Gear ratio query 10 speed Double equivalent of Compact 11-28 any advice please?

[Luba]

Hi I have a bike with a 10 speed Compact 50 / 34 with an 11-28 cassette. To be fair this is ideal for me getting up most hills but can be a little underpowered on descents but it's fine.

A few months ago I got a bike with 9 speed Sora Compact with a 12-32 cassette. I want to convert this to 10 speed having got a good deal on some 105 shifters. I also picked up a new 10 speed SRAM Double 53 / 39 chainset with BB and 10 speed 12 -32 cassette for around £80.

Before I get this fitted, and being a bit wary of the GXP BB, could someone please let me know how similar or different the ratios would be to the 10 speed I already have please?

I have tried understanding what I am seeing on some sites but like Alan Partridge said to Michael "it's just a noise"! And TBH hurts my brain a bit. I am aware that it's unfashionable to have a cassette the size of a dinner plate but I couldn't care less.

My first road bike was a triple which had 52, 42, 30 - and I had to put an 11-23 cassette on that. If anyone can make any sense of all that and explain it in plain English I'd be very grateful, cheers.

 

[Rasmus]

The math:

34/28=1.214
50/11=4.55

39/32=1.219
53/12=4.42

30/23=1.30

So, your lowest gear on the double with a 32 cassette and a compact with a 28 would be practically identical. Both are lower gears than the lowest on the triple with a 23. The highest gear on the double is a little lower than the highest on the compact. Although the full range is similar, the double will have higher spacings going down the cassette on each chainring, depending on which exact sprockets are on the cassette (ie, the range of gears available on the two rings overlap more).

 

[Sharky]

Have a study of "gear tables". There are lots on the web.
http://www.machars.net/bikecalc.htm

It all stems from when our grandfathers rode a penny farthing, having a single speed large front wheel. The bigger the wheel, the higher the gear.
With the arrival of the modern bike, having chains and cogs, you can achieve the equivalent of a penny farthing wheel much bigger than you can possible ride.

They are expressed in inches, which would be the equivalent of the size of the penny farthing front wheel.

If you are considering combinations at the top end (big gears), it's worth remembering that once over 35mph, freewheeling in a tuck position is usually more efficient than trying to pedal. Wind resistance and gravity are much greater factors. So use of little sprockets (11's 12's) don't really make you much faster.

Good luck, Keith

 

[Luba]

Thanks very much for the replies and explaining things. I looked at that site - was one of the ones that appeared Greek to me! Think I need to be in the mood to slowly read an absorb things. Thanks for working out the ratios. I'm pleased that they seem quite similar - saves me some money. Might take a gamble with this GXP BB. It's a 2013 model so am hoping that they have improved over some of the bad reviews I've read.

Thanks again