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Bike Compatibility Question

Old 11-06-19, 10:42 PM
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rhusto
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Bike Compatibility Question

Hey all,

I'm looking to upgrade my mom's old Trek 750 road bike with some drop bars. The drive train is a vintage Shimano STX with 3x7 gearing. I've been looking online at drop handlebars and shifters and have found a good deal on some a070 2x7 shifters.

My question is... How hard would it be (or is it even possible to) convert the 3x7 train into a 2x7 by replacing the front gear set with a 2x? Or is it possible to use the right 7x shifter from the a070 set with a 3x left shifter from an a073 set. (Just go to Shimano's website for more information on the shifters)

Perhaps I should be asking if the a073 3x7 shifter is compatible with such a vintage STX train and cassette. Is it?

Any help would be appreciated.
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Old 11-06-19, 11:10 PM
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Jeff Wills
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The rear (right) shifter will work fine. It's the left (front) shifter that will have problems. Shimano "mountain" front derailleurs use more cable pull than "road" shifters supply. The way around this is to use a "road" front derailleur such as a Tourney or Claris.

Another issue: hybrids like the Trek 750 tend to have longer top tubes than comparable road bikes. When you switch from flat bars to drop bars, the reach to the normal riding position becomes excessive. If you go through with this, you'll want to swap the handlebar stem to reduce the reach to bars to something manageable.
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Old 11-07-19, 11:41 AM
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Thanks for the information.
I have a few follow up questions.
First, would it be possible to replace the front crankset and derailleur with a 2x instead of a 3x? This would fix the issue of MTB versus road bike shifting and would fit the shifters I have now.

Also, any tips on how to find the correct stem size/length?
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Old 11-07-19, 11:21 PM
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Originally Posted by rhusto View Post
Thanks for the information.
I have a few follow up questions.
First, would it be possible to replace the front crankset and derailleur with a 2x instead of a 3x? This would fix the issue of MTB versus road bike shifting and would fit the shifters I have now.

Also, any tips on how to find the correct stem size/length?

I wouldn't replace the crankset- just replace the front derailleur. A Tourney FD-A070 would work with your shifters and should be OK on the STX crank. You won't be using the smallest chainring, that's all.
Here's one: https://www.universalcycles.com/shop...8&category=529

In terms of the stem length... as I said, your bike has a long top tube compared to a "regular" road bike so you'll need to find a very short extension stem. "How long" is a matter for someone who can see the bike and the rider together. (There's also a forum here devoted to "Bike Fitting".) Another complicating factor is that your bike uses a now-unfashionable 1" quill stem as opposed to a 1 1/8" "threadless" stem. Do you have an older bike shop near you? Many times they'll have a stock of stems that you can try out to see what fits right.
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Old 11-08-19, 06:59 PM
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I can see what my local bike shops have. Interesting that the STX crank will work even with a new FD and shifters. Would it be easy to overhaul the crank and BB in the future if I wanted to?
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Old 11-08-19, 08:00 PM
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FD doesn't really matter what crank you use. FD only matters about the diameter and lateral position of the chainrings, and the cable pull of the shifter.

I'm not familiar with your cranks, but a quick internet search seems to say that it uses square tapered JIS BB. You can replace the entire BB (Shimano standard is UN-55 for $12-$15), but need to be careful about the length of the spindle since it would affect the position of the chainrings. They come in several lengths, and sometimes the drive side is slightly longer than the non-drive side. There are some threads discussing the actual lengths of the spindles. If you decide to stop using the inner chainring of the triple and use only the outermost 2 rings, your chainline will effectively move to in between the 2 outer rings (about 4mm outwards). If you are going to replace the BB and continue using the same crank arms, you may consider using a narrower BB length to bring the chainrings a little closer inward. But you have to be careful that there is enough clearance between the crank arm and flanges to the frame so that they don't hit. You also might want to measure the current chainline to see where it is now. Road chainline is 43.5mm (for 8,9,10 speed) on 130mm dropout. MTB is further out for wider dropouts and wider Q-factor. But if you have 7 speed cassette, I don't know how much different the width of the cassette is compared to 8,9,10. You can look up the dimensions on the internet. It could be that 7 speed cassettes are narrower, so that the middle of the cassette is further out, so that you can use a front chainline that is further outward. This will also depend on what your dropout width is.

Some threads seem to say that some models of STX cranks have odd BCD sizes for the chainrings. You'll have to find out what they are in order to get the correct sized chainrings, if you plan to get new ones with the same crank arms. Alternatively, you can get new square tapered JIS triple Shimano cranksets for as cheap as $30, including riveted chainrings (if you're looking for cheap). Also, I remember seeing somewhere cheap Shimano 7 speed RD, FD, and flatbar shifters being only $12 each.

Last edited by tomtomtom123; 11-08-19 at 08:03 PM.
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Old 11-08-19, 09:09 PM
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Hmm, ok. Iíll need to write all this down in a list or something. Thanks for the info.

Iíll also head to my local shop to see what they can tell me.
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