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clydeman 12-25-16 11:18 PM

Crankset/gearing question and opinions regarding drivetrain replacement
 
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Hello everyone,

I have a '93 Mongoose Crossway 625, not a fancy bike by any means but one that I have enjoyed riding and wrenching on recently.

The bike currently has a 3x7 drivetrain with front sprocket in 28,38,48 tooth count and a 13-28t rear cassette. I have a set of Sram 3x9 X9 shifter set with X9 rear derailleur, Raceface crankset in 22,32,42 and an 11-32t rear cassette. These parts were previously installed on a 26 inch wheeled mountain bike I once had.

My question is, what do I lose or gain in gearing by installing these parts on the bike with 700c wheels? I mostly use the bike for recreational riding around the 'hood until I lose enough weight to ride my "nicer" bikes. Will I end up spinning the cranks and not going anywhere due to the smaller sprockets in the front?

Admin, if in the wrong forum I apologize, I wasn't sure where to post this question.

GamblerGORD53 12-26-16 12:11 AM

If it still works, then leave it alone. The mtb set will be slower than school kids in Vietnam on a SS bike with another kid on the back.
They go 8.5 mph BTW.

SkyDog75 12-26-16 10:36 AM

You can plug your cog & chainring sizes into an online gear calculator to figure out exactly what a change in gearing will do for you. There are a few popular ones out there. The one on Sheldon Brown's site isn't the prettiest, but it'll do what you're looking for and then some:

Sheldon Brown's Bicycle Gear Calculator

bulldog1935 12-26-16 10:43 AM

go through this gear calculator, and replace my gears and wheels with yours (make sure you hit save)
then change the wheels and you will see the exact difference

if you're going up in wheel size, though, you're increasing gear ratio across the board - you'll gain high gears and lose low gears

dabac 12-26-16 10:52 AM

You can't put the 9-speed cassette on the 7-speed rear wheel. You'd need another freehub body at the least. Unless you decide to run 8-of-9 on that 7-speed hub.
Going to a 42 biggest will be partially offset by the drop to a 11T smallest.
A 26" bike with a 42/11 will spin out at about 25 mph. On a 700C bike, it'll be about 28 mph. Unless you spend important time in that speed range, don't worry about it.

Maelochs 12-26-16 12:48 PM

I'd swap everything but the chainrings. A 42-tooth top is just too small for the street (IMO.) You might not "spin out" the 42x11 but a more reasonable quick cadence (90 rpm, say) might be unsatisfyingly slow ... though going to 11T in the back will help a lot. Really your call.

If the rear hub is a freehub/cassette and not a freewheel/cluster, you can make the switch there; otherwise you would need a new wheel. Make sure the rear dropout width (distance between the two chain-stay axle mounts in the rear) is 130 mm, or stretch it (cold-set it) to 130 mm to accept modern wheels.

The real issue is, why do it at all? Do you find yourself frequently stuck "between" gears, where one forces you to push too hard and the next one demands that you spin too fast? Do you find yourself switching between neighboring gears frequently, looking for that "right" ratio?

Swapping parts might be really easy, or might open you up to a world of difficulty as stuff which Should fit, just doesn't and stuff which used to work, for some reason doesn't.

If you have the tools, the time, the know-how (or an Internet connection--YouTube has every bike repair video you could want,) a lot of tolerance and patience, and a back-up bike in case the project takes a lot longer than anticipated .....

Homebrew01 12-26-16 12:51 PM

Sounds like a lot of work for 0 benefit.

fietsbob 12-26-16 01:02 PM

Guess by adding more expensive Components, you are going to be Prouder of your Bike ,

downside is someone else can covet that stuff and steal or strip your bike..

48:13 [99.6g"] is too low for you? Is this 'humble bragging'?




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1989Pre 12-26-16 01:14 PM

You being in Los Angeles, I would say that you are already all set with your current setup. 28T in the rear, combined with 28T in the front, should get you up most hills. If you are trying to lose weight, I might recommend staying with your present setup, to make it tougher on you (you don't want to make it "too" easy).
The fact that the 9sp crank and shifters came from a bike that had 26" wheels makes no difference.
I'm not sure how 42T-11T contrasts to 48-13, but your current high gear of 48-13 should give you plenty of speed.
If you are really having trouble with some hills that you can not avoid, I say go for it and put the 9-speed cassette on there, if your dropouts are wide enough.
Gorgeous finish on that bike.

clydeman 12-26-16 09:13 PM

Thanks everyone for your feedback. To clarify why I am considering swapping out the current drivetrain, the current chain is showing wear and I doubt that a new chain will mesh in there well with the current cassette and front sprockets.

Why am I considering using the mountain bike drivetrain? Quite honestly, I am cheap and don't want to spend too much money on the bike. I purchased this bike for less than a 12 pack of Budweiser, it was in sad shape when I picked it up. I just happened to like the color and it made me nostalgic for the 90's, LOL. When I purchased it I knew I had most of the parts I would need to repair it in a parts box. The reason I considered the mountain bike drivetrain is because I have the parts available to me, there is no additional cash outlay. I have the cassette, chain, crankset, front and rear derailleurs and shifters.

A second option would be to use a Sora front 3403 crankset which I also have available but that would require the purchase of a front derailleur because the current one I have I don't believe will work with a 50t chainring, an expense I didn't want to make, as I said, I wanted to do this with as little cost as possible.

The current wheelset will accomodate a 9\10 speed cassette, but currently has the 7 speed cassette on there with a spacer. Yes, I also have most tools required and the experience to work on my bikes. Back in the 90's I had a bad experience with a bike shop and took it upon myself to learn to wrench on my bikes and over the years have acquired a decent collection of tools.

In the end, the updating of the bike has nothing to do with scoring brownie points with anyone or showing off. It will just give me the satisfaction of having a well functioning bike that I can ride for a few years to come. I really like this bike as heavy as it is.

Maelochs 12-27-16 12:29 PM

If you can maker it happen and you want to, do it. Don't ask us ... we know as much as you know in general, and much lass than you about you and your bike and your biking.

I don't see why a derailleur which can handle 48 teeth can't do 50, but I assume you have done the research. I don't think 42x11 will be terrible (I manage to survive 48x14 on my tourer/town bike) and the more cogs, the better, in general.

Plus, the fun of wrenching on your bike (except when it is not fun.)

Post pics.

fietsbob 12-27-16 12:42 PM

If The chain rings are not Riveted on they can be replaced, likewise replace the rear cassette , keep the Speeds Count what it is,
and get a New Chain.. the parts to go to more speeds means you replace the shifters too and all cables and part with a lot more money.

Math ,,, 42:11= 3.8:1 ; 48:13= 3.69:1..






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