Gear inch/ratio question

For 25mm tyres I always take the dia. as being 26.7 if that helps.

I love how someone asking for advice is given lots of conflicting opinions :]

I'm riding 42 x 15 = 75.6 (using big / small x 27) at the moment which I find a pretty good gearing for London as its so flat.

reckon that 46 x 18 = 69 is too low for London

• good for skids but bad for keeping up with traffic.
  
  

I woulxd go with 46x17 = 73

Object17 I love how someone asking for advice is given lots of conflicting opinions :]

Personal preference is difficult to factor out when it comes to gear ratios.

[cite]hassanr:[/cite]... if you can get prime numbers into the mix you get more skid spots.

and by skid spots, do you mean there are places that are more frequently skidded on, wearing the tyre in specific spots? why is this?

lukerative I'm riding 42 x 15 = 75.6 ...
I woulxd go with 46x17 = 73

I'm running 40x15 which gives me about 71gi.
Previously tried 66gi which was too low geared for normal riding but I image it's great for short distances and just messin' about.

Also tried 76gi which is great on the flat, but too high geared for all conditions.

I would second a move to 73. This would make pedalling any distance more comfortable.

Umm why is a maths thing. there's an excellent skid-spot calculator somewhere in the web, it'll let you work out how many spots different rations will give you. Seems odd numbers/prime numbers (uuuh I think) seem to give more skid spots, which is maybe why I never seem to be able to find a 17t sprocket when I'm buying.

Why do some people use 27 as their wheel size when they have 700c wheel and 23or 25 mm tyres? The entire point of using gear inches instead of gear ratios is so you can compare the gearing used between different bikes of different sized wheels. I know this is not as good a the gain ratio as this is dimensionless but it is a silly thing to use 27 as your multiplier as it removes the point and convince of gear inch to use as a comparison tool.

It is very simple to find your wheel size in inches.

700 cc rims are 622 mm in diameter.

Tyres are square so as 23 mm would add 23 mm at the bottom of the wheel and 23 mm at the top of the wheel so the wheel is now 622 + 23 +23 = 668 mm in diameter.

25.4 mm in one inch. (You can use google to look this up) So 668/25.4 is approximately 26.3 inches. Very simple. Now you can use gear inches for the purpose for which they where developed rather than negating their entire purpose by using such an extreme approximation.

If you are going to use an approximation use 26". For a start is is closer to the real value than 27" plus it is alway better to understate your measurement and surprise people when accurate measurement is done than to over state and disappoint.

Skid spots: because you always skid with your pedals is the same position this results in the tyre being in one of a fixed number of possible positions, hence you get skid spots. More spots is better for tyre life. (this assumes that you alwasy put your wheel on in the same position.

carlitos [quote][cite]hassanr:[/cite]... if you can get prime numbers into the mix you get more skid spots.

and by skid spots, do you mean there are places that are more frequently skidded on, wearing the tyre in specific spots? why is this?[/quote]

If you always skid with your right foot forward and had a ratio of 40:20, your rear wheel is going to rotate twice per pedal stroke (ratio simplifies to 2:1), with the same parts of the tyre being in contact with the road when your cranks are horizontal.

Therefore you only get one skid patch that is always in the same place.

With a ratio of 46:17, you can't simplify the ratio, so you get 17 patches so your tyre will take longer to wear through.

lukerative reckon that 46 x 18 = 69 is too low for London

• good for skids but bad for keeping up with traffic.
  
  

I can hit about 40mph downhill in 46x18, and easily notch up 30mph on the flat - anyone can spin at 120rpm in an emergency (24mph), and I don't see many London cyclists going above 20mph...

And I'm sh*t at spinning, just ask anyone on here ;P

Skid spots
http://www.bikeforums.net/archive/index.php/t-241065.html

BringMeMyFix [quote]lukerative reckon that 46 x 18 = 69 is too low for London

• good for skids but bad for keeping up with traffic.
  
  

I can hit about 40mph downhill in 46x18, and easily notch up 30mph on the flat - anyone can spin at 120rpm in an emergency (24mph), and I don't see many London cyclists going above 20mph...

And I'm sh*t at spinning, just ask anyone on here ;P[/quote]

Slap

Brakes ;)
http://www.wiggle.co.uk/CategoryProducts.aspx?categoryName=Brakes%20-%20Rim&cat=cycle

I have a beautiful brake. If you're lucky I'll take a photo of it for you, I just need to go and find it in my spares cupboard first :]

Ouch, my head hurts. I just read this thread to try and figure out the skid spots thing, and it confused me even more.
http://www.bikeforums.net/archive/index.php/t-241065.html.

Anyway, I guess it inevitable I'll get skid spots whatever the gearing, so I'm not gonna worry about this too much.
From the many opinions offered (all gratefully received!), I think 18th might work for me, so I'll give that a go.
Thanks!

oh, thanks 'fc9k'
that's much easier.

It's not fucking 27 unless you ride a bike with fucking 27" wheels!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! FUCKING FUCK FUCK FUCK

eh? I must have missed something.....

50 x 14

no brakes, it's the way forwards.. they say

my wheels are 700c with 23 tyres so I'm thinking............must be 27.....yes ,sounds about right!

RPM 50 x 14

no brakes, it's the way forwards.. they say

At least he got 7 skid patches. To go with the one in his pants.

TheBrick(Tommy) It's not fucking 27 unless you ride a bike with fucking 27" wheels!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! FUCKING FUCK FUCK FUCK

hahahaha what's 8mm? its all scientific up to there, why not include some inaccuracy

RPM 50 x 14

no brakes, it's the way forwards.. they say

Ooooh Controversial :]