jbronson

Hello everyone,

I'm looking for a crankset for an older MTB and I need to get a sanity check/second opinion on what my bike mechanic is telling me.

I've got an old Univega Acera 400 (Bikepedia stats) that's overdue for a drivetrain replacement. I brought it into the local bike shop today and the mechanic told me the crankset would be hard to replace because new MTB cranksets have wider spacing than my 9-year old bike's. He thinks the cogs will need to be replaced one at a time. This is apparently a lot more expensive than just buying a whole new crankset.

Bikepedia says the crankset that came with the bike is a "SunRace Accura, 22/32/42 teeth" - I haven't been able to find this item online anywhere. I did find a Shimano Alivio crankset with the same size cogs on Amazon (here) but I don't know if the thing will be compatible with my bike.

I'd also like to get new Acera front and rear derailleurs so the whole undercarriage of my bike will be new.

How do I know if a crankset I find online is going to be compatible with my bike? And how do I know if the front derailleur will be compatible with a given crankset?

Thanks in advance!

LesterOfPuppets

That's a pretty low end bike to be buying parts for a la carte.

As far as I can tell that new crank you're looking at needs a BB with a 113mm spindle. You might be able to determine your current BB's spindle length with some calipers, or by removing the BB to look for labeling.

Your front derailer probably doesn't need replacing, unless it's been injured or the bike's been left out in the elements for a few years.

reptilezs

they still make cranks with 47.5mm chainline if that is what you are looking for.

desertdork

You'll need a new bottom bracket (BB) to go with a new crankset. That Shimano Alivio crankset is apparently available in two types: one that is compatible with a "square-taper" BB and one that is compatible with an Octalink V2 BB. These are two slightly different systems with different advantages. You can find both cranks here. According to this Shimano document, you'll need either this Octalink V2(68X118mm) or this square taper BB(68X113mm). I'm taking this off Shimano's pdf, with the assumption that your BB is English threaded.

Unless they're dead, I'd skip replacing the derailleurs. With the cranks, BB, chain and cassette, you'll have spent enough money. In addition, your cables/housings may also be in need of replacement. Spend what you need to spend.

jbronson

Desertdork, thanks for the research. It sounds good, but the Bikepedia page lists my bottom bracket's spindle as being 102mm. Maybe this is the narrower width the bike mechanic was referring to? Or can the spindle length be whatever length, as long as it matches the crankset?

king koeller

If I was you, don't replace anything unless it's totally trashed. Just over haul everything and you'll be all set.

desertdork

You need to match the bb spindle length to the cranks you'll be installing.

bigbossman 

It would be cheaper to buy a donor rigid mtb off the local CL, and use it as a parts donor. I just picked up a clean Giant mtb for $30 - wanna buy the drivetrain?

bikinfool

So what exactly is wrong with the crankset that it needs replacement? Same for the derailleurs...

I could see maybe you've got an old worn chain and cassette on a well-used bike more than it needs new derailleurs/cranks....and I sure wouldn't spend a lot of money replacing the whole drivetrain.

jbronson

The chain was slipping, but only when I put medium-to-hard pressure on the pedals (as in normal acceleration from a stop). I did the research and found out that this is what happens when the chain stretches and the teeth wear out. The bike mechanic agreed. I also had him put reinforced, goo-filled tires in. I'm in southern Arizona, where the goat heads apparently shred everyone's tires. So, I've just invested some money into the damned thing.

I'd just received this bike in the mail. I just joined the Army, and I had my former roommate send it out to me. When I put the chain back together, I think I put the rivet in either too far or not enough. I was riding it home this afternoon and the rivet started coming undone, and then caught in the rear derailleur, which was ripped off the bike. So now I've got the problem I was describing before, plus a busted rear derailleur hanger.

