpster bikes you take barhopping, it seems like a good idea.

freeskihp

those types of chain tensioners can't take the stress of backpedaling and so they are fine for SS but not fixed

Eatadonut

Where's the weak point? I can't believe the chain pushes that hard. I could do the math if someone could provide me a ballpark of the coef. of friction between a 23mm rubber tire and smooth concrete.

lunacycle

1.00 - 4.00 Static (your call)
0.80 Kinetic

g4dhatch2

nevermind what these bikenerds say i say do it, with platform peddals.

Eatadonut

Why did I suggest I could do this? It's been 3 years since I did a single math problem more complicated than figuring the tip. It's coming back, but slowly.

heebro

go ahead, but when your chain derails and locks your wheel, don't come running to us complaining how much it hurt when your face skidded down the street. which I really really hope never happens to you or anyone else.

Eatadonut

From some math, I've figured the most a 200 lb rider like myself could push downward at the tensioner, assuming my bicycle geometry (1984 trek 540), with all my weight on the back wheel, before skidding, is 84 pounds.

can anyone tell me the crumple load of a standard road chainstay (EDIT: reynolds 531)? I suspect it's more than that, since I've stood on my chainstays before and had no problems. So I believe the frame, at least, is not the weak point. Then the tensioner can be the only other question - and I don't see how that could be the weak point. Making a steel tensioner that can handle even 100# of vertical force should be pretty simple. A coke can will handle that.

Eatadonut

Why do people make posts like this? Do you think it helps?

And for the record, if I did this and it failed in a spectacular fashion, you can bet your sweet heiny I'll be back on here with pics. I might even mention how much it hurt.

heebro

ok, you are right. go for it!

I did think it was helpful actually.

seriously, if you are going to make the thing urself then it might work. I actually thought you were referring to a surly type tensioner. now realize I wasn't paying attention.

that said, IMNSHO your efforts would be better spent learning to build wheels and making yourself a nice inexpensive rear wheel that you can swap from one beater to another as you grow tired of them/break them etc. A sun cr18 rim, formula hub, and DT spokes won't hurt your pockets too much.

and FYI I don't think reynolds ever made 531 stays. and you did ask for someone to call you stupid. also let us know where this dumpster full of reynolds 531 frames is...

Eatadonut

Well, the frame I based the geometry on is supposedly 531, according to the year and model number I found on a website that may or may not be reputable, and the half-destroyed serial number on my bottom bracket. Maybe the stays aren't 531 *shrug* I don't know.

I actually build most of my own wheelsets these days. This thought arose this morning when I stared dolefully at the fixed (used to be a coaster brake, but it rusted into a fixie) cruiser that is sitting on my lawn. The nuts are so rusted you can't wrench them off, they'll dissolve first, so there's no adjusting the chain tension that way.

And the dumpster full of 531 frames is at any college apartment complex in any college town, I would bet. That's where I've found a decent complete bike, a decent frame with rusted parts (the one I'm using now), and numerous craptastical bikes that I mangle in various ways. Just a quick glance in any time I took out the trash is all it takes.

freeskihp

straight from sheldon

You cannot use a derailer on a fixed gear bike, even as a chain tensioner, because when you resist the rotation of the pedals, you would bend the derailer. This presents a problem if you want to use a frame with vertical dropouts as a fixed gear, because there's no easy way to adjust the chain tension. This is also true of chain tensioners sold for singlespeed coasting bikes, such as the Surly Singleator.

Even the chain tensioners used for downhill mountain bike racing are not strong enough to withstand the stress of resisting the pedals. These tensioners have to clamp on to the chain stay, which is more or less round. There is no way to make one that would be secure, short of installing some sort of brazed-on fitting.

Philatio

I remember seeing a fixed bike from fgg here a while ago (maybe a month or so?) that used a chain tensioner with a piece of rubber + ? added so that when you backpedaled the tensioner would stop at the chain stay and not break off or whatever. I am not really sure that's exactly how it worked, and I looked around the site but I wasn't able to find it again. Just thought I'd throw it out there, so I assume it is possible.

2manybikes

You can make a bike with vert dropouts work without one anyway.
Sheldon Brown has the insrtuctions, it worked great for my friends bike.

Find my post Sheldon Brown.

deathhare

Dude.... decent tensioners cost 5 bucks...which is WAY! too much to spend toward something as trivial as your safety and well being

Moose

Dude...read the thread, he ain't talking about chain-tug type chain tensioners. What he's referring to does not exist, at least not for $5 anyway.

Sheldon Brown

https://sheldonbrown.com/vertical

Sheldon "Shazam!" Brown

AfterThisNap

Newsflash:

The physics are totally different if you use the springy type chain tensioner (surly), but put the pulley on the underside of the chain of a fixie. I'll let you guys battle it out.

Moose

You can't use a spring loaded tensioner because when you apply backward force to the pedals, an inevitability on a fixed, the tensioner will give way and the chain will go slack above the chainring and cog. This can end up bad in many ways. The simple answer is you cannot use a spring loaded tensioner reliably on a fixed gear bike regardless of the direction it pushes the chain.

Now if someone wants to try and figure out a way to make a rigid adjustable roller to take up slack in the chain, there's more room for debate.

Aeroplane

Some folks use a spring loaded tensioner in push-up mode with a cable-tie holding the tensioner to the stay. This would work pretty well in theory, at least as well as a rigid adjustable roller (rollenlager type).

Using any sort of a tensioner is pretty half-assed though. Might as well do it one of the hack ways like Sheldon or LF used.

Moose

These folks are puttin' the "OH!!" back in ghetto.

Analog

I have one of these, and they include a rigid (and beefy) chain tensioner. I would think it could withstand a whole lot of force.

Fixxxie

Correct me if Im wrong but that isn't a fixed gear

Fixxxie

I had an idea for building a tensioner that has two pulleys one above the top of the chain above the chainstay and one below the bottom of the chain under the chainstay that kind of floats there and maybe connect to the chainstay somehow
BUT then I figured might as well just use a frame with the proper dropouts

Moose

No way, I have already had to tweak one of these that had been bent to the point that it would no longer take up tension even at it's furthest adjustment. It is not nearly as strong as it looks, trust me. In fact I was quite disappointed with the rigidity of this thing.

No offense to anyone using this setup, it's fine for SS, the owner of the bike probably bent it while grinding a rail or something. But it was so easy to bend back that I doubt it would hold up on a fixed gear.