aprieto28

fietsbob

1) its a pink tandem .. stoker's crank ..

Back when there were 6 speed freewheels . the wide gaps in a wide range freewheel;
then a narrow difference between chainrings filled in the gap
double shifter pattern was a ZigZag.

The Brompton 6 speed drivetrain is a Half step . 3 widely spaced Internal gears +

a 13 & 15t cog on the driver .. single chainring . same ... functionally .

peterw_diy

https://lmgtfy.com/?q=half+step+bicycle+gearing

FBinNY 

This is similar to how my touring bike is set up.

The logic is very simple. 90% or more of riding is done on relatively flat terrain, with shallow grades in either direction. With the right cassette, this packs a nice selection of gears in a tight range for that 90% of the riding, yet has the "bailout" granny for the hills.

Contrast this with the 39/53 combination so much in favor for road bikes these days. For many people, the upper mid range gears where they do most of their riding tend or be crossed combinations.


BTW- when you see this on a bike, you can win a bar be that the owner is over 50.

cyccommute 

The zigzag pattern was always a hurtle for me to using half step. To do the "zigzag" properly, you have to shift the front and rear at the same time. I never could figure out how shift the front and back at the same time with downtube or barend shifters. It would be trivial with modern STI shifters but we have many more gears now and don't need the half step.

Where you live might have 90% or more of the riding being done on relatively flat terrain but most of the rest of the world is anything but flat. Even Kansas isn't all that flat from the saddle of a bicycle.

fietsbob

there a Shrimp on that Bar be ?

FBinNY 

I know that there's nothing that you can't find an argument in, but I was explaining the logic behind the gearing (for those who use it). If my description of terrain and riding conditions doesn't match yours, then this may not make sense for you.

However, I'll bet a number of beers that the vast majority of road riders here on this forum, do spend most of their riding miles using the upper third of their gear range, riding on "relatively flat terrain with shallow grades in either direction". If this weren't true, many would find it too difficult and give up early on.

Crescent Cycle

The point was to avoid redundant overlapping gears, especially in an age of 52/42 and 5 speed freewheels.

Double density of close gears in the upper range, with a wide range of bail out gears

fietsbob

my 85 Specialized Expedition came as a 48 44 28 crank, stock ..

changed it to a 50,40,24.. soon after buying it ... new.

austex9000

HALF STEP refers to a transmission design where a change between the front chain-rings result in half the ratio of a change between two of the rear cogs. Most times the two largest chain-rings will vary by 6 or less teeth.


Your pictures shows you've got a "granny" chain-ring too. NICE! So you've got HALF STEP PLUS GRANNY. The granny gives you a gear that is a very low one you'd use in a steep climb or when slowly maneuvering.


With HALF STEP you can make smaller changes when that is desirable.


Lets say your rear cogs are generally spaced at 15% between most cogs and you're speeding along in your highest gear when a black cat darts in front of you! You hit the brakes briefly and only slow a bit...the cat is now nowhere in sight. Change to the smallest of the two front chain-rings and you've geared down 7.5% instead of 15% ...just enough to regain your top speed in an instant by pedaling a few revs in the smaller chain-ring then shifting back into the largest chain-ring. You gotta keep that tandem moving, it's like a semi truck.


Now you encounter a steep grade. Use that GRANNY front chain-ring with the largest and second-largest rear cog to spin up that hill in a very low gear, preserving your knees for lifelong riding!

davidad

My 7sp tourer is set up with half step gearing. The front is 44-40-20. The back is spaced at 20% between cogs. A double may be necessary to get the 10% change in gears. The rear 7sp cassette cogs are; 11-13-16-19-23-28-34. I go from a 104" high to a 15" low. I have 18 individual gears that you don't get on a 20 0r 30 gear drive train.

austex9000

Nice wide range and an extremely low gear too! Your setup features the characteristic < 6 teeth difference between the two largest chain-rings producing exactly 10% ratio change...half of your 20% ratio between cogs.

You must have a 56 or 58 BCD crankset in order to have a 20T chain ring and one of the largest total capacity RDs available. Is it a Deore LX or a Suntour XC? Pleaase tell me the other drive-train components.

Kimmo

Because the power required to accelerate increases exponentially with speed, ideally ratios would get closer as they get taller, but the opposite happens once you can't go below a 1t gap between cogs.

The half-step triple seeks to overcome this physical limitation, and bears reconsideration in this age of electronic shifting.

(Green is using the setup as a normal triple, blue is alternating between big rings.)

I spent ages plugging in numbers, and these tooth counts provide the best spread and flattest curve with the least shifting and most sensible use of cogs.



The curve on the graph would ideally flatten towards the right; the curve of the half-step is a lot closer to ideal.

austex9000

That's interesting you'd say this old gearing type deserves a second look because of modern system's shortcomings. I've always liked half step + granny.

Crescent Cycle

You would need a 52/50, and it would more than double shifting compared to current 1t gaps since you have to mess with the front too. 1t gaps are already pretty small too. It would be ideal if they got smaller the higher they went. Maybe someone invent a drivetrain with the cassette in the front.

I think it has been used in some very specialized time trial setups.

aprieto28

It sounds like i should stay in my big chainring, which is 54t and shift to middle chain ring, unk tooth count, to adjust to the terrain for the in-between gears and then shift to lower rear cog as needed, and use granny for hills. Right?

wheelreason

Wow! I just sit on the bike and pedal it. Who knew...

davidad

I have an old Richie (Sugino) mountain bike crank. 94-58 BCD.

Sixty Fiver

I like single malt scotch.

fietsbob

that 20t to 40t upshift will need a down hill slope to make that big upshift.

Retro Grouch 

That's what I think too. With 9 or 10 cogs on the back, I use my chainrings as a series of gear ranges.

Back when 10-speed meant two on the front and five on the back, I used to operate both downtube shifters simultaneously with my right hand.

gsa103

That's probably why compact doubles (50/34) are basically standard. The 50 is a much better gear for the average rider on flats, and the 34 provides better hill climbing.