cynergy

Hi Folks,

I have a Trek FX 7.4 bicycle. I think it's about 3 years old. It's a nice bike and has an 8-speed shimano rear cassette. I'd like to upgrade the current triple crankset and bottom bracket for this bike.

When shopping for a new crankset and bottom bracket, do I have to worry about finding an 8-speed compatible crankset or will most Shimano mountain bike cranksets work with my 8-speed cassette and derailleurs?

Thanks!

Andrew R Stewart 

Crankset upgrades don't provide much actual change of performance beyond the psychological (ring size not changed).

But- The ring c-c dimension is different between the 8,9,10 speed systems. There is some fudge and overlap as well as tolerance variations. So sometimes the miss match will work to a degree, sometimes not so much so. Andy.

Bill Kapaun

The 7.4 FX's I see are 9 speed??

cynergy

Oops - it is a Trek 7.2FX and it has an 8 speed drive train. I think I found some compatible 8 speed triple crank sets. Currently it has a road crankset with a 28/38/48 set of rings. I think I would prefer a more compact mountain bike set (pretty hilly where i ride and i am using it for commuting so i haul my work laptop and other items to/from the office). I think I found some 7/8 speed compatible shimano crank sets online. Thanks!

dabac

While doable, there's really marginal benefits to your suggestion.
Lightly loaded, hills, not mountains, a 28T small at the front should let you deal with almost anything.

If you're still struggling in your lowest gear, another cassette with a bigger biggest, would give you the same benefit at probably lower cost/effort.

Phil_gretz

No, you wouldn't. I'd run your crankset on a touring bike, for loaded touring. But I'd pair it with at least a 32T largest cassette cog, and maybe a Deore-level rear derailleur. What is the 8-speed cassette range? Maybe go to a slightly wider range, depending on your rear derailleur capacity? It's likely that your gearing is fine. How is your conditioning?

HillRider

Your crank has riveted rings so you can't change the current 28T granny ring for a smaller one and it has a square taper bottom bracket so any more "modern" crank will require a new bottom bracket too.

Your best bet is to find an older square taper Shimano, Suguino or SR 7/8-speed triple crank with a 110 mm/74 mm bolt circle. It will take down to a 24T granny ring. It may require a different length bottom bracket too.

fietsbob

So..whats wrong with whats on there? you would not wear out all 3 chainrings in 3 years..

any how, 110bcd 34 t are common as spares from aftermarket sources, likewise 74bcd, from 24t upward..

joejack951

If he is running out of gearing now, then he probably would prefer the smaller MTB rings. A MTB crank like a 44/32/22 with 700c wheels offers plenty of high end gearing, especially for loaded commuting.

HillRider

I agree that would be a better choice but all of the current MTB cranks I know of are 9/10-speed format and require an external bearing (Hollowtech II or similar) bottom bracket. They would also be best used with a 9 or 10-speed chain. This is going to be a relatively expensive conversion, particularly if the OP can't install the components himself.

joejack951

Nothing special but good enough, Shimano, 8 speed compatible and square taper for $41: https://www.universalcycles.com/shop...3&category=380

Bezalel

28/38/48 was a common combination when paired with 7-speed freewheels with a small cog of 14 teeth*. With a move towards cassettes with 11 tooth cogs, 22/32/44 made more sense.


* I know 13t cogs were available but 14 was much more common.

cynergy

Hi folks

I found a shimano crank set with the same compatibility for the square factory bottom bracket. It's an 8 speed MTB set with a 22 tooth chain ring.

I like the small/short gears for hauling stuff around the hills around here (New Hampshire). I am getting back into cycling after a 5 year absence. Trying to ride to work regularly.

Thanks for all of the replies.