Bicycle Mechanics - Newbi Q

Hi,
I notice, as one does, that fromt clusters often have three rings. If I want a bike to carry huge amounts up VERY steep and orrible mountain paths VERY slowly, then I need a small ring. If I want same light unburdened bike to FLY down smooth roads I need a large ring. Now, assuming an SRAM cluster on the rear, is it possible to have Four rings up front so as to get decent spread of gearing and still have a shifter that will shift across them, without having the rings fall apart from chain rub or the chain to fll off when on smal ring and small ring. Say maybe 20, 28, 34, 44 or something
I mean, what do you reckon?

Hi,
I notice, as one does, that fromt clusters often have three rings. If I want a bike to carry huge amounts up VERY steep and orrible mountain paths VERY slowly, then I need a small ring. If I want same light unburdened bike to FLY down smooth roads I need a large ring. Now, assuming an SRAM cluster on the rear, is it possible to have Four rings up front so as to get decent spread of gearing and still have a shifter that will shift across them, without having the rings fall apart from chain rub or the chain to fll off when on smal ring and small ring. Say maybe 20, 28, 34, 44 or something
I mean, what do you reckon?

Well, nobody makes one, or a derailler that would shift it, so you'd heve to hie yourself to the metal shop.

And since currently available combinations span pretty much the entire range of ratios a normal human can reasonably push, I have trouble seeing the need.
Put a 52-42-30 (or 28) on the front, and a 12-32 on the back, and you can go low enough that you'll fall over or high enough to do 50mph on a descent. What more would anybody want?

RichC

Hi,
I notice, as one does, that fromt clusters often have three rings. If I want a bike to carry huge amounts up VERY steep and orrible mountain paths VERY slowly, then I need a small ring. If I want same light unburdened bike to FLY down smooth roads I need a large ring. Now, assuming an SRAM cluster on the rear, is it possible to have Four rings up front so as to get decent spread of gearing and still have a shifter that will shift across them, without having the rings fall apart from chain rub or the chain to fll off when on smal ring and small ring. Say maybe 20, 28, 34, 44 or something
I mean, what do you reckon?


You are obviously new, and havent actually ridden a bike to see how it operates. Just because you have a tiny ring up front dosent mean you can climb up a wall. If your going DOWN a hill you might spin your big ring out, but you wouldent be able to on flat.

You could always do something really insane to get more gears. Here you go:

Buy a rohloff fourteen speed speedhub500 internal hub.
Stick a seven speed freewheel on it.
Put a triple on the front crank.

This should give you 294 possible gears (I think, my math is bad). You might have to cold set the rear spacing out to get this abomination on the back of your bike, though. And you'd have to learn to work three seperate shifters, plus route an extra cable. I'm not really sure it would work, either, because I haven't tried it. If you'd like me too, you can buy me the speedhub and I'll try it out; I have the rest of the parts lying around.

With a four ring crank (no such animal, sorry), you'd probably have some serious chain alignment problems. Many three ring cranks have a granny ring so small you're spinning when you're on the large cog/small ring combo anyway.

Rich is right. The kind of combos he suggests are a nice, wide, range. If you're new to biking, though, I'd downsize the big ring a bit. Finding comfortable gearing is probably one of the first things you'll learn on a bike. Have fun. It's a very individual thing.

Davinci tandems do have a QUAD crank... Well sort of. As part of their ICS drive system...

http://www.davincitandems.com/images/drive2.jpg

What they did was offload the drivechain from the stoker cranks... The "idle" shaft "timing" gear is not 1:1 with the crank timing gears. This means you can use a smaller set of "chain-rings" and therefore you can get a larger range of speeds off of each.

They also point out a benifit of this is you don't need a wopping 54 or 60 toothed ring which requires vast amounts of chain (to go from a 30T to a 60T).

And to top it off they added a 4th ring...

The effective chainring sizes now become:
24T, 36T, 48T and 60T.

This probably works quite weill because all of this mounts on the boom-tube of the tandem. This would be hard to retrofit onto a single... A recumbent could probably use this concept...

SRAM/Sachs used to make a 3pseed internal hub gear with a 7spd cluster. It was meant to be used with a single chainring, but some enterprising riders have used it with triples, mainly for tandems and recumbents.

