Why are Bikes Geared this Way?
 

Maybe a technodweeb question, but here goes: I typically ride my 12-speed with the chain on the smaller front sprocket, and only use the rear derailleur to change gears. I live in a hilly area where I need the lower gears, but I still thought I was a real weenie until I did some calculations: most of the gear ratios actually overlap. On "standard" 10-speed gearing (52 and 40 teeth on front sprockets, 28-24-20-17-14 on rear), all but the largest two gears on the rear sprocket combined with the 40 tooth front sprocket and the smallest two gears on the rear sprocket combined with the large front sprocket do not overlap. All others are redundant to some degree. Couldn't we achieve the same thing with a 5-speed with a 46-tooth front sprocket and a rear casette that was geared 32-26-20-16-12? Seems to me that this would work just fine for most casual riders (most of the folks who buy bikes). What am I missing here? Should I file a patent application?

Yes there are overlaps ... I use the "big ring" when decending and hammering on flats, and the "granny" when ascending (over 10%, to save my knees). I only use the top 4 cogs on the "granny", the middle 7 on the "middle ring", and the bottom 4-5 on the "big ring". The reason ... cross chaining.

Cross-chaining is not as big a deal these days with the new cogs and chains, but using cheap or older equipment, the wear on the chain and the cogs can make for very sloppy shifting, and pre-maturly wear out your drivetrain.

That is as much as I know ... there are certainly peopleout there that will explain it better than me.

Fritz,
As pointed out is is all marketing. I strongly suspect that only a tiny fraction of the users in any given market area really benefit from anything much over 10 speeds. I have ridden 3 speeds for years and in my part of the country I can ride 90% of everything I need to on just that. I do walk the occasional hill but so what, I am no longer a high performance rider;), to me just being able to ride is enough. I own everything from a single speed cruiser, up to a 27 speed German "trekking" bike. My only bike set up as a "road" bike is my Giant Excursion tour bike and it has 21 speeds. Gears allow you to fine tune your riding speed to road conditions but for the average American that is unknown territory. I think if entry level bikes were more user friendly we might get more people to ride on a regular basis.

Aaron:)

Hi,

Here is a basic patent search at USPTO:

(keywords: bike and gearing)

http://patft.uspto.gov/netacgi/nph-P...FIELD2=&d=PALL

Give em a read and ask yourself if what you have is inventive. Even if you don't find anything .. you would likely have the application rejected for being an obvious improvement.

In the search results there are a number of patents on CVT gearing. IMHO that is were future gearing system might/should be headed.

Enjoy,

R.

I don't understand gears well so my bikes have only one chainring and one cog at the rear. :)

Not all are, no overlap with internal hubs.

Back in the day when you only had 5 (or maybe 6) cogs on the freewheel, they had to be spaced pretty far apart to give you any kind of range (like 28-24-20-17-14 as you point out). The front chain rings would either be set up as "half-step" or "Alpine". Half step means that the difference between the two big chainrings (percentage-wise) was half that between the rear cogs. You would perform what's called a "double shift" where you would shift the rear cogs to make large chages in your gears and shift the front chainrings to fine tune the gear. Alpine, which is what you described here with the 52-40 chain rings and also known as 1 1/2 step gave you a bigger range (a lower low if you will) at the expense of a really strange and counterintuative shift pattern. In those days, a typical front derailleur could not handle a difference larger that 8 teeth or so on the front chain rings. All that changed in the late '80's with better front derailleur design and a wider 53/39 14t difference between the rings became common. Today, while that is still popular on racing bikes, we are seeing ever more bikes equiped with "compact" cranks, usually with a 110mm bolt-circle-diameter (bcd) that allow a low of as small as 33t and a 16t difference between the rings. There are often special front derailleurs to match these cranks.

Mine too, but it still has 8 speeds. ;)

Not all bikes are geared that stupidly.Some are worse!

Only kidding on the patent application, but I plan to experiment on my wife's (5-speed) Schwinn Collegiate - it has a 46 tooth front sprocket, but a conventional 28 to 14 rear. All I need to do is find the right rear freewheel or cogs for one I have.

I have the 40/52 out to a 13-30. Most of the time, I ride in the 40. Coming off a hill, I ride in the 52; but if the area stays flat, I can stay in the 52 for quite some time (not too often around here though).

The overlap can be nice feature. Could you imagine going through all 6 gears in the small chainring, then having to upshift to the big chainring and having to downshift all the way on the rear for the next gear ratio? Ie, zero overlap. That would be very annoying, let's say the terrain was alternating between those two gears. You'd be shifting both derailers constantly.

