Front Derailleur problem on 3x7
 

I converted a mountain bike to a hybrid and installed a 52 42 30 road crank in place of the 48 38 28 MTB crank. The problem is I was unable to find an affordable top pull road derailleur to fit the 52 ring. So I ended up with a Diore XT mtb top pull derailleur whose spec lists the big ring limit as 48 teeth.

So what? you say...

Its hard to get the MTB derailleur to work with the bigger rings. The curve is wrong, so it has to ride a little higher than it should (about 2mm), and with the greater difference in ring sizes from small to large, the chain drags over the tail of the derailleur in low gear. When on the big ring, the chain rubs the sides of the derailleur when on 1, and 2 on the freewheel. Very annoying.

I finally got the chain not to rub the front derailleur in 2 or 3 by making it slightly askew over the crankset (so little you have to stare hard to see it). But in 1 the chain still drags over the flat part of the tail because the derailleur is just not as long as a road (iow it does not go low enough to clear the chain). It does shift just fine. No problems with FD shifts at all.

Is there a road (or XC) derailleur for a 52 T triple crankset that is top pull and costs less than $50?

Should I look for a larger ring for 1, say a 32 or 35 so it wont drag over the tail?

Any suggestions are greatly appreciated.

How many teeth are your cassette/freewheel?

What are you trying to achieve by going to 30-42-52? I mean of course you are going for higher gearing, but how many gear inch is your top gear?

I assume you are not concerned with your low gear since you suggested going bigger.

44/12 with 27 inch (700c with tires) wheel yields 99 gear inch.

44/11 with 27 inch (700c with tires) wheel yields 108 gear inch.

42/11 with 27 inch (700c with tires) wheel yields 103 gear inch.

IMO your best option might be to go with a bottom pull road triple (assuming it's compatible with the lever cable pull ratio). Reversing the cable from top pull to bottom pull is easy enough using a clamp-on pulley made for the purpose, or as I do, looping the cable under either chainstay.

If looping the stay you can use a piece of teflon tubing, or as I do, a strip of metallic tape, ir muffler tape. The cable will find the shortest path on it's own and stay there.

An average geared 26" will take a perfectly normal rider to 25-28 mph.
If you're faster than that long enough and often enough to matter, you deserve a full-on road bike.

Mountain shifters have a different amount of cable pull on the FRONT der than road shifters.
So, if you still have the old shifters, that "might" be a contributing factor.

You may also have better luck with one of the "A" FDERS (Altus etc.) designed for a few less cogs. They aren't quite as narrow.
7 speed chain is wide compared to a 10 speed FDER and pretty much asking for chain rub.

IF I were attempting to do the same, I'd use an FD-C102.

Split the difference and go with a 48-36-26 Deore or Alivio crankset. Your FD will work fine and you'll have all the range you will ever need on a hybrid.

+1 It was a mistake going up to a 52 tooth. Even a 52-12 is of minimal use on a full road bike, let alone a hybrid. A 52-11 is just absurd. You'd have to be going 28 mph to even justify shifting into that gear, and that's at only 75rpm. There are pro riders that "only" have a high gear of 50-12.

Is your derailleur designed for 7-speed? I'm not sure how far back XT goes but I can recall 7-speed DX derailleurs. If you are using a 9-speed or 10-speed derailleur the cage is slightly narrower and can result in rub problems which can be worse if your shifters don't have a trim feature.

Could you tell us if you have a freewheel or freehub and what its range is? That would help with specific recommendations.

I ran an old Shimano XT FD-M735 on a Santana tandem fitted with 52/44/28 tooth chainrings and bar end shifters (i.e., the front shifter is friction). That front derailleur was designed for 3x7 MTB, max 48 tooth large ring. However, it is unusual in that it could handle a 26 tooth difference between the small and large chainring; most triple front derailleurs can only handle 20 tooth difference. What I found was setup was critical. Usually the outer cage is set parallel with the large chainring. But you will need to twist yours to minimize chain rub; try turning the back of the cage further inwards. Of course, with indexed MTB shifters, you have limited trim so you may always have chain rub in some combinations. In which case you are stuck and may just not be able to use all the combinations. Seems like you would want to avoid the large chainring small cog anyway. Good luck.

I run friction shifters so there is adjustments done with every shift.

STI as I see It, goes ker-thunk to a spot, then depends on the complex cage shapes to not scrape the chain until you hit cross chain gear combos.






....

Nearly every Shimano STI lever ever made (and every modern one) has built in trimming functionality. Not as much leeway as you have with friction but enough to allow one to cross-chain at will with minimal or no noise. Of course, one has to set it up correctly to work correctly.

The freewheel is 14-28. The 52 T gets me to a comfortable cadence on the flats. The shifters are for MTB, the Derailleurs are for MTB, so I'm sure things are matched. Measured the new crank ring spacing, its the same as the MTB crank it replaced. Maybe I need a thinner chain. Its a KMC X8.99 Chain. And the rear tire is a 26 x 2.185.

Here are the calc results for each crank.

