Cyclocross - Singlespeed issue

I recently got a 19t freewheel for my rob roy and the chain is slack at 2 points in the stroke and tight at 2 points (imagine there's just 4 points and I think you'll get my meaning). If it's tensioned enough so the chain won't fall off at the slack points it's too tight at the tight points- way too tight. In fact it damaged my chainring (but I just flipped it around, it's symmetrical). I tested it w/ my 18t freewheel and a 18t and 20t ss cog on a road hub and the same problem is slightly there, but it's nothing like with the 19t cog.

So I check out the cog closely and it seem maybe, just maybe, at a couple points the troughs of the teeth are cut lower than elsewhere- this is probably the slack points. It's a real small difference, like 1mm maybe, but apparently it's enough so I'm trying to get a replacement and see if it's better.

Anyway, I'm wondering if anyone else with a "true" ss (that is track ends or horizontal dropouts) have had this problem? I'm just curious because I'm wondering if the rob roy is right for me after all, I've been having some fit issues with it, and if I go w/ a different ss bike next year I'm wondering if a regular frame w/ a tensioner is the way to go just to avoid the issue altogether.

These are ACS claws freewheels I'm using, don't particularly want to spend $70 on a white bros. freewheel, and w/ the tuggnut making wheel changes slow and the tension issue I'm wondering if there's an advantage to track ends at all. I know tensioners have their problems but if you keep the chain as short as you can it seems maybe it's the more sensible way to go for racing.

I have not had that problem. I've used a shimano 18t, an ACS 20t, and a white 20t freewheels and a couple of different chainrings.
I would loosen the bolts on the chainring and then make sure that they are tightened evenly. Also try another freewheel since they're pretty cheap.

Someday when I get a custom ss 'cross frame, I'll make sure that the track ends are angled so that I don't have to adjust the brake pads when switching freewheels and that the track ends have built in tensioner screws.

Is your chainring 38T by any chance? That might be a clue as to why the 19T freewheel gives you more trouble than the 18T or 20T.

I've seen variability in the chain slack, but not enough to cause problems even riding fixed.

Check out Sheldon Brown's section on chain tension (http://www.sheldonbrown.com/fixed.html#tension). He seems to be suggesting that it has something to do with how the chainring is bolted on and tightened. It's one of the few places I've seen where he doesn't seem to have explained every detail fully.

I've been trying the chainring trick with no luck. I suppose I should try some more.

It is 38t. What's the deal with that?

It is 38t. What's the deal with that?38 is exactly twice 19, so if the tightest spot on the chainring also happens to be the tightest spot on the cog, you'll be hitting it every rotation of the cranks.

A bit of slack in the chain is no big deal, as long as chainline is correct. I gave up trying to get "perfect" chain tension a long time ago, and my life is better for it.

To answer your original question, yes it will be fine. Mine does the same thing, as does my track bike.

38 is exactly twice 19, so if the tightest spot on the chainring also happens to be the tightest spot on the cog, you'll be hitting it every rotation of the cranks.

Yes, that's what I was thinking.

If that's the problem, you should be able to fix it by taking the chain off and rotating the cog a quarter turn. You'll probably want to mark the relative crank and cog position before removing the chain.

38 is exactly twice 19, so if the tightest spot on the chainring also happens to be the tightest spot on the cog, you'll be hitting it every rotation of the cranks.

A bit of slack in the chain is no big deal, as long as chainline is correct. I gave up trying to get "perfect" chain tension a long time ago, and my life is better for it.

Ah.

It's not a "bit of slack", if it's not falling off the chain then it's so tight it's binding. I've been riding this bike for a year with 3 different combos in the back, I'm used to not having perfect tension but this 19t freewheel in particular is essentially unusable. I'll try the chainring thing some more and rotating the position of the chainring.

take your chainring off and rotate it one hole at a time until(if)the problem goes away(maybe)?

that can happen from chainrings that aren't perfectly round or the chainring shifting a bit when you're tightening it. it's probably more pronounced with that gear combo, as it would create exactly two spots per revolution, rather than some fractional amount that would perhaps be less noticeable.

try loosing all your chainring bolts slightly and turning the cranks until you find the tight spot. you want it loose enough so there is slight play in your chainring, but not super sloppy. then grab the chain from the top and bottom and sqeeze it so it sort of shifts where your chainring is sitting. then tighten your chainring in a star pattern, like you would lug nuts. see if that helps.

i've run into that issue many times on my various singlespeeds; it typically does the trick. however, some chainrings are very unround and it can't be helped. you then just have to find the best average i suppose.

