Converting old mtn bike to single speed, new rear hub has 52mm chainline. My front chainline is about 50mm near as I can measure.

Is the 2mm difference within acceptable tolerance, and if not what is the preferred method of adding 2mm - spacers at chainring bolts?

Depends on the crank. But 2mm isn't enough to worry about.

Which way is the 2mm off ? inboard or further out from centerline?

Yes, there are alloy spacers that go under chainring bolts ,
in several thicknesses, 2 mm included.

there is also a behind the right BB cup spacer available.

you can put it behind the cog on the hub too.

Thanks. yes the front is 2mm inboard compared to the rear. Functionally, it is preferred to use a BB spacer as opposed to chainring bolt spacers, or does it matter?

what BB is in Use?, that may matter in that option, chain ring spacers it won't .

not sure what BB type - inexpensive type installed by LBS.

Are you using an SS specific chain ring and cog? If not, the flex in the frame and the ramped chain ring and cog could be enough to throw the chain under heavy torque (ie:climbing). It happened to me and it isn't fun.......

BMX cog/freewheel in back, regular chainring in front.

my $.02-- 2mm offset won't matter for most "old mtn bikes", but might be an issue with a bike with very short chainstays. I wouldn't eff with chainring spacers unless your drivetrain ends up noisy/crunchy.

You'll likely want to leave the BB alone.

I'd strongly recommend getting a ss-specific chainring. They're not too expensive, and totally worth it. However, i'd run the set-up as-is before getting a singlespeed ring, just to see if you actually do have issues, and to make sure you like the gearing. If you think your gear inches are too steep or too short, you can go down or up a few teeth and upgrade to a SS ring at the same time.

Just an FYI-- singlespeed chainrings have no ramps or pins, which are found on multispeed systems b/c ramps and pins assist in shifting. In SS drivetrains, ramps and pins assist in dropping the chain-- not good. Most SS chainrings also have deeper teeth, which is a good thing for single speed.

-rob

Since it will be my winter bike, I'll probably run it as is and then work from there if I'm having problems.

Thanks for everybody's advice.

That reply is worth at least a whole quarter!! You sold yourself cheap at $.02

It'd be fairly easy to add 2 mm to your hub axle on the drivetrain side and then re-dish. With such a small distance, re-dishing would actually be optional, also. Adding a BB spacer is kind of a PiTA compared to chainring spacers or axle spacers.

Do you really believe that ramps and pins come into play on a single speed? The chain will never touch them.

There's no problem using 3/32" cog, ring and chain as long as your chainline is straight and you keep your chain properly tensioned.

I'd much rather add a spacer at the BB than respace and redish the rear wheel.

If the chain line is off and the frame fflexes enough, absolutely.

That's ridiculous.

When I first converted my Wicked Fat to singlespeed I used a random shimano middle ring- pinned, ramped, etc and an 8 spd chain. The chain fell off all the time. slight uphills w little pressure on the pedals, downhills at moderate rpms. totally unreliable when going any faster than a leisurely pace. A cheapo 6061 unramped and pinned chainring was put on. zero chain derailment issues. Maybe it was just me, but I think think the chain was catching the shifting ramps at just the right time and specially shaped teeth just dumped the chain right off.

I like the Stainless steel single speed chainrings QBP has made under their Surly brand.

My set: 26"wheels, a 110 - 38t paired with R'off 16t. .

I've never personally had the problem with ramps and pins, but then again, i've never tried running a ring equipped with ramps and pins on a SS drivetrain. However, i've known lots of ppl who have had problems with dropping chains, and who solved the problem by getting a rampless/pinless ring. As i said, the modern SS/downhill singlering-specific chainrings will also have deeper(vertically) teeth, which may also help. I've also run old-style chainrings for derailer systems, with shallow teeth but no ramps mand pins, without any problems. To be fair, it may be possible that ramps and pins won't be an issue with perfect chainline and a perfectly tensioned chain, but in my experience, such perfection is rarely achieved entirely.

All that being said, most modern chainrings for singlespeed use are compat with 3/32" chain, as are most freewheels and cogs. Nothing wrong with 3/32" chain, and a case can be made that they're superior for SS use. I like to run the fatter chains, but mostly for emotional reasons.

-rob

+1. These are very nice chainrings, available in a fairly decent range of sizes/bcd's. I miss the days when you could get plain, silver, aluminum, rampless 3/32" Sugino chainrings in almost every conceivable size (for 110 bcd), but those days are over...

-rob

Add a BB spacer:
Remove driveside pedal;
remove driveside crank;
remove bb lockring
add spacer;
reattach crank;
rethread pedal

Add axle spacers
Remove rear wheel;
Remove freewheel;
Add spacers
Rethread freewheel;
Reattach rear wheel;
2mm--perhaps re-dish (maybe one full turn of the wheel, loosening and tightening spokes).


Not sure which is easier, really. To me, having to remove and re-attach cranks and re-adjust bb seems like a lot more work. But maybe it's not. YMMV.

8/9 speed cassette spacing is 4-5 mm. Chainrings are what, around 7mm? So, if you can shift a cassette fully at 4-5 mm, a 2mm chainline differential and a 3 mm flex in the same direction adds up to a dropped chain.

The biggest concern is that the flexing and dropping of a chain due to ramped chain rings and cogs will happen at the most inopportune time, being hi torque. A quick out of the seat sprint, or climbing a hill is not when you want to lose a chain.

Then there is the variable of using a chain tensioner.....

Now you're talking chanline, not ramps and pins. The fact remains that ramps and pins will have no effect because the chain can never touch them.

Your first paragraph shows that you have no understanding of the mechanics of shifting.

The OP is asking about his chainline. That is what the topic is about, not shifting.

if you have a straight chain and no flex then ramps will not come into play. You are correct.

Since you disagree please explain why. I would like to hear your thoughts.

A crooked chanline and frame flex will not cause ramps and pins to come into play because they're on the side of the ring and cog where no amount of crookedness and flex can make the chain touch them.

You brought up shifting in a discussion of single speeds in #21. You seem to think that misalignment of the ring and cog will cause the chain to react the same way it does to a derailer pushing it to the side an equal amount and it won't. None of my single speeds or fixed gears have ever dropped a chain, and I attribute that to paying attention to chain tension more than anything else. I've never used a tensioner and I never will.

Just to add my real world experience ... I currently have a terrible chain line on my twenty 10mm+ out (short chain-stays) and a borrowed 44 tooth multi speed ring on the front

No issues in the slightest .... I've not dropped the chain or heard any unusual noise. I've not pushed it really hard yet though.

I wouldn't worry about your 2mm ... but if you really want you can fit spacer's ... I've got spacer's on mine to improve the chain line while I get the BB sorted.