At some point, with that many ratios available, you are bound to find many duplicates. The only thing that really matters is the gear spread and the ratio drops between them, not the total number of gears.

If you were to use a 4-ring front with a 9-cog rear, you would have 36 ratios to choose from. One would assume that, in such a setup, you would need a fairly close-ratio cogset, unless you also had a custom long-cage rear derailleur made to handle the excess capacity-one which would probably hang so low that it would likely catch objects near the ground and get easily destroyed.

Or, you could learn how to shift between gears properly. You may need to re-think the ratios you have on your cogset, but mostly what you need to learn to do is remember what ratios you have in which ring, and choose the proper ring ahead of time. Front derailleurs aren't exactly quick and accurate, so a bit of foresight is what's needed.

Or, maybe you should look at buying an autobike......

Thanks for the replies, which were mostly pretty polite.
To make myself a little clearer, this setup is designed to replace my ATX 890 which i'd had for some while until it recently got stolen (ff.....), so Im not exactly a newbie bike wise, have ridiculously big thighs and all that. But the technical stuff I have till now left to others, and it is only now I am sorting this stuff out, thus the question.
A few years ago I was in Tibet, in the far west where few people go except for scattered Tibetans and Chinese and, it apears, a few intrepid cyclists. My desire is for a bike that can cope with an extreme I doubt any of you have exerienced, certainly not if you think a Klein makes you a big boy. Carrying self sufficient gear through the Himalaya on your own, remember NOT Nepal, means carrying a truckload. And crossing the passes at over 14000 feet is hard work which I want to make as easy as possible. But on the oher hand, it needs to be an everyday bike, and running out of pedal at 70 KPH (road downhill) on my Giant was a little frustrating, so why not go bigger if possible?
It also has to be enormously reliable, so no internal hub which the incredibly fine dust in the air would destroy soon enough anyway. I had an internal hub way way ago on a Mongoose, and it worked pretty well but not this time. A long chain might be able to be taken up by a second chain tensioner, but it all sounds a little dodgy and unreliable.
Thank you again for your sensible suggestions, Ill take Richs suggestion and run through the ratios,
James

I think the high end is fairly well covered by current large chainrings. And if you travel to Tibet, you won't need your largest chainring, except for those downhills with asphalt. But anyway. There are a few technical solutions that you might do, but they will all require some machining. One way is to adapt a 19 or 20 tooth cog onto a specially designed adaptor.

1. Brian de Sousa shows a tandem adaptation in his cycling Tech pages. See here:
http://www.briandesousa.com/bicycling/index.htm

2. Or if you have money, you could get a Specialités TA crankset. See Peter White's pages here: http://www.peterwhitecycles.com
Basically, one crankset has the following bolt circles: 110 and 74 or 58 mm. You normally can only use the 74 or 58 mm -- not both -- at the same time, but I'm sure a good machinist could find a way to use both, or to install 2 chainrings at the same time on the small bolt circle.

As to why 4 circles, I could say ""Why not?". It would allow close ratios and wide range, a bit like the Da Vinci system, which I consider good but too high-geared. With
19-28-40-50 and a 13-32 custom cassette for example, one would get close ratios from 18,5 to 104 gear-inches. Or you could have an extra-large 37 or 38 cog machined (a Shimano LX or XT derailleur should be able to clear it, if not a 20 year old Suntour would) and could get gear ratios from 15 to 105 gear-inches.

You will need to use a bar-end or downtube shifter (i.e. one which works in friction for the front derailleur) and you will need to fiddle with spacers to get the rings as close to eachother as possible.

Regards,

Thank you for this reply, Michel. It makes the techical sense that seems based on experience, which in this area I lack. I will follow your leads, get together an idea and maybe email the guys at Middleburn. With Rihs suggestion I at least have advanced on where I was.
You are also correct (I think) in saying the largest ring would not be well used in Tibet. The cyclists I have spoken to( and there are more of them on this stretch) say that although its the longest downhill in the world, the road surfaces mean middle range pedalling all the way! Better than pedaling up I suppose. But sometimes, at other times, you just need more go you know?
Thanks,
james