mrtuttle04

I currently have an inexpensive 24-speed hybrid that needs a new crank and front derailleur. I am thinking about replacing the three-speed crank with a one speed crank and removing the derailleur and shifter making the bike an eight speed bike. This brings up two questions.

1. What crank do you recommend? I would like to put on a 36 or 38 tooth crank but I do not want to sink a lot of money into an inexpensive bike.

2. Are there any "gotchas" that I need to worry about. Can I simply replace the crank and remove the derailleur and shifter or is there something else I need to do?

Thanks
Mark

hueyhoolihan

it's tempting to ask what is wrong with the crank.

but i won't.

here's what i did with an old MTB.

i just removed the inner and outer chainrings and front derailleur. you might need new singlespeed chainring bolts, unless you have a few extra chainring washers...

dsaul

Without a chain guide of some sort, the chain is likely to derail off the front chainring when you hit bumps. A narrow/wide chainring like this https://www.wolftoothcycling.com/prod...oss-chainrings will help. You can also just leave the derailleur in place and lock it in position with the limit screws.

cny-bikeman

I would be surprised if that would be much of a problem, but you could also address it by simply getting a bashguard instead of replacing a working chainring, and a chainkeeper for the inside. The bashguard can offer some protection from chain grime as well. You don't have to do anything, though until you determine there's actually a problem.

Retro Grouch 

The trick to a project like this is keeping the cost down.

"What's wrong with the old crank?" is a valid question. If possible, I'd want to keep the old, already paid for crank and it's bottom bracket. There are two questions to be resolved: 1. What's the middle chainring look like? 2. What's the chainline look like? If the middle chainring isn't too worn (my bet) and it's about the right size, I'd try to use it. First, hold a straight edge alongside the middle ring and see where it bisects the cassette. If it hits pretty close to the middle, that's good. If it's off a skosh, plan on using washers to space the chainring more toward the center of the freewheel. You'll probably have to buy a set of single speed chainring bolts but, if you can shorten the chain yourself, that might be all that you'll have to buy.

If you get the chainline right, there's a good chance you'll be able to do without either a bash guard or a chain keeper. There have been lots of 7 & 8-speed bikes made that came from the factory with neither. If you have problems with dropping the chain you can always buy such add-ons later.

fietsbob

'speeds' is a meaningless term of marketing.. a chainring is a thing.
what you need is the bolt circle dameter , and the tooth thickness

a single 'speed" chainring is 3/32 or 1/8" thick and intentionally lacks the shifting aides added recently
to make the quicker derailing work.

teeth are taller , .. no ramps or pins.
chain alignment so rear cog and chainring are in a straight line parallel to the centerline were covered above..

triple crank? maybe the middle ring will line up with the new single cog wheel.
it is the right spot where the rear derailleur is retained and the hub unchanged .
middle ring is in the middle, after all ..

You need to sort that out , maybe a Bike Shop will help.

hueyhoolihan

there is nothing to lead me to believe, after riding fifteen years and tens of thousands of miles touring and commuting on a 1x7 setup, that a single chainring on a road bike is more or less likely to derail than one that has two or three chainrings. but, if one is the type to wear a helmet while on a trainer, i would advise using one if for only for consistencies sake.

mrtuttle04

Thanks everyone for the imput. The primary reason for replacing the crank is the middle chainring is worn and must be replaced, along with the chain. I realize I can just replace just the chain ring but replacing the entire crank gives me the opportunity to go to a longer crank arm.
The reaon I am thinking about goint to a single speed crank is: 1) the derailer needs replacing but I do not want to sink a lot of money into an inexpensive bike , 2) I ride mostly level ground and have little need to use the high or low gear.

Retro Grouch 

OK. The key to this project is going to be the chain line. You'll want your single chainring to line up with the middle of your cassette. If your middle chainring lines up with the middle of your cassette now (usually the case) measure in millimeters from your existing chainring to the middle of your seat tube. That's the chainline you're shooting for. When you buy your crankset, you'll likely need to get a new bottom bracket too to get the chainline to match. Also, my rule of thumb is to always pair a new fresh chain with new sprockets.

So crank & chainring, bottom bracket and new chain. The cost is creeping up.

hueyhoolihan

remember, if you buy the chainring separate from the crank, that you may have issues unless BCD of crank is considered. 110 or smaller BCD for 36 or 38 chainring will do.

HillRider

Depending on the model, a lot of cranks on "inexpensive" bikes have their chainrings riveted in place and they cannot be replaced. If this is the case, you need a new crank anyway.

mrtuttle04

Thank you Retro Grouch. You told me exactly what I wanted to know.

mrtuttle04

I just checked the bikes and ther are riveted in place. So yes I am replacing the entire crank regardless.

hueyhoolihan

if that's the case, i would just shop ebay for an inexpensive used road crank with the arm length you demand, it may not be possible to find one with the chainring you want. if so i would get a crank without chainrings and buy a cheap chainring with or without shifting pins and ramps or all the other shifting gadgetry on it, doesn't really make any difference one way or the other, IME, in 3/32" width. remember the singlespeed chainring bolts will still be necessary. don't forget that the crank must fit on the BB that you have. there are many (incompatible) attachment methods that can bite you.

crank about... 30

chainring... 15

go to LBS and ask the salesperson with the most body piercings and tattoos (BMXer) where the singlespeed chainring bolts are, they will know... 5-10 (hard to believe, i know).

for chainline issues it's possible to mount the chainring on the inside of the spyder or outside, whichever's best.