phughes

smd4

I had to scroll back up to see if he really wrote what you quoted.

Ooof..

livedarklions

What are you doing about drag?

This big chain ring thread has always gotten stuck on the particulars.

JohnDThompson 

The larger the jump between rings, the more trouble-prone the shift.

It's been tried, with the SunTour Cyclone "extended" front derailleur:

https://www.velobase.com/ViewCompone...m=113&AbsPos=4

The results were underwhelming, and the extension was soon dropped from the product line. This was before indexed shifting aids (ramps and pins), so it might work better now. But is it really worth it, when you can now get rear clusters with more than ten sprockets and more than 40 teeth on the largest sprocket and down to 11 teeth on the smallest?

Miele Man

I believe that another problem with such a jump between such a small chainring and the proposed large one would be the required shape of the derailleur cage to match the 70 teeth chainring. Then I expect that the cage would snag the chain on the small chainring.

Cheers

79pmooney

It is verboten to mention the word "drag". (Florida will be banning all public university engineering textbooks that mention it next year. )

And "seriously" - why is a triple such an impossibility? 34-52-70? All existing rings (I am assuming you have a source for that 70). An 18 tooth jump is a lot but has been done. (I have a 24-42 jump on one bike. An ordinary SunTour AR derailleur.)

Another consideration - the rear derailleur. I know you said you wouldn't use many of the cassette cogs while on the big ring. I've heard that a few times before. I've also seen what happens with a momentary brain fart when gassed at the top of big hills and starting the roll into that wonderful descent. (That is why you are doing this, isn't it?) That one little pedal stroke while you remember "Oops! Gotta shift in back first!" will only cost you a rear derailleur, a couple of spokes (maybe enough to make the wheel unridable), probably the derailleur hanger, maybe the dropout, probably chainstay paint and maybe deeper. I know, just incidental stuff.

My advice? Get a rear derailleur that can handle that entire range from the biggest ring to biggest cog all the way to the small ring and the smallest cog; at least without destroying itself. If you are smart, you won't push the gear limits beyond what your rear derailleur can do. (You can consider inventing/making derailleur modifications or buying them such as a longer derailleur cage arm.)

Kontact

I have a bike with more with larger total chain wrap than the derailleur has capacity for. But it isn't dangerous because the chain is long, not short. My chain will lose all tension if I shift into one of the three highest cogs while in the granny gear up front. I never do that, but it is rideable that way if needed.

I don't think that was all that unusual in the past, given that Deore DX and XT were available with short cage derailleurs.

79pmooney

+1 I do the same. If I hadn't ridden so hard this afternoon, that might have come to mind. Thanks.

Trakhak

Threads about 70-tooth chainrings are always fun.

SPOILER ALERT!!!!!

This guy was convinced that with a 70-tooth chainring and an 11-tooth sprocket he'd be able to hit 50 mph. Argued with those who were skeptical that he (or anyone) was capable of doing that on a conventional road bike on level ground without coming off a ski jump first or otherwise cheating. A few other posters chided the naysayers, arguing that "If he believes in himself, he can do anything!" trumps physics. The ending was predictable.

PeteHski

I read the first couple of pages, which went as you would expect. But I was a bit disappointed that the thread got closed down before the guy rode it (unless I missed his record attempt somewhere along the way).

livedarklions

Not sure why that post was aimed at me. I was responding to a post that claimed one would be able to ride at 55 mph with a 70t. That raised the drag question.

livedarklions

The thread got closed after several days of him promising to post a photo of the bike which never materialized and his refusal to answer a whole bunch of simple questions. He repeatedly challenged someone to bet him, and wouldn't answer questions about what he was actually going to do. It got shut down with the mod basically telling him he could start a new thread if he actually had something to report. He has never returned.

​​​​​BTW, he casually claimed that he was likely the fastest rider on the planet.

BobbyG

I bought my Nishiki Blazer in 1997 for commuting. I needed higher gearing for faster cruising and put a 53-tooth chainring up front on the triple. It is the biggest ring that would fit due to the widening of the chain stays just behind it. In fact, there is no room for the chain to slip between the cog and the frame at that point, which has let to some epic chain jams over the years. I also fitted longish crank arms soon after that.

I forget what the smallest rear cog is, but I don't spin out on the bike until about 36 mph or more which I used to hit regularly on a long, steep downhill.

I was more of a low-cadence masher 25 years ago, but I still ride it regularly, mostly in the winter when it gets studded tires.

Pics from 10 years ago:



The bike last year or so:

Trakhak

He had another thread going on the same topic. You can find it by using Advanced Search with his user name (tellmethetruth) and selecting "Search for threads" at the bottom of the search page.

That thread is also a fun read. Unfortunately, after he finally got to where he had the bike set up to his liking and was about to make his record attempt and thus shut up all the doubters, he abruptly stopped posting. I wonder why.

livedarklions

He couldn't show us pics of the bike because he couldn't get the computer to work, as I recall.

Trakhak

I think he later said that he returned the nonworking computer and got a new one, so he was all set. Then, silence.

Frog Man

I just set up a new road bicycle yesterday so I've been learning a bit more about how the front derailleur works. The ramp up makes sense as I see it working.

I'm curious about front derailleurs, how they work and why they are the way they are which is why I made the thread.

I'll be lowering aerodynamic drag to get to higher speeds.

Thanks for your replies.

smd4

They push the chain one way, then the other, usually using a cable to make it move.

livedarklions

Seriously, it sounds like an interesting project. Hope you post about it as it progresses. I was actually disappointed by the last 70t chain ring when it became obvious that the guy was just BSing.

Are you doing an upright bicycle with fairing?

livedarklions

Was that sarcasm necessary or deserved? It's also not even correct--they don't just push, there's also lifting the chain to the bigger ring The height of the 70t is the tricky bit here..

smd4

Seriously? The FD "lifts" nothing. Do you even understand how a derailleur works?

Of course the sarcasm is deserved! Did you read what this guy wants to do?

livedarklions

You're actually claiming that the top of the big chain ring is not higher than the top of the small one? You do know where the FD is in relation to the chain ring, right? How is the chain getting up there? Magic? If it makes you feel better to call that "pushing up", that's fine, but that's a pretty weird use of the word "pushing" and it just results in a synonym for "lifting". You could also use the phrase "picking up" if you like, but that's also actually a synonym for lifting.

People are actually giving him decent advice on how it might be done. He may possibly decide to go with a single ring up front to make this work, but I don't think there's anything ridiculous in thinking about doing it this way.

The guy is being polite, admitting he doesn't know what he doesn't know, and is trying to learn stuff that you obviously aren't familiar with. You're just making yourself look stupid.

smd4

Yep, I was right. You don't understand. The FD does nothing but push the chain one way or the other. The reactions of the chain due to that pushing are what causes the chain to move from gear to gear. Next you'll be telling me the RD "lifts" the chain from the smallest to the largest cog. (spoiler alert--it doesn't).

Anyway, now that I think back on it, I didn't use sarcasm at all in explaining precisely how a FD works. Perhaps I should have. There's nothing wrong with calling a spade a spade. "Do you guys think I can increase my speed by tying a hydrogen/LOX rocket pack to my back?" would get the same kind of response from me.

genejockey 

I can't believe nobody posted this - the power required to go 70 mph on flat, on aerobars, is >4500w, per Bike Calculator.

big john