What Kind of mountain bike Chainrings Do I need to use with vintage topline cranks?
I thought I could just put them on in place of my shimano crank arms but they have two drilled holes for two bolts. what would you call the chainrings needed to fit these arms. Where can I get some?
Well if your cranks are similar to these http://www.bikepro.com/products/cranks/topline.html then you might run into some issues.
First it is the furthest outward hole that will take the Chainrings – the inner is just for design/ weight saving not for the Chainrings.
Next, part of your confusion there may be because the Bolt Circle Diameter of those 5 outer holes is 130mm (shorthand is to say 130 BCD) Where nearly all older mountain bike cranks are 110 BCD and newer compact cranks may have a 104 BCD. So what that means is you won’t be able to use regular mountain bike Chainrings on your Topline cranks.
There are still some possibilities. Really there is nothing particular about MTB chainrings that is really different from road chainrings. Chainrings are chainrings as long as they are modern enough to be designed for narrower 7,8,9 speed chains there isn’t any real design difference between a road chainring and a MTB chainring.
130 BCD is a very common size chainring for Road bike cranks. All the Shimano cranks up until the compact drive stuff used 130 BCD so you can use just about any Shimano chainring. The problem you will run into with this solution is that the smallest 130 BCD chainrings you can find are 38 teeth (or 39??? I think 38 but you don’t see them often) . A typical mountain bike would run 46 / 36 / 28 or maybe even 44 / 32 / 24 but you won’t be able to get a 36, 34 or 32 chainring to fit on 130 BCD and even 44 and 46 aren’t that common in the used parts bins.
So if you are planning to ride this as an Urban Commuter or such, or if you use it mainly on flat paths and smooth trails this can actually be good. You could run 48 / 38 or 50 /39 and gain a bit of long legged speed. If you need to gear down you could look at your rear cassette and change.
http://www.sheldonbrown.com/gears/ Here is a good tool for looking at your gear rations
The Headbadge: Shimano 600 (RD-6401) RD shifting