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Road vs. MTB cassettes?

G-Cracker

Monkey
May 2, 2002
528
0
Tucson, beatch!
A friend of mine who is starting to get into biking (and who knows even LESS than me) needs to know the difference between road cassettes and mtb.

He wants to know why he can't use a mtb cassette on his new TREK road bike. Someone told him he can't and he came to me with the question why? I know they're typically geared much differently, but technically they are interchangeable as long as he keeps the same number of cogs, yes?

Or no? I think he wants to move up to a larger cog so he can climb easier, but I'm not sure.
 

Westy

the teste
Nov 22, 2002
36,651
3,475
Sleazattle
I think the only problem that could come into play is if the cage length of the derailleur wasn't long enough to handle the difference in gear size. He may also have a problem with the idler pulley on the deraillur being too close to the largest gear, but I see this problem in MTB too, it is a hanger locasion issue.
 
J

JRB

Guest
It works fine if you run a mountain derailleur. The real problem is the huge gap between each gear.
 

Pau11y

Turbo Monkey
Yeah, I tried it. Shifting is soo much more sloppy w/ the bigger spread compared to the tighter jumps of the road stack. It is cheaper than swaping everything out to go triple. But if you don't fuss over super crisp shifting of the road stack and budget is an issue, then it's a descent work-around. The other option is to go compact in the front, altho there may be some issues if you have braze-on frt der (I ran into this one w/ my steel frame roadie).
Another option is to go longer crankarms. But if you're short, this isn't a good option. And, by going w/ longer crankarms, it'll make 90rpms more difficult to maintain - bigger raidus = more motion => more hopping in the seat when the revs get high.
 

LordOpie

MOTHER HEN
Oct 17, 2002
21,033
1
Denver
i think the largest cog a shimano rear-d can handle easily is a 27t. Campy goes to 29t. Don't most mtb cassettes go to 31 or 34?

Couldn't you just put small rings on it?
 

sanjuro

Tube Smuggler
Sep 13, 2004
17,412
0
SF
LordOpie said:
i think the largest cog a shimano rear-d can handle easily is a 27t. Campy goes to 29t. Don't most mtb cassettes go to 31 or 34?

Couldn't you just put small rings on it?
It is more than just total gears. A road der will probably hit the big cog in the rear, no matter how much you adjust it.
 

LordOpie

MOTHER HEN
Oct 17, 2002
21,033
1
Denver
umm, that's what I'm saying... you could have one cog, but if it's over a 27t, you're gonna need some serious adjusting and I'm not even sure if a standard road read-d will even get to a 31t. Maybe I'm not being clear and just my usual dumb self.

just re-read the OP... NEW road bike? Didn't it come with a triple? If he can't climb in a 30-27, then he needs patience as slow hammering in that gear is, what, 3-4mph?

Help him develop a good spin, in and out of the saddle, and he won't need an easier gear?
 

Pau11y

Turbo Monkey
You may get a long cage DA (for triples) to go 32, but it'll be a stretch...no pun. The XTR SGS will go (designed to go 34 in the back) but then you'll have the spread in the front to deal w/. It shouldn't be too bad tho. The M96x series of transmission parts are REALLY nice, but again, the jumps are kinda big for super crisp shifts. Oh, another thought... how will rapid rise feel on a roadie...lol!
 

G-Cracker

Monkey
May 2, 2002
528
0
Tucson, beatch!
Well, I'm not sure he wants to do anything to his bike. I think he's just trying to gain some knowledge. And I can't help him develop his spin as he lives about 1200 miles away from me. :)

Thanks for your help, monkeys!
 
J

JRB

Guest
I can tell you that it works, but there is a huge gap. It rode and shifted fine, but it had huge gaps and I spun out a lot.
 

igxqrrl

Chimp
Oct 1, 2005
9
0
With a standard 105 RD (circa late-90s?), I can shift my 11-32 cassette just fine with DT friction shifters. I assume a standard DA RD might not work quite so well.
 

sanjuro

Tube Smuggler
Sep 13, 2004
17,412
0
SF
Here is an evil trick: take the B screw out, and screw it in on the other size, so the head is hitting the hanger stop. You can get another 1mm that way.