Instead, on most bikes, sure, the front chainrings is worth say 2 gears in the rear, which would mean 3 or 4 overlapping gears. But that overlap means that you don't have to always shift both derailers at the same time. Hopefully, both chainrings have a couple of overlapping gear ratios that you cruise at, that way, small terrain changes won't force a FD shift.

I have 24 "gears" on my hybrid and it still isn't enough gears.

When I am up to speed and cruising, I am constantly switching between two gears and I wish I had one in between because that "one more gear" would be perfect.

The larger the gap between the gears, the less "smooth" the shift between them will be.

Look at the cassette (The rear gear set) on the high end bikes, and you'll notice that it's a very tight group. This isn't JUST for fine tuning (although it is nice to be able to fine tune your effort like that) but also to promote smoother shifts.

That being said, there is a small but growing movement towards internal hubs in back (which shift well over large gaps) and a single chainring up front.

-- James

A lot of more experienced riders are going to a 46 tooth for their largest front chainring. They can still pedal at 25 mph at a comfortable cadence and if they need a burst of speed they just pedal faster. A second chainring with 36 teeth (or third with 26 teeth) can be used for steep hills or hills when you're loaded, towing a trailer or a trail-a-bike.

If a downhill is faster than 25 mph you're better off from an energy management view to stop pedaling, tuck-in, and let gravity do its thing. The only other use for a big chainring is if you have a really strong tailwind but once again, you might want to save your energy for the trip back.

None of this applies to racing where tearing around at 25 mph is the norm and big chainrings do have their place.

I think that's the case no matter how many gears you have. One always wants a gear "in between":)

Here's a chart I made for a math lesson I taught on gear ratios:

Gear Ratios

The gears are off of my beater Bridgestone X0-4.

After the class learned about how the bike gears worked, I used a blank version of this chart and had the students find the values. The resulting epiphany was something to behold! Plus, I got to teach about bikes!

In fact, it was the work I did on this lesson that led me to buy a new bike, buy a bunch of tools and build my wife a bike. I just got back from my lunch break riding around town looking in thrift stores for fixies! This information is what also convinced me that front derailleurs/multiple chainrings are overrated. My next bike, after a fioxed gear, will be a REAL 10 speed!

I've switced from 21 (?) to 9 gears which I like, but I find that I am quite jarred when moving from the hardest gear to the light one (sorry for my improper terminology) as I prepare to go up hills. Any tricks to that? Seemed to me that the shift on the other bike was smoother since the middle ring softened the jolt. Eg. 3, 2, 1....
Don't mind so much going into the higher gear. It's that sudden whip-around that really disorients me. But maybe I'm just not used to it yet.

Marketing has little or nothing to do with it. A single front chainring paired with a multispeed rear hub only gets you so many gears. Adding a second or third front chainring expands the total range of gears. It's all about the extremes -- very low for hill climbing, very tall for high-speed pedaling. The best way historically to achieve that without excessive numbers of rear cogs is to add chainrings in front that multiply the rear cogs into different ranges. So what if they overlap? Nobody who has 18 or 21 speeds actually uses all of them, but at one time or another they will use the highest high and the lowest low.

I can not only imagine it, I once actually built a bike that way. I don't remember for sure what the front chainrings were 28/38/48 maybe but the 5-speed freewheel was 14/18. The concept was to run through the freewheel in one chainring and then make a big double shift all across the corn cob and do it again.

It was a PITA to ride. Imagine cranking up a hill, running out of freewheel cogs and trying to make that double shift. It was impossible to maintain any amount of momentum. Overlap is good because it gives you options as to when to make the front chainring shift.

OK, I think I get it. On a 10-speed, as I reach the top of a hill, I am obviously on the smaller front sprocket, and keep shifting to the smaller rear sprockets as the hill gets less steep. When I am going down the hill and can't keep up even on the smallest rear sprocket, I simply switch to the larger front, and there is not a drastic change in gearing. Certainly makes more sense than having to go from smallest front/smallest rear to largest front/largest rear to go to the next gear ratio. Conversely, at the bottom of the hill going up, I have to change from largest front/largest rear to smallest front/largest rear, rathe tahn having to switch totally over both front and rear. Somewhat counterintuitive to an engineer such as myself, but mnakes perfect sense when you see it in terms of what you actually do when riding. However, I still thing that most of this could be done with a 5-speed, either with rear derailleur or Sturmey-Archer type rear hub. Would be much simpler to operate for the vast majority of riders.

