Frog Man

It is common for front derailleurs to have 16t to 22t capacity.

Why not more?

Suppose I wanted a 34-70t double chainring?

Could a front derailleur with a long cage be used to get more than 22t capacity?

An extra chain tensioner to increase capacity.

Where would I find someone that knows the answer?

bulgie 

Not a framebuilder question. I mean, some FBs might have opinions on that, but I'd rather not water down this forum with bike-mechainic questions. Try the general forum, you'll get more eyes on it there.

Mark B

Claude.fr

IMHO, that’s the rear derailleur, all things equal-no chain tensioner, cage length which sets the max capacity of a gear train.
x2 road 42-52-with corn cob cassette: short cage derailleur.
x 2 30-52 : medium cage.
x3 regular MTB cassette: long cage.
Ditto x1-12 sp.

Andrew R Stewart 

Why? One reason why is that huge a jump (34-70) will have really challenging shifting. Another, and perhaps the best answer, is that companies only want to make what a lot of people will want to buy (be the motivation from marketing or experience). But if you were able to order enough a quantity (like many thousands I suspect) I bet a company could be convinced to make them for you.

I sense an underlying aspect is at play here but not yet reviled, the OP's cadence preference Andy.

unterhausen

People are always inventing bicycle parts, even bicycle parts that nobody needs. I bet someone has invented a front derailleur that shifts an outrageous number of teeth. Probably weighed a ton.

Frog Man

Why in the frame builder section? I didn't see the mechanic section. I figured a fame builder has faced problems related to custom frames and solved them so they have the right frame of mind for answering my question.

If the front derailleur chain cage is longer will it not shift?

I have a 11-34t on my bicycle. I have another 9-50t cassette but that is on another wheel that I don't want to dismantle ($$$). So increasing the range of the front derailleur seems like the cheapest option if it works.

Regarding shifting into top gear, I would have to shift down 4-5 gears when moving to top chainring.

Why more range? So I can go faster while still being able to climb a hill.

unterhausen

Is this for a folding bike?

Andrew R Stewart 

One thing frame builders have to sometimes do is explain why the customer's wants might not be able to be done. But shop mechanics do much the same too and likely more often.

When I was young a mentor of mine told me that if I wanted to go faster I had to pedal faster. The human body can produce more power as the cadence goes up, to a point. This point is not finite and can be changed with some training or motivation. A different mentor (and at that time future boss) told me to choose one gear easier that I thought was right. This small instruction has resulted in my being what we call "a spinner", cadence on level ground generally between 85 and 100 rpm and higher rpm's when the going get hard.

Pushing a bit of math I find that an 11/70 combo, when riding at 25mph, would result in a cadence of about 50 rpm.(given a 700c wheel). In my world I would be in my 14/44 combo and turning about 100 rpm's. Andy

Frog Man

25-622 Drive wheel.

34t Chainring for hill climbing and 70t chainring for a speed of 90km/h (56mph) at 110 cadence.

The vehicle should be able to go around 70mph.

https://www.gear-calculator.com/?GR=...N=KMH&DV=teeth

But that's beside the point.

unterhausen

Moved here from the framebuilding forum. The mod staff decided that general would probably be best for this.

Kontact

The drop from the big chainring onto the small would be like jumping off a cliff and hoping you will safely land in a bucket. The downshift is fairly uncontrolled and the greater the distance the less likely the chain will hit the small chainring teeth squarely rather than falling off toward the inside. And then upshifting is tough because the difference is like a wall.

Andrew R Stewart 

Frog Man- Are you dealing with a solely human powered vehicle (dare I say "bike")?

Here's the speed/gear ratio/cadence calculator I've used for over 40 years. Andy

DangerousDanR

You use'ta could. It was called a triple. That middle chain ring either gave another set of ratios (often near duplicates) or acted as a chain guide. Kontact described it quite nicely. Just too much gap to reliably make the jump, but the chainring in the middle made it work.

My last triple was a mountain bike and I bought a steel ring for the largest ring and used it as a bash guard since the big ring kept loosing teeth.

CrimsonEclipse

What the hell is a front derailleur?

dsaul

I'd hate to hear the awful sounds of the chain trying to climb up the side of the 70t chainring in order to make the shift from 34 to 70. When you understand how front shifting is accomplished, you will understand why this is a bad idea.

seypat

I think some of the mtb triples fds had 24-26t ranges bitd. A 70t ring would put the fd way up on the ST though.

Suntour had some that would do 24t.

https://www.radiolabworks.com/bikes/...%20Catalog.pdf

Maelochs

If you think of the way the front derailleur works .... the length of the lever arm determines how wide a change the derailleur can handle. To cover the range in diameter from a 22-tooth chain ring to a 70-tooth chain ring (a couple of inches) would demand a derailleur lever nearly two inches long, which means it would swing through an arc almost two inches wide. If you were not concerned with q-factor, you could run an 11-111 chainring set.

The alternative would be to totally redesign the front derailleur so that it didn't swing in a simple arc perpendicular to the frame.

seypat

That pretty much explains it.

PeteHski

Double and "bash" instead of big-ring was a popular choice in the days of triple mtb groupsets. Much prefer 1x mtb today.

njkayaker

Shifting works better with smaller gaps.

Rear shifting works better because the derailing occurs on the side without load. It also works better because there is more space for the more gears, which allows for smaller gaps.

Front shifting occurs under load.

It’s already kind of mechanically unstable (you don’t really get dropped chains in the rear).

Derailleurs mostly work by pushing the chain horizontally. Things work better if they can also add a vertical movement. Wider gaps would require a much larger vertical movement, which might not be possible (or easy/cost effective).

You also want the shift to happen quickly. Front shifts are already slow. A wider gap would make shifts even slower.

It’s not just a matter of a longer cage. What gets the chain up is little nubs (pins) on the larger chain ring that snag the chain being pushed against the chain ring. It’s not exactly an elegant or clean process. A wider gap would require much more out of this process (and it might be asking too much).

A larger gap in the front would make things worse.

Front shifting (with the current gaps) is something that is already on the edge of not working well.

PeteHski

There's not a lot of call for 70T chainrings on a bicycle.

DaveSSS 

A 70 chain ring would be worthless and not make you go faster with the limited power available. The largest stock chain ring difference that is a 48/31 shimano grx. Couple that with a 10T sprocket and you get the same ratio as a 53/11.

In the days of 10 speed, I used a 53/39/28 triple for the Colorado mountains, but that was to go slowly up steep grades, with a 12-25 or 13-29 cassette. I can get more top and low gear with a 48/31 and 10-36, 12 speed cassette.

DangerousDanR

I completely agree. I suspect that the big steel ring was made to be a bash guard and not a chainring, but it had teeth when it was new. You won't find a front derailleur on any of my off road bicycles. The 1X stuff just works better for that application.

big john

People who do land speed record stuff use a jackshaft to get super high gearing.

PeteHski

I had an alloy bash ring without any teeth. I missed having teeth marks in my calf!