Germanicus

Good day. I have a newbie Single speed conversion question that maybe some of you grizzled vets can help me out with.

I am converting an old geared MTB to a SS townie bike and have basically stripped it down to the frame and am building it up. I am getting a new drive train and have decided to use an omnium track crank with a cassette freehub and try and take care of chain tension with adjusting cog/size half links because I really don't want a derailler style Chain tensioner and don't want to spend the dough on building up a wheelset with an ENO hub.
But I want to know if there is a way to calculate the chain tension based on the size of the chainring and cassette choice, distance between BB & rear axle, chainsize etc. before I plop down the cash to buy everything and experiment. I have read other people say that you have to experiment to find out if it will work, but I certainly don't want to buy a crankset only to find the chainring is incompatible or buy half a dozen cogs to find the right fit that works around a desired gain ratio. I don't know if this makes any sense but is there some online chain-tensioning calculator somewhere that I can plug variables into and it will tell me whether it will work? Or at least some mathmatical formula or rules of thumb?

Thanks for being patient with my silly request.
-g

Dr Fu Manchu

f = mx + b

where y = chainlength
m = slope of steepest incline you will face
x = number of colourways on your bike
b = hipster quotient

dbgray21

actually its y= (mx+b)s

where s= skidding distance desired

are you trying to get this guy killed? i can't believe you would leave out such an important variable.

filtersweep

Why do you want to calculate the tension? (or do you mean chain length?)

And why are you using a decent crank on such a cludged-together setup?

Mongoose441

X= Question
y= Google
x y=

https://www.eehouse.org/fixin/formfmu.php

Germanicus

https://www.eehouse.org/fixin/formfmu.php[/QUOTE]

Thank you mongoose. That is closer to what I am looking for than I thought I was going to get. I'll have to play around with it a bit to see how usefull it is.

Germanicus

I guess maybe I didn't ask the question right. This is the first time I am building up a bike and don't know the appropriate jargon yet. I am looking to find a setup that makes the chain tight enough not to fall off the crank or cog when I jump off a curb or something. I don't care about the actual chainlength- just the ability for it to stay on. So I'm assuming what I want is to know the number of chainlinks/halflinks with what sized cog to work with the Omnium chainring at a gain ratio of somewhere between 5 & 5.5

Why should I use an indecent crankset? From the limited searching I did, The Omnium is one of the cheapest 144 BCD Track cranks I can find that I like. If there is one cheaper and better let me know and I will look at it. I'm using a cassette hub with a cog and spacers because I thought it would be a cheap easy way to deal with chainline issues of putting a track crank on an old 1990's MTB frame. Is there a SS MTB hub I could swap that would be compatible with the Omnium chainline. I was under the impression that SS hubs didn't give you as much lateral flexibility in regards to the placement of the cog. This is why I was looking for a chart or something that would show me what kind of hubs work with what kind of chainrings etc. As i'm doing this I realize the more I learn the less I know. Appreciate any suggestions.

Andy_K 

I don't know of any reason to use a 144 BCD crank on a bike like that. You can get a decent 130 MCD crank for $30-$40.

If your old MTB frame doesn't have horizontal drop outs (forward facing -- it doesn't need to be track ends), trade it for an older one that does. That will take care of the chain tension issue. Magic gear (what you want to calculate) isn't a great solution.

Look for this:

ismellfish2

You're overthinking the chain tension issue. The chain falling off is a function of chainline, not tension. If you have the rear cog spaced right (which you shouldn't have any trouble with, using a freehub and a spacer kit) then it'll stay on there. There's not a good way to calculate the links you need because distance from the center of the bottom bracket to the center of the rear axle is not constant. However, you really don't need to figure this out ahead of time. Get the chainring and cog that give you the ratio you want, then wrap the chain around the two. It will be substantially longer than you need- take it down to as few as possible with it still reaching.

If you were going fixed it would be more critical because play in the driveline is really annoying on a fixed gear. Not so on a singlespeed. Both my housemates ride singlespeed mountain bikes and both have fairly loose chains. It's not a problem.

Germanicus

Yeah my bike has vertical dropouts. If it had horizontal dropouts there wouldn't be an issue. I'm not going to buy another frame though. This project was born because my old bike outlived it's heyday and I got a new one to replace it but I had been riding on my old one since 1993 (All over the northeast) and grew attached to it and so wanted to reuse the frame again in a new incarnation. (Kind of like a ummm phoenix-type thing) So I thought about a heavy-duty, bomb-proof, fast, SS, CX-type roadie cruiser for flying around my town on errands while hopping off of curbs and junk.
Anyway, I want to use the omnium because it is very stiff and is rounder than a 130 or 110 crank so should help a little bit with chain tension issues. Also because I wanted to use a larger size cog so there would be less wear on the chain. But most importantly because it looks really cool; very simple, stout and industrial looking crank and I think my phoeni.. I mean my bike would like it. But like I said: If it doesn't work I'll use something else, just want to find out before I buy it and "experiment" with it to see if it will work.

ismellfish2

That's great. You'll be fine. What kind of frame is it?

Germanicus

Thanks ismellfish2,
As I said, I'm still trying to figure all this stuff out. After reading other people's comments online I was under the impression that unless the chain was almost perfect it would derail if you hit a bump hard. Good to know.

