Crankset compatibility...

Hello... newbie here. I am bidding on a compact crankset on ebay to upgrade my 1989 Trek 760, 6/7 speed 126mm hub length; current set up 53t/39t. The write up says it's 8, 9 speed compatible. Now will this fit my bike assuming the bottom bracket length is the same 68 x 108 mm? If not, why. Any suggestions besides getting a new bike. Thank you in advance...

 

In addition to the Shimano square-taper BB spindle that is undoubtedly on your bike, there are at least three other "traditional" BB types where the crankarms are bolted onto the BB spindle:

1) Campagnolo/Sugino taper
2) Shimano Octalink (two types: XTR/ROAD & XT/LX)
3) ISIS

Many square taper cranks are Shimano "compatible" ... but, many are not (e.g., Sugino & SunTour).

A majority of square taper BB spindles seem to have been asymmetrical, some are symmetrical (particularly, later Campagnolo Record/Chorus BBs) ...

I think that 68x113 is actually the more common length, and (AFAIK) is asymmetrical. I suspect the 68x108 is symmetrical; but, trying is the way to find out if it fits.

You should anticipate the possibility of needing to buy a new BB.

 

....I forgot to mention that I will change the bb to ISIS... I will stay w/ current components (Suntour Sprint rear derailleur, Superbe front derailleur, Suntour Winner 13-21 freewheel) ... is there still compatibility issues? Tech guy fr Nashbar says they're not (compatible). ...

 

The Tech guy is mistaken ...

You should be good to go ...

Just buy a new chain (I recommend either a Shimano or SRAM 9-speed even though you are using a 6-/7-speed freewheel) + new chain tool which can handle the narrower chain (I recommend the LIFU because of its cost).

You'll probably need a new BB tool -- the ISIS BB uses the SAME tool as the Shimano cartridge BBs ...

FWIW/BTW. I found that with the one ISIS bottom bracket that I have that the spindle was slightly longer than either the PARK or LIFU BB tool that I have is deep! I think I used a needlenose plier for the final ~1/8 (?) turn, but I may have used something on the outside of the cup (e.g., rubber band used for bundling Broccoli ... don't recall which, but I know I considered the rubber band + slip joint pliers). Maybe, the current BB tools are deep enough not to bottom out against the spindle!

 

I agree with Alfeng. The 8s cranksets work fine with 6/7s. I also agree with the suggestion for a new chain. You probably need one anyway and 9s will work fine with 7s (in my experience quieter than the old 7s).

 

True. Keep in mind though that old cassettes hate new chains and vice versa. You may need a new cassette as well.

 

Sheldon Brown says that the only things that are "speed specific" are:

- Indexed Shifters
- Cassettes
- Chain

So, you should be good to go.

http://www.sheldonbrown.com/speeds.html

 

Can recommend the SKF BRI series of Isis BB's. They use rollers bearings on the drive side rather than the traditional round bearings. This is so the forces imposed on the bearing will be spread across the whole roller bearing given it's cylindrical shape rather than just at on point with a round bearing. Cost is between USD$60-130 depending on the model.

 

Matching components is generally TRUE with indexed shifting, but not always, and less so with friction shifting.

Matching the front derailleur to the chain will be beneficial, but is probably not necessary if the original front derailleur is adjusted properly.

The teeth on older freewheels are more generally robust than the teeth on Shimano cassettes because the freewheel cogs are thicker, so I suspect there will not be signficantly apparent wear.

BTW. The SunRace freewheel cogs are spaced (and, ramped) to index with Shimano 7-/8-speed indexed shifters.

 

I wonder, if the hub and frame spacing will allow, would the new Shimano 7 speed 11-28 MF-HG50 Freewheel be an advantage to him?

 

I would think so ...

A future upgrade!

BTW. There are 135mm Shimano hubs (Alivio, I think) which have 7-speed (i.e., shorter) freehubs ... remove a couple of spacers & shorten the axle to get it to fit a 126mm frame OR respace the frame to 130mm when the time comes.

 

This is slightly off topic but didn't want to start a new topic...
If one goes from 53/39 crankset to compact do they need to change the front deraileur and shifter as well (as the crankset)??
Thank U!

 

You do not need to change the shifters ...

Shimano says that you do not need to change the front derailleur with THEIR compact crankset ...

Others, Campagnolo & SRAM, have front derailleurs which are optimized for 50/34 chainring combinations.

You may need/want a rear derailleur which has more chain capacity, however ...

Try what you have & see if it works before ponying up more money beyond the cost of the new crankset.

