elemental

After deciding against my last bolt-on cog idea (the bike it was going on was just too sketchy), I have another one: I have a rigid 29er with an eccentric bottom bracket sitting around an is only missing a crankset, a partial cockpit, and a set of brakes. For a very small investment, this could make a very fun around-town fixed cruiser. Here's my question:

Who is making six bolt fixed cogs? I found VeloSolo and tomiCOG, but am I missing any? A VeloSolo cog with shipping would be almost $40, and to give you an idea of what I'm looking to spend on this build, I am currently eying a $49 front disc brake setup. It would be nice if I could find some less expensive options.

mihlbach

If you have the tools you could drill your own. Otherwise, I think the tomicog is presently the cheapest option.

wroomwroomoops

There used to be a Hungarian fixie rider on these fora, some time ago (from Budapest, in fact) who drilled his own cog, but it was a b*tch. Cogs are usually made of very hard carbon steel, and he had his drilled at a workshop, where they had to replace the drill bit twice during the session (IIRC, he just bought a couple of bears to the guys, who didn't much care about the drill bits, as they weren't their - it was "on the house").

Anyhow, what I mean is, drilling the cog with the necessary accuracy might be a bit of a challenge.

wroomwroomoops

Well, there's the Boone cogs, but you won't really miss them, considering the ridicolous prices they go for. (or Boone asked for - not sure how many he really sold)

Brian

Did the bears run the drill press?

day1si

wroomwroomoops

Did I type bears? I mean beavers. Yes, beavers. Sorry for the typo.

Brian

I got called out for a typo today too. Don't feel bad. But I though beavers were extinct. You rarely see one on the internet any more. Unless she's French.

mihlbach

Don't bother. I ordered a Boone cog once and I waited for almost two years for them to deliver, and it never came. When the tomicogs became available, I canceled my order and got a refund.

day1si

How do you "though" a beaver? I must be doing it all wrong.
Called out twice! (now I should shut up before I pay for this tenfold)

As to contribute to the real topic
https://www.bikeforums.net/showthread...oSolo-Disc-Cog
That is all I have.

bleedingapple

my one question has always been, do these hold up to higher gear ratios and torque of pedaling said ratios?

mihlbach

You seem to have it backwards. Lower gear ratios yield higher torque. Yes they hold up.

elemental

I don't see why they wouldn't. If anything, the design seems stronger than a traditional cog and lockring fixed hub, and it's certainly simpler. If a six bolt design is strong enough for a 203mm rotor on a downhill bike, I think it's plenty for a fixed application.

elemental

It looks like my options are pretty much limited to the ones mentioned, but it turns out a local shop carries the tomiCOGs. Good news for me.

I think the bike this is going on is going to be a lot of fun.

c double

https://www.wired.com/gadgetlab/tag/fixies/

elemental

Whoa, that's pretty neat. Definitely not pretty though.


In other news, I picked up an XT brake and 203mm rotor tonight. If you only get one brake, make it total overkill, right? Still a little creeped out by the process of setting up hydros, but it seems to pretty much adjust itself.

Brian

You'll want to chuck that 203 and get yourself something smaller if you're only using a 9mm QR and not a 20mm thru-axle.

elemental

If you're referring to the idea that disc brakes exert forces that can eject a quick release from the fork, it seems that this force is actually greater with a smaller rotor (though that is all somewhat abstract).

As I understand it, the failure potential is primarily on the fork, as the larger rotor increases leverage against the fork leg. A number of riders around here used 203s before thru-axle forks were even available, and I have never heard of issues (though you could definitely break a weak fork leg). This bike will be used for commuting and general city cruising, so I don't plan on exerting a ton of force with the front brake anyway.

I am doing more research to try to verify this, though.

Brian

The brake issue is outlined here, under Disc brakes.

Excerpt:

Since about 2003, riders have reported a dangerous problem using disc brakes. Under hard braking, the front wheel has come out from the dropouts. The problem occurs where the brake pads and dropouts are aligned so the brake reaction force tends to eject the wheel from the dropout. Under repeated hard braking, the axle moves in the dropout in a way that unscrews the quick release. Riders should make sure the skewers are properly tightened before riding.[20] Forks that use different brake/dropout orientations or through axles are not subject to this problem.

The issue is made much worse by the larger rotor.

wroomwroomoops

Well, I went to check my winter commuter, to see how the dropouts align with the tangent of the circumference having the center in the pads, and yeah, they align really nicely. Now I am ever so slightly worried about something that's probably nonsense. Thank you Brian, thank you very much. Jerk.

Brian

Actually, Scrod's the jerk. And unless you're running an 8" rotor, (203mm for you metric folks) it really shouldn't be a concern. Check it once in a while anyway.

elemental

I knew this was an issue with discs, but I have yet to find good evidence that increased rotor size increases the forces involved. Actually, I found one reference that says it's the opposite, although it's an unsubstantiated forum post.

Let's be honest, a lot of people have run discs, even really big ones (and so far, I have seen this issue presented independently of rotor size) on quick releases. While this does appear to be an issue, it just doesn't seem to happen that much. I will double-check and supertighten my front rotors from now on, but from the way I've beat on disc brakes on mountain bikes (and watched others do it with 203s), I am just not that worried. I double checked, and I can't find any indication that my quick releases have ever rotated at all on either of my 29er forks (which have had plenty of hard front disc braking). I am going to keep looking into this, but for now, I think I'll be alright on my city bike.


I am still somewhat concerned about the fork though. Seeing how a 203mm can twist a Reba 29 proves that the force against fork legs is indisputable fact. A 2+ pound unicrown chromoly fork should be fine, right? I mean, intuitively that seems about as strong as a fork can get. Any thoughts?

wroomwroomoops

EDIT: Previously I wrote here something that, at better inspection (had a shower and made a static diagram in my head) turned out to be false.

So: the forces acting on the fork exerted by the axle are the same, regardless of the radius of the rotor.

The problem is in the curvature of the circumference, which in the case of a larger rotor, is longer, and potentially increases the likelihood of the axle dropping out from the dropouts.

elemental

[Quote removed]

. . . I should have added "with the dropout-ejecting issue," which doesn't seem completely 100% accepted yet in itself (though I don't doubt it). Bigger rotors definitely exert more force in a number of directions (definitely on the hub and fork leg), but all I have found so far is an argument that the ejecting force is greater with smaller rotors. Other than that, all I have is a free body diagram drawn with MS Paint. Annan himself makes no comments on rotor size on his site. While my intuition says a bigger rotor would be a bigger issue, the argument for smaller rotors sounds as good as mine, so I call it a wash until I can find better info.

I really don't doubt my fork and hub's ability to handle a rotor that big, I just can't figure out for sure whether the bigger rotor makes the quick release thing a really serious threat. Like I said, steep nasty terrain has made 203s a fixture here about as long as they've been around, and I think people bombing rocky steep ridge descents are using at least as much force as I will be cruising downtown.

wroomwroomoops

^^Sorry elemental, I had to retract my previous statement. Your quote of me is no longer current.