SRAM P5 setup

I've had a SRAM P5 internal hub (w coaster) built to fit my only bike which gets used for just about everything.

I test rode a Velorbis Arrow which has 61 gi and loved the bike so figured that making my direct drive gear (3) something close to this would be sensible.

If I use the 22 tooth sprocket I have with a 51 touch chainring then I get gears of 38.6, 47.6, 60.9, 78.0 and 96.2 according to Sheldon

These numbers are dutch to me though - does it sound like a useable setup for a london bike?

Once I've read up on chain lines, I'll be asking questions about them too. If I can get this set up with the crankset I have at the mo (shite steel with cottered cranks) I will but will upgrade to maybe the Sturmey set up when I have the available money.

Thanks

It would depend on the bike/ riding position I guess but just to give you some perspective: I can climb comfortably on 38gi on my road bike and very rarely go over 95 unless its a massive downhill, I probably wouldn't be able to use 78 much unless its a very long flat road and I can get up to speed (note: I'm a girl and bad at pushing big gears) . Personally if its mainly a town bike I would gear down a little, but that would give you a very spinny direct drive gear.

<-Also girl bad at pushing big gears!

I'm sure I read on the internets that 4th gear is the most lossy and therefore best avoided. I think what I want is a 2 speed hub, one that will get up Highgate Hill and one for the rest of the time - but I've got what I've got so I'll make it work!

The numbers refer to how many inches forwards the bike will travel with one revolution of the cranks, so a low number is easier to pedal and a high number is harder to pedal.

If I'm reading your numbers correctly, your 3rd gear is 60.9 which is so close to 61 you are never going to notice! Any difference in the way it feels is probably down to something else in the setup.

From what I can gather, most people's GI falls between 65 and 75 on fixed and maybe up to 80 on single speed. I ride a 65 GI brakeless and a 71 GI fixed with brakes if that helps. Oh and I go up Highgate Hill every Tuesday for North Drinks! If you're in the neighbourhood, swing by and say hello to everyone.

The numbers refer to how many inches forwards the bike will travel with one revolution of the cranks, ...

Nearly. You're out by a factor of pi. What you're describing is development. Gear inches traditionally refers to the diameter of the equivalent penny farthing wheel. Development = the circumference of the equivalent wheel, i.e. pi x diameter.

Gear inches are really just an almost arbitrary scale that gives comfortable size numbers for people to talk about (increments small enough that you don't loose much by rounding to the nearest number, but large enough that you don't need many digits).

I think the middle gear of a P5 hub is direct drive - i'd aim to make that the gear i use most for cruising along, as it should give least friction and least wear on the hub. The choice of 60.9 looks good to me. I think the high friction gears will be 2 and 4 as they impose skew forces on the compound planet gears.

i have just been through all this with a 3 speed going from a single speed.

my orig set up was 72 which is great for town riding.

the dilemma is which gears are you going to use for flat and which for climbing, caning it. with a SA 3 speed i went for 18x42 giving 47, 63 and 84 on the 3 gears. i think this is nice - middle direct drive for 80% of flats on my commute, good for getting away from lights quick, top gear for long roads where you can get up a bit of speed, nice to feel that you can go fast and then low for climbing. Arguable with a five speed you could have gear 2 as your cruising one and 3, 4 and 5 for climbing but if gear 2 is putting stress on the hubs seems rude not to use the direct drive of 3

61 sounds a touch low for main gear, i'd go for nearer 65 and agree with person above that much above 85/90 is maybe not that much use in london with lots of traffic lights, however it feels great to have a top gear to crank out..

i am a rank amateur tho, so this could be rubbish mind..

Gear inches traditionally refers to the diameter of the equivalent penny farthing wheel.

Holy smokes - I'd just assumed. I'm totally going to find out how far forwards my bike travels on one crank stroke and start a revolution.