FWIW. Because weight is NOT a factor to me, when I have the choice, regardless of the crank-type & rear cogs, I generally choose derailleurs which are triple-capable ... a legacy from years of following the Boy Scout motto to "be prepared" (after being caught short one-time-too-many!).

 

Thank U... Here is another (ignorant) question.
I quess that I might need a rear deraileur for the chain slack that will be caused by the smaller diameter front disks? If yes wouldn't taking a couple of links of the chain solve it? And you mean triple-capable rear deraileurs I guess?

 

Well, in theory, you could just remove some links to take up the slack OR you could even leave the chain slack (with a road bike ... too much jostling with an MTB, however), but the slightly short chain would mean that if you accidentally shifted onto the largest cog (or possibly, the second largest on some cassettes) while the chain is on the larger of the two chainrings, then you could cause all sorts of problems ... sometimes, a derailleur will simply balk at making an impossible shift (i.e., where the chain is too short), but there have certainly been instances where the chain does make the connection, and consequently the wheel is pulled out of the dropout OR other things may occur ...

Personally, I generally get both triple-capable FRONT & REAR derailleurs -- in the past, I ended up getting XTR rear derailleurs to use with (first) my Shimano shifters, and later my Campagnolo shifters because I wanted to use a cassette with a 32t cog and the Shimano 6503/Ultegra could only be tweaked to go to 30t.

Yes, I am one of THOSE people who show little regard for what the techs at Shimano & Campagnolo say with regard to compatibility.

BTW. If you put a 13t lower pulley on your rear derailleur, that will take up one inch of the chain's slack. The UPPER 13t Dura Ace pulley was actually cheaper than the lower, for some reason, when they were readily available. I think that the cost of replacing BOTH pulley wheels with 13t pulley wheels would be close-to-OR-greater-than the cost of an LX rear derailleur.

BTW2. The thing about using a front derailleur intended for a double with a compact crank is that the trailing edge of the cage of SOME front derailleurs may allow the chain to be dragged across it in some gearing combinations ... the trailing edge on the front cage of a derailleur designed for a triple is lower. Also, the back plate on a triple is extended to ensure more links of the chain are pushed toward the larger chainring. Does THAT make a difference? It may not with a 34t as much as it does with a smaller chainring, but other than the slight additional weight and dorky-to-some-appearance, it can't hurt.

 

Wow... I was under the impression that you couldn't use a triple deraileur with a double chainset. I do have a triple 105 deraileur and for the small and middle ring it has 2 positions for each. so essentially there are 5 clicks to go from the small ring to the big one and not 3. Am I right in guessing that this has to do with the shifter and not with the deraileur itself? The next question is how can you make a front deraileur intended for a triple work for the double? Is it by adjusting the inner and outer screws?
I am sorry if I ask too many questions but other than changing a tube and adjusting cable slack I don't know anything about bike DIY.

 

The shifter determines the indexing. The shifters for a triple crankset have extra "notches" inside the mechanism to allow the shifter to engage in additional positions.

I've heard/read people say that there is a difference in how the front derailleur responds to various shifters ... supposedly (for example), the MTB front derailleurs don't index properly with Shimano's ROAD shifters ... that may be true if you are trying to set up a triple; but, I have successfully set up a standard road double (52/39) with an 8-speed LX front derailleur and 105 9-speed shifters!

I currently use 3-clicks to move the LX front derailleur sufficiently to shift a 9-speed chain.

Will those 105 shifters & LX front derailleur work with a triple? I don't know. Probably, not.

Much greater intended-compatibility between components is required to properly set up a triple than a double crankset.

Regardless, you are correct in surmising that you simply need to adjust the derailleur's STOPS to prevent the cage from swinging too far outward/inward ...

So, if you currently have triple front & rear 105 derailleurs, after you install your new crankset, you just need to adjust the stops for the front derailleur so it won't swing too far in either direction, move the front derailleur's clamp down a bit, and retension the front derailleur's cable.

Put the chain on the big-big chainring & cog combination and see if there is too much slack OR put the chain on the small-small and see if the chain rubs against itself as it snakes through the pulleys. Remove links if you feel so inclined (one inch == 2 links).

BTW. The Shimano 8-/9-speed MTB rear derailleurs did not change their mechanical geometry.

However, the Shimano ROAD rear derailleurs changed their mechanical geometry a little when the 9-speed groups were developed; and, more so with the 10-speed Shimano rear derailleurs. Nonetheless, I feel that I was able to successfully index a Shimano MTB (XTR) rear derailleur with Ultegra 6500 9-speed shifters & 9-speed Shimano cassette -- close enough, for me, at least ...

 

Thanx a lot. You clarified a lot of things...