I had the same problem when using a cheapo chainring that for some reason didn't quite sit flush on my crank's spider. If you are using a cheapo chainring, just toss it and if possible get a chainring made by the same manufacturer as your cranks. 130mm, 110mm, etc. should all be the same in theory but different manufacturers have different production tolerances I'm sure.

that can happen from chainrings that aren't perfectly round or the chainring shifting a bit when you're tightening it. it's probably more pronounced with that gear combo, as it would create exactly two spots per revolution, rather than some fractional amount that would perhaps be less noticeable.

try loosing all your chainring bolts slightly and turning the cranks until you find the tight spot. you want it loose enough so there is slight play in your chainring, but not super sloppy. then grab the chain from the top and bottom and sqeeze it so it sort of shifts where your chainring is sitting. then tighten your chainring in a star pattern, like you would lug nuts. see if that helps.

i've run into that issue many times on my various singlespeeds; it typically does the trick. however, some chainrings are very unround and it can't be helped. you then just have to find the best average i suppose.

This is on the right track, and it is described in more detail on Sheldon Brown's website

Yeah, the first thing I would do before spending money is trying to center the chainring. if that doesn't work, i'd replace your chainring first, then your cog.

i have the same problem on my rob roy, but i'm also running a biopace, so it's to be expected. no problems with my old round ring.

The chainring is a sugino and they normally make good stuff, messing with it last night though it's clear the 19t freewheel and the chainring are both out of whack. It doesn't come up as severely with the other freewheel or cogs because they're not as bad as the 19t but it's still there because the chainring is apparently slightly oval. There's no adjusting how it sits on the spider, it fits perfectly. I suppose I could file down the flats where it mates with the spider to give it some leeway, but we'll see.

Perhaps your bottom bracket spindle is bent?

I've had the same problem with an ACS Claws freewheel. With a Dura-Ace cog and another freewheel, my chain tension was consistent. With the ACS freewheel my chain tension was all out of whack, just like yours. The ACS and cog were both 16T, so I knew it was the freewheel and not a weird ratio between it and my chainring.

I've had the same problem with an ACS Claws freewheel. With a Dura-Ace cog and another freewheel, my chain tension was consistent. With the ACS freewheel my chain tension was all out of whack, just like yours. The ACS and cog were both 16T, so I knew it was the freewheel and not a weird ratio between it and my chainring.

What other brand of freewheels have you used?

If it really is the freewheel being warped or out of round, you should be able to see it with your bare eyes. Just spin wheel (without chain of course) and sight from different angles. Also, check for any excessive play in the mechs.

I've used a 19t ACS freewheel without issue in the past. AFAIK they're as good as anything short of a White Bros.

What other brand of freewheels have you used?

Shimano. I used a claw once for about 20 minutes. I did not like how it felt on my feet while coasting and the racket it made drove me crazy.

Hmmm.

Well threw my chain today w the 19, cost me a place. Not a great place but I coulda been 3rd to last instead of 2nd to last.

I'm prolly gonna sell my ss cx bike (iro rob roy) at the end of the season if not sooner. I raced my geared cx bike today for the first time and it fits me correctly and I had a much better time on it. I think I got too big a frame on the iro so prolly gonna sell it.

same story on my singlespeed 'cross bike. i'm running a 42t salsa chaining and 16t shimano freewheel. when the left crank is pointed forwards, the chain is tight. when the right crank is pointed backwards, the chain is slack. good thing is i can tense up the chain before drops and stuff. bad thing is it feels like i'm not getting all the power into the rear wheel.

i plan to fix this by getting a steel surly chainring and white industries freewheel next summer. we'll see what happens.

38 is exactly twice 19, so if the tightest spot on the chainring also happens to be the tightest spot on the cog, you'll be hitting it every rotation of the cranks.

A bit of slack in the chain is no big deal, as long as chainline is correct. I gave up trying to get "perfect" chain tension a long time ago, and my life is better for it.

would the 38/19 problem still be an issue with a der mounted tensioner? 38/19 is what i use for dirt