As for why: I got this bike 8 years ago and I've had other, more expensive ones, but I really like this one. So, I'd like to replace the drivetrain and hopefully get another 8 years of riding out of it. I'm looking at about $90 in parts from desertdork's suggestions, which is a lot less than the $300 I'd need to spend to get a new ride with equivalent specs. What do you think?

bikinfool

That the chain was skipping was more likely from the chain/cassette needing replacement rather than anything to do with your crank (altho it is possible your chainrings, or at least your most used chainrings, are worn). If your crank spins smooth and there's no play in the bottom bracket, leave the crank as is and perhaps replace the chainrings if needed (or are they not replaceable? On some low end cranks they're permanently mounted). I'll help you out with a derailleur from my parts box, call it a thanks for serving (PM me), but you better make sure you can get another hanger for that bike first. Try derailleurhanger.com if your local bike shop can't help. Gotta be careful when putting a chain together as you've found out (I've done it, too, learning can be expensive sometimes).

jbronson

The chainrings are replaceable. But can I get three chainrings for cheaper than the $50 it would cost to get a new crankset?

bikinfool

That indeed could be the dividing line...if you need all three. Crank and bb don't forget (not likely to find another crank using a 102 spindle too easily if your bb doesn't need replacement). This is a good deal on a crank with bb https://www.jensonusa.com/store/produ...t.aspx?sc=FRGL.

cyccommute 

Unless the crank is broken, there's no real need to replace it. A 2001 bike should have similar cranks to what you can find on new bikes. A 102mm bottom bracket is very narrow. I doubt that you even need new chainrings...they don't wear that fast.

If you want new derailers, there's nothing wrong with that. Your chain and cassette is probably worn as bikinfool alluded to. This bike might have a freewheel, however. In any case the chain and the rear gears need replacement. The derailer could probably use some minor tweaking as well.

I'd suggest addressing the chain slipping problem first. Get a new cassette (or freewheel) and a new chain. Those are relatively cheap fixes. You don't need to spend more than around $30 to $45 to fix that problem.

Keep the crank and change the derailers if you want. Those are pretty cheap fixes too but more expensive than a chain/cassette. You could even step up a level or two in quality. Go with a LX derailer. Good price with good quality.

You can get rings for as little as $15. If the rings are worn, it's probably only one. Look at the teeth and if the teeth are shaped like this



they need replacement. But if they are fairly symmetrical in shape, they are fine. You'll likely find some of the teeth are appear flattened. This is normal too. It aids in picking up the chain for crisper shifts.

Keep the crank and concentrate on fixing the problem

jbronson

Here's what the crankset looks like:


Looks to me like the two outer rings need replacing (and the whole thing needs cleaned up). What do you think?

The thing is, all the chainrings I'm finding are $25 apiece. This makes me think I might as well get the crankset/bb combo desertdork linked above, which will cost $50. Then I can get a cassette and a chain for about $20 each, for a total of $90.

cyccommute 

The plot thickens

That isn't a Sunrace crank. The Bikepedia listing for your Univega should be a 4 bolt 104mm bolt circle diameter (BCD). But that's a Shimano STX crank which dates from 1995 to 1999. Is it a replacement? Those were 95/58mm BCD (for the most part). Look here for good selection or try googling "chainrings 94mm". I may have some in my parts been that I'll never use. Let me check tonight.

LesterOfPuppets

STX is definitely a step up from stock!

jbronson

It is a replacement! My apologies for not mentioning this - I had some kind of issue when I first bought the bike and the store owner hooked me up with the STX crankset. The bike actually has a lot of upgraded/replaced parts (the rear wheel is a rhyno lite with a deore hub, the brakes are newer shimanos, I don't know what model), which is one reason I want to hang on to it. It's my beloved frankenbike.

I measured between two of the crankset's bolts and they were 55mm apart, which according to this chart, means it's a BCD of 94mm - just as you said, cyccommute. If I buy the outer two rings new, it's going to end up costing me about $50, but I guess it'll be an easier operation putting them on, right?

cyccommute 

There is more work involved in changing cranks than in just changing rings. All you need to do to change the rings is take the left crank off, remove and replace the rings and reinstall the left arm. To change the crank is more involved.

Unless you are having the shop do the work (it's pretty easy to change rings), look around some more for rings. Cambria has a 42 outer for $5. You can find inners for around the same.