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Converting my CX

Old 01-27-20, 03:18 AM
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seingru
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Converting my CX

Hi everyone!
In the last month i went riding a few times whit fixie guys, and got immediatly hooked with the community and the bikes.
So i decided to convert my CX bike, which i'm not using anymore since i stopped doing that kind of races, in a fixed gear.
The first problem is the rear wheel: i saw a site that sells an adaptor that you put in the place of the cassette, it makes the wheel fixed and spaces the the pinion correctly (or at least that's what the site says).
Since the bike has vertical dropouts, I looked as well for a chain tensioner, and found one that consists of a piece of metal and a pulley, and is bolted where now there is the group.
I haven't found something to convert the double chainring to a single one, if you have any suggestions please tell me.
Anyway, you think those changes will be functional and safe? Thanks
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Old 01-27-20, 09:03 AM
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There's no safe way to use a chain tensioner on a fixed gear. You'll probably need an eccentric bottom bracket or hub if you want to use a frame with vertical dropouts.
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Old 01-27-20, 10:21 AM
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+1, don't do it! Only use a tensioner for single speed drive-trains and only with brakes. As for your cranks, you can remove the chain rings and install one of suitable size. Depending upon a lot of factors, you may need to install the ring on the inside of the crank spider to improve the chainline. If you use 3/8" chain it is a bit less important.

Look up "magic gear" on the search function. You "might" be lucky and find a combination that works with your dropouts. If you are set of using that frame, I would just spend some bucks and get an eccentric BB and a crank set designed for fixed gears.
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Old 01-28-20, 02:30 AM
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thank you, i've "studied" a bit this topic and decided to build a single speed instead, i don't want to spend too much money. i'm going to use it in a flat city, but i also want to be able to go up climbs, also long ones...will 46x16 do the job? i'm 19 and pretty trained (i raced xc until last october, then i started university and rode a bit less)
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Old 01-28-20, 08:39 AM
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Originally Posted by seingru View Post
thank you, i've "studied" a bit this topic and decided to build a single speed instead, i don't want to spend too much money. i'm going to use it in a flat city, but i also want to be able to go up climbs, also long ones...will 46x16 do the job? i'm 19 and pretty trained (i raced xc until last october, then i started university and rode a bit less)
In that case, a tensioner will be sufficient.
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Old 01-28-20, 09:23 AM
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46x16 is a pretty big gear, especially if you keep the knobbies.
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Old 01-28-20, 09:25 AM
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^^ Yup. A good way to figure this out is to just go for bike ride with only one gear. Find one where you can accelerate pretty well from standstill, maintain a good speed, and get up any climbs you encounter. Then try to replicate that with your single speed gearing.

As for what to do with the double, the easiest solution is to buy some single speed chainring bolts and just use one of the chainrings you have already, assuming your setup allows for it.
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Old 01-28-20, 03:44 PM
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Originally Posted by seau grateau View Post
There's no safe way to use a chain tensioner on a fixed gear. You'll probably need an eccentric bottom bracket or hub if you want to use a frame with vertical dropouts.
This gets repeated a lot around here, but is it true? Wouldn't most spring-loaded chain tensioners just retract when the lower run of chain goes taut?
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Old 01-28-20, 04:22 PM
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Originally Posted by ThermionicScott View Post
This gets repeated a lot around here, but is it true? Wouldn't most spring-loaded chain tensioners just retract when the lower run of chain goes taut?
With a single speed, It doesn't matter. The chain just freewheels backwards & the bike is stopped by actual brakes. With a fixie or a hub equiped with a coaster brake It'll work for the fraction of a second before the tensioner is pulled straight like a guitar string until the hanger gets ripped from the frame.
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Old 01-28-20, 05:01 PM
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Originally Posted by base2 View Post
With a single speed, It doesn't matter. The chain just freewheels backwards & the bike is stopped by actual brakes. With a fixie or a hub equiped with a coaster brake It'll work for the fraction of a second before the tensioner is pulled straight like a guitar string until the hanger gets ripped from the frame.
So you've witnessed this happening?
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Old 01-28-20, 06:18 PM
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Putting aside that possibility, it's still not safely functional. Any time you stop putting enough force into the drivetrain to keep the chain taut, all the slack your tensioner was taking up is suddenly introduced back into your drivetrain. There's no good time for that to happen.
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Old 01-28-20, 06:32 PM
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Originally Posted by ThermionicScott View Post
This gets repeated a lot around here, but is it true? Wouldn't most spring-loaded chain tensioners just retract when the lower run of chain goes taut?
Bad things will happen and I've seen it with someone who's freewheel seized up due to age/rust/neglect which approximated the exact same situation.

If you have a shimano or sram thru axle crank you can get a BB by Phil Wood or I believe Wheels Manufacturing which will function as an eccentric, it will not work with any square taper or 30mm axle. You can also get a white Industries eccentric hub with 135mm spacing that also solves the whole chain tension issue.
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Old 01-28-20, 06:56 PM
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Originally Posted by ThermionicScott View Post
So you've witnessed this happening?
No, but just for giggles once I mounted a roller brake wheel into a derailleur equipped frame. It didn't take long to see where the folly lay.
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Old 01-28-20, 08:48 PM
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Originally Posted by seau grateau View Post
Putting aside that possibility, it's still not safely functional. Any time you stop putting enough force into the drivetrain to keep the chain taut, all the slack your tensioner was taking up is suddenly introduced back into your drivetrain. There's no good time for that to happen.
Now that objection sounds legit... a drivetrain that goes from fixed to slack to fixed again all the time is hard to work around.

But the whole "ripping off derailleur hangers" sounds like an old wives' tale. Provided a sprung chain tensioner has enough travel to allow the lower run of chain to pull straight, it should be out of the circuit at that point, posing no danger to itself or the hanger. Perhaps the warnings come from devices that are bolted into place and aren't able to move, so they bend under the chain tension, but I don't know why someone would choose that solution...
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Old 01-29-20, 09:22 AM
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Originally Posted by seau grateau View Post
^^ Yup. A good way to figure this out is to just go for bike ride with only one gear. Find one where you can accelerate pretty well from standstill, maintain a good speed, and get up any climbs you encounter. Then try to replicate that with your single speed gearing.

As for what to do with the double, the easiest solution is to buy some single speed chainring bolts and just use one of the chainrings you have already, assuming your setup allows for it.
-----------------------------------

This is some of the best advice I've read on these forums. I've heard it put simply this way, "If you want to know what it's like to ride a SS, just don't shift!".
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Old 01-30-20, 01:24 AM
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Thank you all very much. In the end I think that for what I will do with the bike, ss is more functional than fixed, I'll play around with gears until I found the perfect one.
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Old 01-30-20, 11:46 AM
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FWIW, single speed cross racing is a thing.
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