mrmb

Well, I do. Am I alone?

My question to those that have experimented with a magic gear and have found combinations that work is this:

Suppose you find a combo that gives good chain tension/slack. Now suppose this combination is not ideal for riding on this particular bike. You need to subtract teeth from the chainring and leave the cog alone (for this particular combo).

Is there a formula that will tell me how many teeth need to come off of the chainring for every link that I remove from the chain when starting out with a combo that gives perfect chain tension?

I currently have 48/22. Chain tension is spot on. If I leave the cog at 22 and remove one link from the chain, how many teeth need be subtracted from the chainring to end up with the same chain tension I started with?

veganbikes

I used to use Magic the Gathering cards...still have them should probably sell them since I don't play anymore. If you are interested I could cut a deal plenty of foils and fun rare cards and an Elf deck that maybe could have won a tournament if I had ever thought of myself good enough to play in a tournament.

I use a bike with proper track ends so I have some tension-ability. I don't want to have to do math to ride my bike.

mack_turtle

https://eehouse.org/fixin/formfmu.php
pretty darn accurate.
what's your current exact chainstay length?

acoustophile

While waiting for road parts to come in, I messed around with magic gears for a little bit on one my road bikes. I found it less than ideal as the gearing was either too big or too small. Even the ratios that were close to what I wanted just felt off. https://eehouse.org/fixin/formfmu is really accurate but just a huge pain. I ended up buying a $30 eccentric bottom bracket and it fixed all of the headaches of magic gearing.

caloso

I did once. It was a fun project, but in the end, it's better to just get a frame with proper track ends.

ofajen

Four. And a link is an inner link plus an outer link and is 1” long.

If you want math:

You want to keep the remainder

(chainring count plus rear cog count) modulo 4

constant by changing chain length.

Otto

mrmb

Double chaecking...

You are saying that if I currently have 48/22 and chain tension is perfect, I can remove one link (inner+outer) from the chain the reinstall with a 44tooth chainring and chain tension will not change from what it was with the original 48/22 set-up.

Am I following you?

I have the parts to do this, so I will give it a go this weekend.

mrmb

According to Surly...460mm

Using that figure I was able to find what appears to be a good combo. I will try it this weekend.

Thanks for the link

79pmooney

ofajen got it right. Downsize the chainring 4 teeth. One "link" - inner and outer - equals 2 teeth. So you want to downsize the chainring two teeth but that is two teeth in 24, not 48 since we are talking top of chainring to bottom, not all the way around. Two teeth in 24 is four teeth in 48. This is not entirely accurate since the chain upper and lower don't run quite parallel but then, nothing here is exact. Few chainrings are really round, chains stretch, there's slack in the chain ...

79pmooney

I'd measure my bike rather than rely on a published spec. That spec was probably published before the first production bike of that run. As lond as the bikes come out close, no one cares. And if they don't, well I'm sure they have fine print to cover it. But close (as in say 1/8" of chainstay length) might as well be a mile and a half for what you are doing.

ofajen

Yes. 👌

Otto

Dylansbob 

I've done it alot over the years. Usually, I'll run the chainstay length through the magic gear online calculator. Sometimes I'll guess. Half-links help give you more range to work with. Several of my fixed gear mtbs would let me run ~63gi, with bolt-on cogged rear wheels flipped.

I've also tried the Sheldon trick of cutting off the ends of a qr axle to gain a couple mm of play. Never rode it hard that way, but probably be safer on a singlespeed than fixed.

Lakerat

My favorite fixed gear lured me into the world of magic gear. I had a wonderful 1981 Trek Pro Race 750 frame that I wondered if it would make a useable fixed gear. It has what I think are called "semi vertical" dropouts, so there is some adjustment. A more successful experiment has rarely been realized. I love this bike and it mostly keeps me off geared bikes if route and wind allows. I clean and lube chain every ride always anyway, and that seems to minimize chain and component wear. Location and winter means I haven't ridden it in a few months and I miss it.

ofajen

Both of my frames are older and have semi-horizontal dropouts, so I have some flexibility.

Since the math is modulo 4, there are four positions corresponding to remainders of 0, 1, 2 and 3. I’m running 42/16 so that would be 58 mod 4, or 2.

My Schwinn can run that in two places on the dropout and I use the one that is further back to give my Superbe sidepulls a better chance to work on a frame that was intended for 27” wheels, not 700C.

My RockHopper has a rear U-brake, so I have to consider how the four positions work with brake pad clearance. I know I can run with remainder 1, 2 or 3, but remainder 0 is either too far forward and not properly in the dropout or too far back and would cause brake pads to hit the tire.

Otto

mrmb

UPDATE:

The chart by EEHOUSE was very handy and I found it to be accurate as I was on the hunt for the magic gear. But it is not as simple as look at what is listed and pick one. Too many variables, but it is a great resource and a great starting point. It will give you the "most-likely" combos to give good results.

As for the 4teeth = 1link formula........sort of. I started at 48/22 and I wanted to keep the 22 and lower the 48, basically. This was the goal. So, I started by putting on a 44 and removing a link. It worked, but there was noticeably more slack in the chain. For experimental purposes I went to 40/22 and removed ANOTHER link. Now there is way too much chain slack. So, it would appear that 4teeth = 1link has it's limitations.

After all was said and done, I went with 44/22 and removed a link. Now, as I mentioned, when I did this before I noticed more slack. Since I was already on the edge of an acceptable amount of slack, this extra slack was a little troubling. What to do......? I decided to try a different chain. I had been using a KMC410 1/8in chain (new). I decided to try a KMC410H heavy duty chain which in my experience had always been a little tighter (shorter) for a given amount of links. As in 100links of a 410 is longer than 100links of a 410H. And...........came out perfect. Buuuuuut, 44teeth isn't exactly what I wanted. I wanted more like 39 or 40. So, I had to improvise. I went with lighter, faster tires and wheels to get the bike to ride the way I wanted it to. I would rather have had the heavier duty rims and tires but with magic gear, the gearing has to pick me and not the other way around. I am still happy with it though and plan to keep it this way. If I am going to enjoy the benefits of magic gear, I have to also live with its limitations and I am fine with that.

FTB

Only big issue is chain stretch, which won't even have to be much to greatly affect chain tension.

Not to be that person or anything but I strongly suggest to run a magic front brake before you have a magic crash

Edit: I understand that the calculator take into account stretch but I was meaning just check your chain tension regularly so you don't have any issues since the ratio you currently have that works may not next week

Dirk de Chablis

If you use a quality chainring and sprocket then there shouldn't be tight spots. Cheap rings aren't very round, I run Sugino, Campag and Dura Ace rings on Shimano or Miche threaded sprockets (don't used pressed sprockets), for the hack/commuter bike I run 39x17 (63") I run my chain "Track Loose" a little bit of slack reduces drag and wear.

mrmb

Well, if you can convince me to use front brakes, I'm in.

I am using the KMC heavy duty chain, which has never exhibited any stretch with the the kind of riding I do.

mrmb

Who said anything about tight spots?

The issue is achieving proper slack via chainring and cog sizing.....all the way around.

We are not talking about tight spots.

veganbikes

I believe they were mentioned here by the person you quoted
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