jar351

I've seen these on several frames on which I've worked in the past, and I've always wondered what their purpose is. Why wouldn't you want your wheel to be seated as far back as possible in the dropouts? I suppose I could ask the same about the adjuster screws that go through many rear dropouts, but those I've always understood as a way of getting the wheel centered easily in the dropouts.

Bianchigirll 

Thye are the cheap version of dropout adjustment screws. the help ensure your wheel is placed in the proper place

for proper shifting the center of the freewheel needs to be aligned over a certain axis of the RD, these help keep everything aligned

jar351

Aha! Thanks for schooling me on this. That makes perfect sense. One question, though: should there be an identical doodad in the opposite dropout? I seem to recall seeing them in pairs but this frame came with just one.

auchencrow

If you had a fixed stop on the left as you do on the right, you would have difficulty centering your wheel between the stays.

bibliobob

The derailleur usually shifts better with your wheel as far forward as possible...

old's'cool

That's not the first time I've heard that concept espoused, and I'm sure there is a sweet spot for RD longitudinal positioning relative to the axle. I'm very curious to know the theory why having the wheel as far forward as possible is better for shifting.

jar351

+1 And, what's the point of having all that extra space in the dropout if it's never to be used?

oberthecat

I guess Ill leave the one in one of the bikes Im working on.
Being able to read sure is useful.

jimmuller 

I've been wondering all this myself, since for the first time in recorded history I have a bike with an integrated RD hanger and adjustment screws. With DOs that require a claw the rearward limit is set by the QR skewer nut hitting the claw. In any case, I can see how the longitudinal position of the cogs w.r.t. the jockey pulley would make a difference, but I see no arguments leading to what is better or worse!

What seems more significant is that running the screws in or out changes the effective chainstay length and the wheelbase. Surely this would have an effect on handling, so in theory one could set it as desired for handling. But since the temperature is now supposed to be much below freezing for several days I will not be doing any experiments on that for a while.

acoffin 

Well, I can tell you if there is a bolt in the braze on you will not be able to shift into your lowest gear with the axle all the way back in the drop out. The bolt and nut will get in the way of the chain. However, I have seen bikes with adjuster screws and no braze ons, so it's not the only reason for the spacer.

neurocop

I've often wondered about these DO axle spacers. I understand why you would want to be able to "fine adjust" the rear axle position in the DOs, but for that the old fashioned DO pacing screw bolts seem the most logical solution (the bolts that screw into threads tapped into the DO). Most of the time these DO spacers are just placed way back into the DO axle slot, in which case they merely hold the axle a few mms forward of where it would be without the spacers (yes, they should be in both DO slots). However, they are designed to be set at whatever position you lock them into, to take full advantage of their adjustability (to fine tune axle position for optimal RD position and to compensate for less than perfect DO geometry). Compared to the more "classic" threaded spacer bolts, they are "sturdier." Most folks don't understand this and just use them as fixed spacers set as far back as possible, obviating their usefulness.

jar351

Glad to find out I'm not the only one befuddled by these darn things!

gaucho777 

Regarding position of freewheel relative to the RD (or chain gap), I refer you to T-Mar's excellent post HERE.

old's'cool

Thanks for the link to the other thread. I think I read it before, but I guess it didn't sink in a the time. Now I get it.

Doohickie

I had the opposite experience on my Super Course. I threw that thing away and slid it all the way back and it shifts much better.

dbakl

Not if they were set to center the wheel! That's how they're used. Also, one on the left with a claw is used the center the wheel. Actually a good system, I use them all the time on old frames.

xB_Nutt

Where can I get some of those? My Handsome Cycles Speedy Devil has horizontal dropouts but is not threaded for adjustment bolts. I could use one of those to make it easier to get the wheel back in the same position after a wheel change.

auchencrow

That only works as long as neither one is positioned full-rear (unless the DO's are perfectly aligned with each other).

neurocop

Excellent reference. The main advantages of the DO adjustment screws/spacers is that they allow you to: 1) fine tune your axle position in DO's that are a bit "off," and, 2) they serve the same function as the "B" limit adjustment screws on older RD's that don't have "B" adjustment screws.