It is a good question. And if you want to use your gears effectively, you probably would want to map out what are the best combinations for smooth operation, efficient shifting, and minimal wear due to cross-chaining.

If you do the ratio math on a typical 21 speed mtb setup (14-34 rear - 7 cogs, 28/38/48 front chain-ring), you will find that you only have 11 - 12 or so actual distinctly different usable combinations.

With the chain ring in 28, I cycle thru all 7 gears. Next I shift the front to 38, and the rear thru gears 5, 6 & 7. Then I shift the front to 48, and use rear gears 6 and 7. Essentially 7 plus 3 plus 2 for a total of 12 needed selections. Everything else is redundant. This can vary slightly depending on the exact number of teeth and spread between the 7 in the rear, but that's about all I typically use.

If you didn't mind slightly wider ratios (jumps) in the upper mid range, you could even skip over the middle chainring, and go 1 thru 7 with the small front cog, maybe 5 thru 7 with the big front cog, for a total of 10 selections.

Why? Several posts before mine attempted to explain it. Mainly it has to do with the ability of the front derailleur to shift the chain between excessively different sized gears. A zero overlap front setup with a small chain-ring of 28 teeth (1x) would have the next gear at 56 teeth (2x). These two front gears paired with the 7 spd rear referenced above would give 14 distinct ratios, and no overlap. It would start at the same low ratio as before, but the top 3 (+/-) would be way higher than before (larger total spread).

There would be two problems with this system:

1) The front derailleur would have a daunting job to shift between gears so differently sized. Probably not possible with current designs. It would be jarring at best. Adding a ring between these two for the chain to climb makes shifting easier, but gives you a bunch of redundent combinations.

2) In this 2 ring 2x system, to smoothly go from ratio #7 to ratio #8 would require a 'two handed' technique of taking the rear from gear 7 all the way down to gear 1 while taking the front from the 1x gear to the 2x gear. I have often though that this might be an interesting place for a very wide ratio 'internal' gearbox up front in place of the conventional bottom bracket cassette. If this internal hub interfacing the pedal crank to a single external gear either had an automatic fast sequence or skip shift feature, it would achieve the goal. Technically feasible, but complex and expensive.

With today's technology you could easily use something like a single 42t chainring in front and 9 or 10-spd cassette in back with 11-32 range and have similar range and spacing to the old "10-speeds" without the need for a front shift at all but why limit yourself?

If you want to get creative, SRAM makes an internally geared 3-spd cassette hub that they normally spec with a 9-spd cassette and a single front chainring effectively giving you a "triple" with 27-speeds. You could then mount a double chain ring crank on the front and come up with all sorts of creative combinations. Sheldon Brown has done something like this on a few bikes i believe

Don't forget, having two (or more) chainrings does allow you to have two (or more) optimum (or nearly optimum) chainlines. On my bike, with just a six speed, I tend to use all 6 gears in the 40T chainring, but the chainline is about optimum inbetween 4th/5th. Which is nice, as when I'm on flat ground (rare around here) that is right around where I would be. The 52T chainring, while I haven't looked, is probably optimum around 6th, so when I'm cranking on it for maximum speed, (hopefully) pretty efficent. A triple might be nice, as again I might have better chainlines for those "crank up the hills" rides.

With just a single chainring, odds are, the chainline will be perfect somewhere in the middle of the cassette, which is hopefully where you tend to cruise at.

Someday, I'd like to try out one of the newer internal hubs. I think I'd love to, this winter, other than the whole money bit. I'm not sure about total range, though: my cheapo bike runs 36-108 gear inches currently, and I find that I have used all of those gear inches and wished for more (well, it's a bit crazy on the top end, but you only live once). I'm sure the expensive hubs do have wide ranges, perhaps at the expense of closeness; which then illustrates a bit of how you are stuck with whatever gear ratios they pick for you. With cassettes and chainrings, you do have some easier options to play with gearing.

My biggest grief would be the "normal" trend for these hubs seems to be grip shifts, which I hate--and would have no idea how those would work out on a road bike with drops anyhow. Maybe if they could get that to work with a brifter, then it'd be something I'd really drool over.

Great.... Is that all you got out of the discussion? Thanks for contributing.

What is "typical"? I have 6, 7 and 8 speed rear clusters on my MTB's the 6 is a freewheel.

Aaron:)