Crank length on converted roadies

[bene313]

What crank length are people running on converted roadies?

[GaryF]

I suppose the old standard for a track bike is 165mm. This was to give the pedal clearance from hitting the track when rolling around the velodrome. The pedals had the outer part of the cage removed for the same reason. Track bikes tended to have a higher bottom bracket as well.

Is it easier to spin shorter cranks? Do you get more leverage from longer cranks? Do you want toe clearance so you don't hit the front wheel as a track frame is probably short in its wheelbase? Is the chain line straight (crank cog and wheel cog alignment). These are other related questions.

But your question is about a converted road bike. Where are you riding it? If it's going to be raced on a velodrome you are going to consider some of the questions above (clearance). Your answers will take into consideration your frame geometry. Your choice of crank length will come from your needs.

An important consideration may be chain line. What cranks are you going to use? Converted road cranks may put the chain out of alignment with the rear wheel which brings up the question of rear spacing. What is the rear spacing of the frame? Are you using a converted road hub? Will you need spacers to align the cogs? If the rear stay length is long the chain line won't be as important to get right. Is the rear hub a track hub (120mm)? The chain - is it a 1/8" chain or a 3/32"? 1/8" is the track standard but it probably has less lateral flex. Chain width will have a bearing in crank/chainring/rear sprocket choice.

There are a lot of questions that you could ask when converting a road frame to a fixed single speed. Or you could just ignore all that stuff and convert the bike and have fun on it. If it wears more quickly and isn't as efficient - so what.

I suppose the answer to your question depends on how the bike is to be used. If it's just a fun bike the crank length wouldn't matter. If it's going to be used in 'anger' other things need to be considered.

[bene313]

Well Gary that's a lot of questions. But I get your point.

The bike is for street. Commuting, fun, some longer rides.

The rear spacing - 126mm dropouts, and the track cog is 42mm from the centre.

The chain I'm using is 1/8" mainly due to the track cogs being 1/8".

I've got the 170mm original cranks from the old road bike (obviously 3/32"). With my current bottom bracket, the measurement from the centre to the wheel cog is 44mm. So 2mm out from the back measurement of 42mm (but sometimes I think it looks a little more than this - not sure why). The wheel cog has room to move in towards the frame by about 5mm (which can be achevied with a shorter BB).

Now that I've said all this, I also should ask - is 2mm difference in chainline is an issue?

[europa]

When converting a roadie for the road, just use what comes with it. The chainline issue can rear up and bite and this is easily fixed by spacing the rear hub across to line up the cog and crank and then redishing the wheel.

If you're investing in track wheels, it's worth investing in a track bottom bracket and crankset at some point - this puts everything in line without dramas. But note the use of the term 'investing', you don't HAVE to do that, it just makes it easier and neater. Depending on the cranks, sometimes you can whip off one chainring and be left with a clean look, then get the right sized bottom bracket to line that ring up with the cog, but this is a suck it and see effort.

My Europa spent much of its early fixed gear life wearing two chain rings, 27 x 1 1/4" wheels and a suicide hub. Cost me a track hub and 1/8" chain only and justified spending the money to do it properly (over time, the best way to do it). None of that matters if the set up works and to hell with the fashion Nazis.

1/8 chains run on 3/32 cranks without a problem. Don't worry about that and don't bother trying to source 3/32 cogs (though I've got one here somewhere), they're made out of unobtainium.

2mm out? I'd run with that for the time being. I've read arguments that claim it has to be perfect and read opposing arguments from long time fixed riders who reckon it can be 5mm out without worrying. It's a bit like the suicide hub argument - if you're into extreme riding with lots of skids etc, do it properly. The more relaxed you are, the more you can get away with.

Richard

[tallywhacker]

2mm won't make any difference to the bike "going" but you will notice a bit of grinding

[europa]

2mm won't make any difference to the bike "going" but you will notice a bit of grinding

Yes, it's one of those things where irritation will spur the effort before guilt will

Richard

[GaryF]

Hi Bene313,

I agree with Europa and Tallywacker, they have a wealth of experience. I think the 120mm road cranks will be fine. If you get behind the bike you can visually determine how far out the chainline is. If it's not too bad, it should be quite OK to ride. As Tallywacker states, there might be some grinding of the chain against the side if a cog but you are using a 3/32" chainring with a 1/8" chain so there is a littie slop there - which may help.

Fixed wheel bikes are fun, as you no doubt know. I remember, when I was much younger, trying to ride my old fixed wheel backwards. I had seen others do it but no matter how much I tried I just couldn't do it. I'd still like to be able to do it.

Have fun with the bike,

Gary.

[bene313]

Thanks all.

So has anyone gone as short as 160mm?

Ben

[GaryF]

Not me. You should give it a go and compare with your 170's.

[vee_dub]

Depends how low is ur BB to the ground....Road bike tend to be lower on the ground hence shorter cranks is a better choice.

[tallywhacker]

I use 175s, high bottom bracket but ocassionally you forget, lean too much and dig a pedal in. Nothing like the rear wheel lifting off the ground when cornering.

[vee_dub]

I use 175s, high bottom bracket but ocassionally you forget, lean too much and dig a pedal in. Nothing like the rear wheel lifting off the ground when cornering.

Rear wheel lifted off the ground is the scariest thing in fixed gear riding. I was riding out of a carpark got too excited, lean too much and lifted my rear wheel up.
Toe overlap is another concern, even i got a big feeet i tend to have a medium cage for my sz11 feeet. No toeover lap at all. (Road Bike have higher rake hence less likely to happen).

My preference is 165 to 170mm. Right now I am running 170s