Bicycle Mechanics - Old 105 vs modern Tiagra

I've got a 1980s vintage Trek 1400 with Biopace rings on on an old square taper Shimano 105 crank in 170 mm. I have long legs and my other bike has 175mm cranks. I want to get rid of the Biopace rings (I'm not trying to start a debate on the merits of Biopace). The 1400 is also in need of a new BB.

My options are to put in a UN54 square taper BB and new chainrings or to replace the crank. As budget is an issue, my LBS mechanic suggested that I replace the crank with a modern Hollowtec Tiagra compact double. I can get the crank length and chainrings I want with the appropriate bottom bracket for about $110 or replace the chainrings and get a UN54 for the 105 for about $85.

He said that the modern Hollowtec Tiagra crank and rings will definately be lighter and are probably as good or better than the old 105 crank and vintage rings.

I'm not sure which way to go. A modern 105 compact crank is outside of my current budget. Thoughts and suggestions welcome.

Thanks in advance.

The performance difference will be negligible and I am not convinced the tiagra will be lighter. For $15 difference in price I would say go with whatever one you like the look of better.

BL

Tiagra is a good product line and your mechanic is correct it will be lighter and the chainrings better shaped than your old 105. However, if your bike is 7-speed and you are using a 7/8-speed chain, the new Tiagra crank (9-speed, or even 10-speed for their newest ones) will have the chainrings closer together and your chain will rub on the big chainring while in the little one. So, the Tiagra crank is likely to be incompatible with the rest of your drivetrain unless you have updated it to 9 or 10-speed. If not, refurbish your current 105 crank.

Tiagra is a good product line and your mechanic is correct it will be lighter and the chainrings better shaped than your old 105. However, if your bike is 7-speed and you are using a 7/8-speed chain, the new Tiagra crank (9-speed, or even 10-speed for their newest ones) will have the chainrings closer together and your chain will rub on the big chainring while in the little one. So, the Tiagra crank is likely to be incompatible with the rest of your drivetrain unless you have updated it to 9 or 10-speed. If not, refurbish your current 105 crank.

+1

I thought it was the rings, not the crank that determined 8S, 9S, 10S, etc. Therefore if you put old rings (like Biopace! haha) on a new Hollowtec crankset they would work just fine with the old drivetrain.

Personally I'd go for the new crankset and see how it is rather than spending $85 for a new BB and rings. Worst case you have to get a new (old) little ring.

I thought it was the rings, not the crank that determined 8S, 9S, 10S, etc. Therefore if you put old rings (like Biopace! haha) on a new Hollowtec crankset they would work just fine with the old drivetrain.
Yes, it is the rings that set the spacing but the OP wants to replace the Biopace chainrings on his 105 crank so, buying a new 9 or 10-speed Tiagra crank would require also buying the older 8-speed rings for it. That way he gets to pay for both the new crank, a new bottom bracket and replacement chainrings. Doesn't sound like a good deal to me.

some chainring spacers should solve the spacing issue.

some chainring spacers should solve the spacing issue.
Yes they would but the OP has to know about them.

My options are to put in a UN54 square taper BB and new chainrings or to replace the crank. As budget is an issue,

You have a lot more options than that. You can buy a myriad of used cranksets. Or you can buy a myriad of other new cranksets with bb.

As far as needing a new bb, a UN26 will set you back $10 on line.

In my experience with new/old hybrid drivetrains, the chain rub isn't much of an issue. If you don't cross chain, you will be fine.

I was looking at Sheldon Brown's site and he says that there is no difference in the crank spiders between 7/8-speed and 9-speed versions, it's all about ring width and design. He further said that a 7/8-speed chain should work OK with a 9-speed double as long as it I don't severely cross chain in the small ring. If I do end up using small/small combinations (unlikely as it doesn't make sense) and rubbing becomes an issue, I have the two options of a larger small chainring and/or a 9-speed chain, which Sheldon says will work OK with a 7-speed cassette though it may not shift as smoothly.

If you have time to look around you can find new 105 compact cranks cheaper than you might think. I saw a black 105 crankset the other day for about $65.00 or so. I believe it was compact, not sure.