Problem with Swapping Rear Racing and Training Wheels
 

I have a set of racing and training wheels for my road bike. Now that the road season has ended I put the rear training wheel back on. Both wheels have the same cassette and ratio. A problem I'm having is that the chain isn't working well in the easiest gear in the back but it's working well in all the other gears. When I first put the racing wheel on, I had the opposite problem; the chain didn't work well in the hardest gear. I did adjust the tension, and that helped somewhat. However, I think I need to adjust one of the derailleur screws. Which screw do I need to adjust, and in which direction?

It sounds like you need to adjust the "limit" screws which set the inner and outer derailleur travel limits. When you said the easiest (largest) cog isn't shifting well what is the symptom? Is the chain not going all the way onto the cog or is it overshifting the cog? If the former, loosen the low limit screw a 1/2 turn and see if that is enough.

BTW, your problem is very common. Even "identical" wheels with the same cassette, etc. often require minor limit screw adjustments when swapped.

Or minor indexing problems. I switch between SRAM and Shimano cassettes, and find the Shimano ones to stick out a few hundredths of a millimeter more, enough to diminish crisp shifting. I just loosen or tighten the adjuster on the derailleur one or two clicks as needed and I'm good to go. The limit screw is already a little slack on the fast end and negligible on the slow end. If the cog sizes are different, you may have to adjust the B screw, too.

The key to using multiple wheels on the same bike is matching the "freewheel position offeset" on all the wheels. That's the distance from the inside dropout face to the high gear sprocket. These usually vary slightly between brands, and can throw off indexing if not matched.

Matching is achieved using a shim behind the cassette to bring it out to the needed offset from the axle face. I've done this with most of my personal rear wheels, and BITD when working with race teams, matched 100% of the wheels used, both on bikes and spares. This ensured that any wheel swapped out in a race would be compatible with the bike.

To do this, you'll need an assortment of shims and a depth gauge to measure the step from axle face to sprocket.

On limit screws - on lesser derailleurs, the high gear (small cog) limit screw is usually labeled "H" and the low gear limit "L". On better derailleurs, I think it is assumed the mechanic knows and frequently they are not labeled. To find out which is which, screw one screw in two turns. Now pedal and shift. Note which cog you cannot get into.

As said above, cassettes/FWs on different wheels often do not exactly line up with the previous. Making the switch might be as simple as screwing in the "L" 1/4 turn and the "H" out that same 1/4. Once you find what your wheels need, doing the switch without issues can become simple. (Now, bikes have enough vagaries that I could be totally out to lunch. :))

Edit: I hadn't heard of FB's shimming to make wheels consistent but it makes sense and I like! I raced with close to Japan's finest; identical hubs and freewheels BITD and did the swaps, training to racing and back doing nothing.

And funny because it is a little related and I just did it. How to make thin spacers. (The shop I went to had only 1mm ones.) I took aluminum sheet, nailed it to a piece of plank, secured the plank under my drill press, hole saw drilled the ID then did the same with the OD. So easy I made four. These to use on a fix gear where I am running the cog flipped so the cog surface butts up to the spokes. With a low flange hub and 19 tooth or larger cog, just fine but smaller cogs require spacers to keep the chain off the spokes. I want it dialed in to the bare minimum to allow as many lockring threads as possible.

Note, if the cassette position varies enough to require adjustment of the limits, it's very likely that it will also require adjusting the RD trim to index properly.

Also note that while makers of index cassettes are very diligent about sprocket spacing on all the inner sprockets, and likewise the lever's index cam, they allow themselves greater margin of error for the high gear sprocket. The index system may allow an overshift, or slack cable condition with the assumption that the RD position will depend of the limit rather than the lever.

Before turning screws, I'd try turning the barrel adjuster 2-4 clicks and see IF that works. Just keep track of how many to restore it to the original position.
You may still have to adjust a screw (try 1/8 turn at a time until it "just" reliably works. Keep it as minimal as you can and it might work OK in both scenarios.
IF you end up having to turn screws a bit every time you swap wheels, you may want to invest in a JIS Phillips screw driver. It fits the screws properly.

So you trained all year on your race wheel? What's the point of having training wheels?

I have a Phillips that I ground down the tip of. Works beautifully in those JIS screwheads.

Where does one source these very thin shims? At the hardware store? I've tried to do this once and only managed to find 1mm being the thinnest, which was still a bit too thick.

Wheels Manf. sells them to the rich-
https://wheelsmfg.com/products/hub-p...e-spacers.html

Else, a pop can, heavy scissors and a 7/16" (for a 10mm axle) one of these.
https://www.harborfreight.com/9-piec...EaAtllEALw_wcB

If the amount it’s off by is as small as I suspect you could probably shim under the drive side locknut as well.
Shim stock, tin snips, a nibbler, and for thinner things a throwaway pair of scissors works fine. For those wanting to buy, the links above cover it. I’ve gotten many a provision shim washer from both McMaster Carr and Misumi, though that may be overkill.

I kept the racing wheels on since April because I didn't know to adjust the derailleur when swapping wheels. My thought process was to at least have the training wheel on from now until next March or April. Next year, I might swap the rear wheels during the racing season more frequently.

My problem was the chain wanted to from Gear 1 to 2. I loosened the L screw and the shifting was working better on the stand. Hopefully, it will work better on the road.

I'm a bit confused. I thought that only front derailleurs had trim.

Sorry to confuse you. Yes, it's derailleur trim I'm talking about. The problem is that if the cassette is in a slightly different place, either a bit more inboard or outboard, then the RD trim won't be right, which is what this thread is about.

You're both right. Only front derailleurs have 'trim' positions but you can refer to adjusting the rear derailleur indexing as 'trimming' the derailleur. There are so many good videos on how adjust derailleurs that you should be able to swap back and forth whenever you want/need to. I'm really surprised that you haven't done this yet, it's very easy. It's a much faster way to learn things like this than coming to a forum and getting all sorts of random suggestions. At the absolute worst you totally f it up and have to make a trip to the bike shop.

Ok, so what you did was go the entire season without being able to adjust your bike as needed? You raced all summer and never asked anyone for help? I ONLY put the race wheels on when I got the race. Training wheels went in the pit or in the follow car. They're called 'race wheels' for a reason.

Used that, but getting a proper JIS screwdriver was a very nice change - even compared to a ground-tipped phillips screwdriver.
It works nicely with phillips screws (no worse than any Phillips screwdriver), and it works wonderfully with JIS (Shimano and Yamaha stuff mostly).

+1. Works for almost anything Asian made which is........almost everything.

I bought a nice SET of Hozan JIS screwdrivers for less than $20. I've always gotten by with Phillips screwdrivers on my Shimano equipment, but it seems like these fit the screws better.