Yeah, I am actually a little more concerned with chainline but was saving that for another post. I would like to use a single speed MTB hub to keep everything simple and minimalist but not sure it would work with a track crank. And I really don't mind using a cheap freehub cassette and spacers, because I know it shouldwork.

Germanicus

Sorry ismellfish2, I missed your second post.
It's a 1993 Giant Iguana MTB frame. It's all steel with no suspension front or back. Just an old fashion "skinny" tube MTB frame. Nothing exotic but it was my steed for the better part of two decades.

Brian

Am I missing something here? The chain is going to be the least expensive part of this project, and you need to buy one anyway. It's not like they're sold by the inch.

Scrodzilla

They're sold by the link, right???

Germanicus

BTW, here is the bike so far:


Besides stripping the old paint off and repainting it I haven't gotten much further. I have a headset, thread-less stem adapter and an ahead stem, seatpost and saddle but haven't installed them yet. I need to get the drive train and wheels figured out first. The orange is glossy enamel and the grey is a textured hammered paint (and is darker in person- camera flash made it look a bit lighter for some reason; maybe the specular reflection off of the texture) Looks terrible on the sage green couch I know but it will be cool when I'm done.

Brian

That would be sweet. And by the pin too. Where's that guy who does his chains with a punch?

Scrodzilla

Hahahahaha

Germanicus

It's not the chain itself I was thinking about, but rather what cogs will work with what chain rings to get my tension close enough that a 1/2 link could make it close to perfect. I had read that unless the tension is perfect, the chain will derail if you hit a bump or something, but another poster hear is saying that it is not as critical as chainline so I will just try to get it as close as I can. The bottom line is I don't have horizontal dropouts, I don't want to spend the cash building up an Eno-hubbed wheel and I don't want a Singulator type device to replace the derailleur I just got rid of so I was looking for the best way of eliminating chain sag with just chain link & cog adjustments.

operator

There is a lot of bogus information in this thread.

Chainline, chain tension, frame flex, quality of the drivetrain parts ALL contribute to whether or not the chain will fly off. The closer all of these variables (chainline = straight, chain tension variation = low as possible) the better chance of your chain staying on. Your requirement of not running a chain tensioner is silly - you will have a VERY annoying time trying to find the cog combos that give you perfect chain tension for your particular frame (at which point you have NO control for microadjust).

Chain tension is not constant throughout the entire rotation. Even *extremely* high quality parts/frames have spots where the chain is slightly looser or tighter, with a setup like yours you will NOT be able to correct for this. Theoretical is fine if everything is theoretically perfect but it isn't. Even if you can get this adjusted properly with a magic gear you still have to deal with chain wear - ANY chain wear will mean your chain loses tension from it's original setup - wear enough and you will throw a chain over bumps.

This is exactly why vertical dropouts blow for a fixed gear (need eno hub) and why removing the derailleur from a vertical dropout bike for ss = very difficult. Think hard about this.

Germanicus

Thanks operator.
As I said before, a vertical dropout is what I have to work with. (And it really is perfectly vertical-unfortunately.)
I don't think I would use a chain tensioner though unless I absolutely could not get the tension fixed with 1/2 links and tall toothed cogs. But first I would try other cogs and chainrings. I know they work fine but there is something really irritating about removing a clunky derailler and then putting something that looks just like a derailler right back on the bike. And I have read enough people's reports of succesfully using the magic gear that I will at least attempt it first. If the chain stretches too much; I'll get a new chain.
I can always put a Singulator on later if it doesn't work- or if I plan on taking the bike off of my home town boulevards out into the wild. I'm assuming they're just bolt on devices. Maybe it's something I can chuck in my toolbox and only use when seeking mischief.

kyselad

I'm not trying to sound too harsh, but you've got some good advice in this thread, and you seem not to want to hear it. You have vertical dropouts, so you either need to (a) run a tensioner, (b) get an eccentric hub, or (c) have a suck setup you'll later regret. I know you want to do this on the cheap using the frame you have, but you should either choose (a) or (b), or else find another frame with horizontal dropouts.

I'd also forget the omniums. If this is a townie, they're complete overkill, and you also won't realize how "stiff" the cranks are on that frame -- the bike will flex long before even the cheapest cranks, and you're just burning cash. If you're looking for something responsive, putting that money into a different frame geometry would make much more of a difference

Germanicus

Thanks Kyselad,
I am listening to everyone's advise. And just because I didn't automatically agree to do what the previous poster suggested doesn't mean I'm not considering everything. I've already spent a lot of time looking into the eccentric hub and chain tensioners and I know they will work, but I'm still trying to gather information about all options before purchasing everything. The Eno is great and it would be my first choice, but I could buy an entire wheelset for the price of just that one hub. Also, only one other person here suggested using a magic gear was a definite bad idea and I have heard other people here and elsewhere say that adjusting the chain length also works fine if you can get it tight. And as I said in the last post that you captioned, if I could not get a magic gear to work, I could always retrofit a tensioner later very easily.

I posted all the reasons why I preferred the Omnium & why I'm keeping this bike frame above.
Thanks