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Can you substitute a 39 tooth w/ a 30 tooth?(10 posts)

Can you substitute a 39 tooth w/ a 30 tooth?js5280
Jul 25, 2002 7:19 AM
I have a friend with a double 53/39 with an 8 speed 12/23 in the back. It's older Trek 5200 (Carbon). The gearing is a little too tall and she ideally wants a few gears below 39/23. I plugged in 54/30 w/ 12/23 in Sheldon's gear calculator and it seems like a good gear inch combo. The bottom/top two gears overlap and gives her 3 additional lower gears. Will a 53/30 work?

Other options I've come up with are:

1) Swapping the cassette to 13/26, but that only gives her 1 extra low gear at the expense of an upper. Couldn't find a lower cassette than this.

2) Upgrading to a 9 speed double, which gets expensive (Cassette, R. Der., STI, Chain)

3) Upgrading to a 9 speed triple, which gets really expensive (Cassette, R. Der., STI, Chain, BB, Crankset)

Any other options I'm missing?
Short answer - no.BipedZed
Jul 25, 2002 7:30 AM
The crank probably has a bolt circle diameter (BCD) of 130mm. The smallest chainrings available in this BCD is a 38T.

Her best bet would be to swap her double with an 8spd triple. Check bike shops to see if they have any 8spd triples laying around that were swapped off of other bikes.
That was my suspicion.js5280
Jul 25, 2002 7:56 AM
Thanks for the fast feedback! I have to tell her to look into these other options.
How about a mountain bike cassette?Spoke Wrench
Jul 25, 2002 7:33 AM
You need a cassette, rear derailleur and new chain. Best feature is this is a proven solution. I wouldn't bet on getting good shifting from a 30 to 53 chainring. Worst feature is the gaps between gears are going to be wider.
re: 12-28?Chen2
Jul 25, 2002 8:23 AM
Sheldon Brown lists a 12-28 8-speed XTR cassette for $59.95. That should work with the existing derailleur. You might need to adjust the "B" screw and maybe add a link or two of chain. Replacing the 39 ring with a 38 would also help. I like those options best. You can't put a 30 on that crank set.
~Al
just buy a mtn bike cranksetstr8dum1
Jul 25, 2002 8:48 AM
LX crank and BB would be under 70$
then sell the road cranks for $70 on ebay
good idea but won't workChen2
Jul 25, 2002 10:43 AM
We're talking about a Trek OCLV with a "braze-on" front derailleur. The road derailleur won't go low enough for good shifting performance and even if you could the curvature of the derailleur would be way-off. And you can't put a mountain clamp-on der on a CF bike (in my opinion). And the small and middle rings would be too small. But it was a good idea.
~Al
Get a mtn cassette !PeterRider
Jul 25, 2002 1:10 PM
get a 11-28 MTB cassette - I did it with a XT cassette (also on a Trek 5200 8spd), you can also use LX or XTR. You need an additional spacer and lots of derailleur adjustment.

I have a XT 11-28 (M750 - 8spd) cassette sitting in my closet, brand new and unused, if you want I can sell it...

Pierre
additional commentsPeterRider
Jul 25, 2002 1:15 PM
if you have a shimano 600 derailleur (this is what I had), you need only a cassette, no new derailleurs/shifters...

Some people use a 11-32 cassette, but you need a MTB derailleur, and most likely a new set of shifters

What you were saying, 30/53, would result in bad shifting and drops of the chain. In the shimano line, I think the biggest teeth difference accepted is 22teeth for a triple setup.

Pierre
re: Can you substitute a 39 tooth w/ a 30 tooth?kcd
Jul 25, 2002 4:06 PM
I recently switched an RX100 8-speed 12/23 to 12/32 (for the big hills) by replacing the rear derailler, cassette, and chain. The total cost was about $50 (using PC-48 chain, Performance cassette, and Sun Race derailler). It was a suprise to find that this new setup shifts much better, probably from the Sun Race derailler.