Gearing Question

McTiger

I'm still not grasping the relation between gear choice & speed.

Currently I have a Marin San Rafael sit up and beg city bike, there's a 11-32 cassette on the back and a 48/38/28 crank.In perfect conditions (yeah, it happened once this year) on flat, smooth tarmac I managed a pedalling top sped of about 22-25mph at a good cadence for my ageing knees. This was in top gear, 48-11.

What I'm wondering is - Will a cycle cross style bike, such as the Focus Mares AX1 Ultegra, with a 12-27 on the back and a 46/36 up front have a slower top speed?
Would a Focus Cayo 105 with a 12-27 back, 50/34 front have a higher top speed?

anto164

yes and yes

unixnerd

Most folk use other measures but I'm a simple man so I just divide to calculate ratios.

So 48/11 = 4.36
46/12 = 3.833
50/12 = 4.166

If it were me I'd prefer a cassette with an 11t top cog on the last two.

But it's not all about gearing. I run 52/12 (4.33) on my road bike, I could go faster with 52/11 but I'm trying to up my cadence and pedal faster instead. I run out of steam above 75-78rpm and I'm trying to change that.

McTiger

Thanks chaps, I guess I understood the basics better than I thought.

I''m sure the more aerodynamic riding position of the Mares would allow a better average speed over the bolt upright position of the Marin.

bobones

Play around with http://www.gear-calculator.com

It will show speed for a given gear at set rpm

Secteur

All your answers in this thread

http://www.bikeradar.com/forum/viewtopi ... highlight=

Speedmenace

46/12 @ 100RPM is about 30mph, I doubt you will ever be doing much more than that on a flat

McTiger

Thanks for the help and advice.

I was naively hoping that a new bike with slightly different gearing would net me a substantial increase in average speed. I now think that to get that substantial boost I need to focus on getting the riding position right, getting the weight (my own) down, streamlining the bike (no panniers etc) & most importantly increasing my fitness & stamina.

New bike wouldn't hurt though eh?

optimisticbiker

A better aero position and streamlining will help - 80% of your effort goes into overcoming air resistance and that increases with the cube of speed... weight has no impact on speed except when climbing hills and in terms of acceleration to the desired speed...

So to go faster - better aero or more input power...

Gearing wont change that as power comes from you... when I started seriously measuring I was putting out about 210watts, now I'm doing around 300watts if i push hard (as well as being 6kg lighter and shed loads fitter). It takes 280watts roughly to maintain an average bike at 20mph on the flat - I can manage this on my hybrid. On my road bike I am closer to 23mph due to the better aero. In a group I can manage 25+ simply because of the reduced drag (but I die miserably when I move out front for anything more than a few minutes!) Point being I am nowhere near utilising my top gearing yet except downhill.... my power to weight ratio is 4.2W/kg... it doesnt even come close to a pro rider (about 5 - 7W/kg) which is why they can maintain >30mph individually for long periods of time.


But to answer your most important question.... you can't have too many bikes lol