Gear Inches MTB. Big ring is 48T
https://lh3.googleusercontent.com/Rf...L=w721-h672-no

Cadence at Speed MTB
https://lh3.googleusercontent.com/Ad...z=w716-h629-no



Gear Inchs Road big ring is 52
https://lh3.googleusercontent.com/E7...8=w720-h669-no



Cadence at speed Road
https://lh3.googleusercontent.com/RP...R=w720-h626-no

The charts posted above say its 97.

Use a down pull road derailleur with......

http://problemsolversbike.com/produc..._clamp_-_17312

Shimano indexed road and mountain front derailleurs use a different cable pull ratio, and that's gonna be a problem for you: MTB derailleurs don't exist with proper cage radius for a 52-tooth chainring. Road derailleurs do, but they won't have the correct cable pull to index with your MTB shifters.

In addition, while there are top pull road derailleurs for cyclocross bikes, I'm not aware of any that have a shaped cage for a triple. (If the derailleur has a big enough swing and a long enough cage, you may be able to make a double front derailleur work.)

The spacing between the rings on the road crank is the same as the MTB crank. 7mm. Besides, the problem is not shifting, the problem is rubbing from small rear cog to large rear cog, and not enough drop to low on the crank. It shifts just fine. Now medium and high rings work fine, just the low ring drags.

I realized I had checked the wrong crank to get the MTB tooth counts. I have updated the calcs above to reflect the MTB is 28,38,48T. With the MTB crank, I felt I was pedling too fast at my common flats speeds of 10 to 15 mph. This 52T slows the cadence down a few rotations and I feel about the same as when riding my old 2x5 road bike.

The change in Gear Inches (48 vs 52 with 14 cog) is 90.7 to 97.42. Change in Cadence for 10 & 15mph is 37,56 to 35,52. Guess that is not a really big change, but it sure does make my 59 yr old knees feel better.

Funny thing is if you read Shimano Derailleur setup tech document, it says that rubbing is normal when at the extreme gears. So, seeing as rings 2 and 3 are running quiet, and its only ring 1 that drags the chain off the tail of the cage, I'll look into modifying the cage to be longer .

Chainring spacing may be the same, but the geometry (lever/arm length) of road and MTB derailleurs isn't. It takes a slightly different amount of cable pull to move road and MTB front derailleurs the same distance, hence shifter/derailleur incompatibility between the two product lineups. But hey, I'm not one to argue with results.

If you only wanted to change the gear inches in your top gear from 90 to 97, you could have just replaced your 14-28t freewheel with a 13-28t.

Shimano MF-HG37 7-Speed Tourney Freewheel

Yeah, you're right. But I also wanted to get rid of the steel arm crank, and get the benefit of the 52 across all gears, as well as raising the second ring ratio too. So I have shaved off a lb or more with the alloy crank and chain rings and got the higher ratio across all gears by changing the crank set.

You can accept that explanation or this one: DOH! :mad:

I've got 10 years on you + 2 bad knees, 1 bad ankle, emphysema and a bad back.
Pedaling that slow would kill me on about 3 counts.
I went to 165mm cranks and LOW gears and my knees no longer have chronic pain.
I spin 80-85 and have far more endurance than trying to mash at 60, which was near my max with 175mm cranks. (ROM issue with the knee and my foot would be coming off the pedal @ 12 o'clock @ 63 RPM.

Ouch! I'm glad you found the right setting for your knees, etc. I wonder why the differences in our preferences. I was at the surgeons office the other day for a simple follow up exam. He said casually that my knees were low on fluids. Like they are a car. Said next visit hell inject some more lubricant. My knees don't like rapid motion. And my right ankle is nearly fused, I have about 5 or 7 degrees of vertical motion in my ankle. Just enough to walk normal, but running I look like I have a stick up my ass. Thank you mr drunk driver for all my physical problems. grrrrr.

Slow grinding kills my knees. It also puts me in oxygen debt much faster.
Basically (compared to you) I spin 4/3 faster with 3/4 the force to get the same work done. (or so when you are "maxed") I can also go out and ride an hour instead of about 45 seconds.

I broke my tib & fib some years back and now have a "nail" (actual medical term that we would call a rod).
Pounding the nail in (they don't come from the bottom) didn't mess up my knee a bit except for the scar. It was already messed up. However, my ankle, which "should" have been unaffected, feels like all my tendons were shortened a bit. They get really pissed when I try to stretch them.

The other pic is my hybrid crank with a gawdawful looking, but functional FDER and a 22-32-36. I run either 12-23 or 13-25 Cassettes, depending on my conditioning. Note The Tourney FDER shifting a 3X9.

Wanted to give an update on the situation. Just to recap: Have MTB shifters, deraillerus and installed a road bike size tripple crank. Finally after about a solid hour of riding with my tool pouch and stopping and adjusting every hundred yards or so, everything is working really great. Except for when in low front gear the derailleur still rubs the chain a bit when in the high gears out back. Otherwise I am very happy that second and third front chainrings run very silent and shift perfectly. Persistence pays off.