I'm riding a '71 Le Juene converted to SS. The rear hub is a flip-flop with an ACS Claws freewheel (20t). I just put a Surly track cog (20t) on the flip side, and find that the track cog is about 3mm inboard compared to the freewheel cog. So with a perfect chainline as a SS, my chainline is too far inward at the rear if I run it fixed. I don't understand how this could be, I thought a SS freewheel would be symmetrical with a track cog for chainline. Otherwise it couldn't be run as a flip-flop.

I know for a fact that Surly is redesigning their track cogs, but I wouldn't think a dimension this critical could have been screwed up.

Thanks for any advice. This is my first SS conversion and I'm anxious to try it fixed too.

I noticed the same condition when going from a Shimano 16T Cog to a Surly 19T.

The Shimano used a 1mm spacer and Surly required 3mm.
Be careful of the Lockring and be sure that it locks against the Cog as designed as this was another condition for me when I switched Cogs.

Hey, you can try flipping the cog over and screwing it on, you know, so the 20T stamp faces your wheel. My bike shop did this with mine because I had a chainline issue due to using a 135 hub on an originally 126 spaced frame.

I thought about reversing the cog but that actually moves it inward and makes the chainline worse. I'm still wondering if this is a design defect with the Surly cogs.

I'm not an engineer, but if it was 0.2-0.3mm off from a measurement stated in some sort of documentation, I would say maybe that was a design defect. When it is 2-3mm off from a product made by a different company, I would call that a manufacturing difference.

It would be cool to run two different chainrings bolted to each other, one for each cog, but that really depends on your chainring setup I guess. That way you could run a slightly larger chainring with the smaller cog so your wheel is still in the generally same spot in the dropouts when you flip your wheel around (to pull this off your chainring/cog combos must add up to the same number of teeth, I believe).

It's not a desin defect. It's just that the parts were not designed to work together.

The track cog is designed for a track hub. The freewheel is desgned for a BMX bike. And the flip flop hub is designed for any freewheel/track cog combination so there is no way to be right on target for all combinations.

However, there is a solution. You could use spacers to slightly offcenter the hub so each cog is the same distance from the outer axle nuts. By doing so, you can flip the hub and have either cog on the same chainline. If you want a 'perfect' chainline, you will then need to adjust your chainrings to the chainline you have created in the rear. you can do this with a combination of BB width (preferred), chainring spacers, and crankset selection, depending on how far you need to move the chainring.

Okay, so you must've had it backwards to begin with...

Here is a good source on cog widths and chainlines. https://www.sheldonbrown.com/chainline/
The EAI cog is almost 2mm wider than the surly. Also, you might want to run the chainwheel on the inside of your spider if that is an option. You can always put a spacer on the drive side of your bottom bracket to get the chainline perfect. I believe this is safer than putting spacers between your hub and cog.

Thanks, I believe that explains it exactly. The "Chainline (From shoulder)" measurement according to Sheldon is 5.25mm for the Surly cog, compared to 7.96mm for an ACS freewheel. So 7.96 - 5.25 = 2.71mm, which is just about the 3mm I'm off by. I'll use some BB spacers as suggested to get things symmetrical on the flip-flop.

Thanks for all the good ideas guys.

I thought the sugestion for spacers was between the cog and the tracknuts/dropout. That's where I've always put mine... I also keep the cog on with the wide side in - my chain would rub up against the hub if I had it the other way...

No, the spacing of the hub to dropout is fine. The problem is that the track cog and SS freewheel are not symmetrical on the flip-flop hub with respect to centerline of the hub. The track cog sits 3mm more inward. I can space it out with BB spacers to make things symmetrical.

It is not a Surly Cog Design Flaw. It is a Design Feature. It's adjustable. . .

However Surly should include 1mm and 3mm spacers and documention with the Cog. As I changed the cog on my Pista when it was newly purchased, so the LBS supplied the spacer. Otherwise I would been on the hunt for Cog Spacers. It's foolish of Surly to assume that we all have supply of small parts. Also Surly could have mentioned Cog spacing and placement within "documentation" of which there is none. . .

I recommend the EAI cogs. No spacer needed, more threads engaged.

Yes, EAI for sure! Why? Because of more threads engaging. Why is this good? Because it distributes the force applied to the cog over more threads, which will keep the threads on your hub from stripping. Secondly it makes a nicer chainline and the lockring will press snuggly against the cog.

Surly's cog is not a design feature it is a design flaw. Burn yours today and get an EAI or Dura Ace

What you say makes practical sense and is repeated by the LBS mechanic. . .

I will take your advice and get an EAI Cog and remove the unnecessary spacers.

I also have very noisey ACS Freewheel. It rattles, Clinks and Clanks. The engagement is slow.

The White Industries ENO is over priced.

Shimano doesn't make a 19T.

Do you have any suggestions or advice for a 19T Freewheel?

No experiences with freewheels. I have a 17t shimano on the flop side of my flip flop but have never used it. Currently it is just keeping the threads clean. You might find the same thing happening to you.

I am quite sure you could fit the largest shimano freewheel (i think 18) toghether with your 20t cog without needing to change the chain lenght when flipping the wheel. Depends on your dropouts though.

Also 20t cog is pretty large. I run an 18t and it is definitely on the spinnie side. What chainwheel size are you using?

The chainring is 48T - It's a Bianchi Pista Bicycle

I guess I'll just live with a little ACS Freewheel noise or maybe try a Shimano 18T

Jim - no other reasonable options, either live with the ACS or step up to the ENO.

ACS freewheels clunk. It's a fact of life. Lube the innards with some heavy oil or a 50/50 mix of grease and oil and it will quite it down for a while. I've run mine hard in lots of harsh conditions (on my MTB - mud, water, etc). Despite the noise, they hold up pretty well. But I must say, the ENO is pure sex on wheels.