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I currently have a 7 speed Shimano RD (rear derailleur) that is working perfectly with the stock 7 speed freewheel which is 14-28. Shimano has a MegaRange 7 speed freewheel that is 14-34 but the problem (for me) is I don't really like the huge jump from 34 to 24 teeth on it. I want another gear (cog) "sandwiched" in between those. I haven't found that in a 7 speed freewheel but some 8 speed freewheels have it (34-28-24...). So my question is, if I upgrade my RD such that it can handle the 34 tooth largest cog, can I reuse my 7 speed indexed twist shifter and chain and just not use the smallest cog? I don't really need the 13 tooth cuz since I am rarely in the top gear and the 2nd smallest cog is 15 which is good enough. My dropouts on my new MTB are already factory set at 135mm so there should be ample room for an 8 speed freewheel.

So my question is does this have a good chance of working or might there be problems? What pitfalls do I need to look out for? I made a spreadsheet of the gear ratios and they look very good. There is no large gap in the lowest 2 gear ratios and the middle gears that I mostly use are also very nicely spaced (between 11 and 14% gap).

From the combination of widening the cog ratios (from 14-28 to 13-34) and widening the front chainrings (from 24,34,42 to 20,34,42), my potential spread of gears will go from 3.5 to about 5.5 which is about 57% wider range.

From my riding experience so far, this will be a nice upgrade if it works.

Also, just recently I rode a friends bike that already has the MegaRange freewheel on it and it shifted fine. However, when going from the 24 to 34 cog it felt like it dropped 2 gears based on ratio. That is, I would prefer another cog in between the 34 and 24 such as 28 or 29. Upshifting from 34 to 24 feels like going from 1st to 3rd gear with a missing 2nd gear.

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In short, no. Bad idea. But if you get lucky a couple of times, yes, it could work like you want. 8spd freewheels cost about the same as 7spds, so not much risk in 'going for it' and failing.

Realistically, stick to 7 spd and upgrade the RD for the extra capacity to deal with the big cog if you must... but I'd try it out first, they usually ship long cage derailers on those bikes, so you are most likely fine as-is. I liked the megarange gear selections when I swapped to it.

Potential problems:

-There is a chance that your old 7spd and the new 8spd do not index the same... 4.8 vs 5 mm

-You may need to re-dish your rear wheel to deal with the extra room for the extra cog. Again, you'll have to fit the 8spd freewheel to see if this is the case, but I'd say there is likely a problem here... and that may mean new spokes and a wheel re-build. (or forking out $11 for a 7spd freewheel)

-Freewheel stackup is different between the 7spd and 8spd.. the 8spd is longer... This can cause problems like your RD getting into the spokes to hit that last gear... so even if it fits, you may have to use the smallest 7 and not the largest 7 to keep your RD out of the spokes..

The issue isn't so much the O.L.D. (135mm) it is the stack-height of the 8spd freewheel and where the 'extra' height of the 8spd vs your 7spd goes in relation to the spokes (e.g. that 8th gear is closer to the spokes)

Back when 8spd was new I did see a lot of these conversions work just like you want this one too... some were super picky and the RD would need near constant adjustments due to temperature changes throwing off the indexing (5mm vs 4.8mm gear center-to-center) A lot of pain when you could just pay $23 for a 8spd right-only shifter that would index properly... But the gears have to fit and the RD has to stay out of the spokes!

Good luck!

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Yes - 24 to 34 is a big jump, but in practice it just takes another half to one second to complete the shift. Also helps that you'll be riding relatively slowly by the time you want that gear, probably ~10 km/h and likely half that speed.

A freewheel is not cheap but is not ridiculously expensive either. You should buy one, fit it, and try it out.

  • Yes but the question is can I get the 8 speed freewheel that has a cog between the 34 and 24 (specifically a 28), and just use 7 of those 8 cogs (not using the smallest 13 cog). The reason for this is I am biasing my bike more towards lower gears since most of my time is spent riding at 10 MPH or less and I don't want that big gap and I have read reviews on the Shimano MF-TZ31 MegaRange 7 speed freewheel and some people were complaining it is not a smooth shift and it is over 40% change in cadence. My idea will get me going quicker cuz less parts to change but still 8 speed capability later. – David Jan 29 '16 at 14:35
  • Another thing is I want a smooth shift from 1st (lowest) to 2nd cuz I have accidentally shifted while going up a hill cuz I have twist shifters. Also with the super low gear (20/34), I may begin to spin out even up a hill (such as up a spiral ramp for a crosswalk), and I want to have another close gear so I can continue building speed easily. Later after I play with the 7 speed and build up my strength, I can get an 8 speed shifter and get that last cog functional. That gear will be 42/13 which is 3.23 which is a great downhill gear at 84 gear inches. – David Jan 29 '16 at 14:40
  • @david Go for it! My only reason for suggesting the 7 speed is that its an easy change with no follow-on costs like new shifter. Plus it gives you new spare parts for your brothers cruiser conversion. – Criggie Jan 29 '16 at 23:42
  • But I wanted that 28 cog so that is why I may have to go for the 7 to 8 speed conversion. I rode a bike that jumps from 34 to 24 teeth and it feels like shifting from 1st to 3rd gear, skipping over a "missing" gear. I figure if I have to change my RD anyway for the MegaRange freewheel I may as well upgrade to 8 speed to get that 28 cog and even possibly get a higher gear of 13 instead of 14 teeth. High gears are fun too for downhill or for a very low cadence on flat pavement. Also if the bike is lifted off the ground as an exercise bike a very tall gear helps. – David Feb 3 '16 at 13:57

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