Gear ratio and speed inconsistency

Hi, would anyone know why this might be….

I’m spec’ing the parts for a new fixed and am trying to work out a good gear, something similar to my geared road bike. Unfortunately the figures I get from my Sigma computer (which has the correct wheel circumference entered) are very different from http://software.bareknucklebrigade.com/rabbit.applet.html or Sheldon Brown’s site.

53 x 19, 700 x 23, 175mm cranks = 73.3 gear inches. @ 90rpm = 19.6mph

My Sigma tells me I’m doing 89rpm and 22 mph.

Also, my geared bike is quite light with fairly good wheels (Mavic Kysrium SSCs), if I do choose a similar gear ratio for the fixed will heavier wheels (probably Open Pros) have much of an impact on the gear I’ll be pushing? Are fixed’s, because of the fact you’ve got momentum from the wheels carrying your legs around, slightly easier to push a larger gear or is this wishful thinking? I’ve been doing my daily commute in all weathers at 73.3 and except for a full on 20 mph headwind I’m usually OK.

Getting a close match to my current 73.3 gear inches would be a 46x17 = 71.1.

Many thanks if anyone has any thoughts.

The wheels might be heavier but the bike itself will usually be lighter (unless you have a light roadie already).

My thoughts on the matter are: you like numbers too much. Aim for gear inches in the high-60's, low-70's and ride it.

pick the roadie :p

Are you sure you're on the 19t sprocket?
If you're actually on 17t, that gives you about 22mph @90rpm

Defo 19t - I put tipex on the first tooth to make sure

Yeah, I think you're worrying too much about the numbers. Just approximate it, ride it, and see how it feels.
Your riding style will be a little different fixed, so you might not even want the same ratio as your roadie.
There's always people buying/selling sprockets on here, so just dive in, give it a go, and swap the sprocket if you don't like it after a while.

OK, thanks - you're all probably right. This is what happens when instead of getting a cheap runabout to see what's what you go completely overboard and order a custom frame then realise you've got 4 months to kill - thinking every little detail over and over and over.......

No more daft questions 'til after it's built.

Sure sure. Just wait until you realise the choice of hubs and rims and spokes available... and the permutations of all of them!

if you like numbers go and study on sheldon brown's site all the different threadings for headsets and bottom brackets and track hub lockrings, hub spacing, bb widths and chainlines, seatpost, handlebar clamp, steerer diameters we shall have a quiz with you very soon :)

so what's your favourite thread pitch then edmundane? ;-)

which part are you talking about? :P

Sure sure. Just wait until you realise the choice of hubs and rims and spokes available... and the permutations of all of them!

Oh, I'm already there - some nights I can't get to sleep 'cos I'm thinkin' about what should go with what. Fun tho'...!

Oh, I'm already there - some nights I can't get to sleep 'cos I'm thinkin' about what should go with what. Fun tho'...!

Welcome to the world of bike building. It's nearly as addictive as riding fixed (ask dogsballs!)

My next bike will have Chris King Single-speed ISO disc hubs, anodised red with Zipp 505 Clincher rims and Sapim CX Ray spokes, 32h front and back in crows foot pattern.

It's strange how many months of thought and dreaming can be reduced to one sentence.

It's a cross/tourer if you're wondering why the disc brakes. Oh, and a spare rear wheel will have the same rim and spokes but a Rohloff hub.

Oh, and a spare rear wheel will have the same rim and spokes but a Rohloff hub.

Nice.

Yeah, I want their new one that weighs the same as a regular gruppo.

CX-Ray Aero Spokes........ Mmmm, been looking for something similar to my SSCs for my new fixed and these look sweet. Do you think they'd look OK in Open Pros in 28 front and 32 back?

They look awesome with anything. Already have them on my Bob Jackson with CRP33's and Phil Woods, and they are most bodacious.

48x17 with 25mm tyres gives about 74". I can get up any hill I need to although Highgate area is a push and 30 mph is quite achievable on the flat without spinning out. I would try around 74" unless you are going for long rides with steep hills. I dropped down to 71" once or twice but just couldn't stop myself putting the 17 back on a week later ....

Here is my new front wheel

its not radialy spoked
phail

jokes! much envy lol

:) even worse than that it's not dura Ace track!