tarquin1234

Hi there,

I have a roadbike with 42 & 52 chainrings and a 12-23 rear cassette but I am going to do some touring in a month carrying a heavy load so I need much lower gearing. My plan based on my research into costs and ease of installation is to change my inner ring to the smallest it will take (38tooth) and change the cassette for a 12-30.

I think I will change the rear derailleur to a "Shimano Deore Rear Derailleur M591", do you guys think this will be OK for this set-up? Also for someone carrying a tent, sleeping bag, and water etc through mountains do you think this will suffice?

I will also buy a new chain and chaintool.

I am also considering changing the outer ring to something slightly smaller, like a 50 or 48 as I'm not keen on cycling mega-fast.

Many thanks for any time taken in giving me feedback and advice!

William


Chainring: https://www.bicyclemagic.com/products...113p2797&rs=gb
Cassette: https://www.cyclerepairman.co.uk/details.asp?ID=194
RD: https://www.chainreactioncycles.com/M...le-Products-UK

berner

Hello William, I've been planning for some touring in mountains also. From what I've learned so far, to haul loads up mountains gearing of under 25 gear inches is best with fairly close gear spacing. Usually people choose triple chainrings for this with, 48 T at the top and 24 T at the bottom. The cassette would typically have a lowest gear of, say, 24 T. Check out this gear calculator : https://www.gear-calculator.com/# . You can move gearing around to find the combination that will work for you.

For myself, I've changed out a triple with 52-42-30 to 50-39-26. The original casette had a lowest gear of 26 T that was changed out for a lowest gear with 30 T. This produces a lowest gear of just under 25 gear inches which should be fine for an old guy with no more than 30 pounds on the bike in mountains.

You will, no doubt, get more and more experienced input here but keep us posted on what you changed and how it worked out.

bradtx

William, While your proposed changes will help, be aware that with mountains it'll still be a struggle. Recommended low gearing for loaded touring bikes is often around 20 GI, your changes will be be over 30 GI.

Brad

Mark Kelly 

Here's the science behind it. The gearing you require is the product of four things: your comfortable cruising power ouput, the lowest cadence at which you are comforable with that power output, the total weight of you and your loaded bike and the slope of the worst hill you expect to climb.

The first is a personal thing and could be anywhere from 100 watts (your geriatric aunt) to 300 watts (superman). The second is also a matter of personal style, the third you know already and the fourth can be found by mapping your route on Google Earth and selecting the "show elevation profile" option: the little cursor will give you a computed grade. Let's say you weigh 70 kg, your loaded bke weighs 30 kg, you can comfortably sustain 200 watts at a cadence of 60 and you have grade of 10% on your route.

The combined weight of you and your bike at 100 kg gives you an effective power to weight ratio of 2 W / kg which means you can achieve a vertical ascent of 0.2 metres per second (gravity being about 10 m/s^2). On a 10 percent grade that's a total speed of 2 m/s. Knock off about 10% to account for other losses such as tyre drag, call it 1.8 m/s. Assume your wheel has a rollout of 2.15 metres (700c wheel with 35mm tyre), 1.8 m/s / 2.15m gives you 0.84 wheel revolutions per second. At a cadence of 60 you therefore need the smallest chainwheel to be 0.84 times the largest rear cog. For a rear cog of 30 you would need a chainwheel of 25 teeth.

A cadence of 50 would give you a ratio of 0.72 x (60 / 50) which is 1 so you could use a 30 tooth chainwheel.

That's just an example, for your situation you need to put in your own figures. To put it all in a couple of simple formulae:

(Power x 9)/(total weight x slope) = speed (in metres per second).

Speed x cadence / 129 = ratio of chainwheel to cog.

fietsbob

A change to a triple crank would be advised, long mountain passes would make a 1:1
gear desirable, if not lower..

tarquin1234

Thanks a lot for these responses guys, I didn't realize the extent to which I need to change my gearing. Unfortunately my two companions will undoubtedly not take the time and money to invest into this kind of equipment, so I think we might have to skip the mountains and take the train, which sucks as it was the highlight of the trip for me. They have normal double road cranks, and one of them only has a 5 speed cassette

I might still change the cassette to a 12-27 and put a 38 inner chainwheel on anyway, as my bike is too fast for me for what I do with it. I think I'd additionally only need a new chain in this case? Thanks for your help guys, I'll let you know what I do!

tarquin1234

Or maybe I'll get a compact double 34-50 and a 13-32 cassette.

bradtx

This would be a more versatile drivetrain than what you now have, which happens to be my favorite 8 speed gearing that I still have on my rat bike, but there are assignments it's just not quite appropriate for.

Brad

davidad

If you are going to skip the mountains then don't worry about the gearing. If you plan to do serious touring you need gearing at or preferbly below 19 or 20 gear inches. Bottom line is if you have had a long or a bad day you don't need to try to fight the bike uphill and wear yourself out to the point that it will take morew then a day to recover.