Slower with bigger chainring?

billm

1 mW
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Jul 1, 2018
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I have a tsdz2 that I'm running with an alfine 8 speed rear hub. The cog on the hub is 16T, and I have been, until recently, running a 46T chainring. I damaged that chainring, and looking for a bit more speed, replaced it with a 47T.

I expected that the bike would be a bit faster, but in fact according to the speedometer the top speed is about 3mph slower, and it feels like more work to achieve even that.

What could be going on here? Have I "topped out" the amount of assist the tsdz2 can supply or something? It feels like its giving less assist than it did with the smaller ring, and the speedometer readings seem to support that too. Thx.
 
billm said:
What could be going on here? Have I "topped out" the amount of assist the tsdz2 can supply or something? It feels like its giving less assist than it did with the smaller ring, and the speedometer readings seem to support that too.
You are running out of torque. The larger chainring reduces the torque available, and you thus cannot make enough torque to get to the speeds you used to.
 
I think billvon is probably right. And in that case, you might be able to reach a higher top speed with a smaller chainring instead.

Don't bother with one tooth jumps-- that's only a little over 2% difference in gearing. I say get a 44t and a 42t ring (both common) and try out to see what works best. Get a half link so you can adjust your chain length appropriately.

I think it's strange to use a crank drive without having multiple gear ratios to reap the benefits of crank drive. [Doh.
Alfine 8.
] But at least being able to fine tune your gear ratio is a benefit you can realize.

EDIT:
It occurred to me that it would be more cost effective to get a 17t and 18t freewheel and try those instead (although that implies lengthening, rather than shortening, your chain).

I have no direct experience with TSDZ2, but my BBS02 cruises at about 16.5 mph with it's 46/28 gear and at close to 20mph with its 46/24 gear. (It's difficult to pedal along with these speeds.) With 47/16 gearing, your motor may be radically overgeared for optimum performance. Something between 2:1 and 2.5:1 would probably give you the best compromise between motor effectiveness and pedal effectiveness.
 
Watts vs speed equation might be your problem. I mean, there is a max speed that a given amount of watts can achieve on your bike. There is a limit that you can only beat with more watts, better efficiency, lesser weight, lower aero drag.
 
Try testing the theory of too much torque reduction by using lower gear selection on the 8 speed hub and seeing what kind of speed you get. Frankly, I find it odd that a one tooth difference on a 46 tooth chainring would be perceptible or even noticeable on the speedometer. The difference should be pretty small - on the order of 2% or so as Chalo said - and I don't think I can feel or notice a 2% difference in watt input on my bike other than by seeing it on a meter. I'd expect other variables to swamp that small amount and make it very hard to notice.
 
MadRhino said:
Watts vs speed equation might be your problem. I mean, there is a max speed that a given amount of watts can achieve on your bike. There is a limit that you can only beat with more watts, better efficiency, lesser weight, lower aero drag.

Worth noting: Max wattage almost always occurs at 50% of motor free speed.
 
billm said:
I expected that the bike would be a bit faster, but in fact according to the speedometer the top speed is about 3mph slower, and it feels like more work to achieve even that.
What could be going on here? Have I "topped out" the amount of assist the tsdz2 can supply or something? It feels like its giving less assist than it did with the smaller ring, and the speedometer readings seem to support that too. Thx.
billm,

What happens when you drive a car up a hill with a manual transmission (and the car can maintain speed but not accelerate) then you shift from second gear to third? The engine runs at a lower RPM, it produces less horsepower, and the car slows down. The car may even stop, with the throttle wide open. Engines need to run at higher RPM to produce more horsepower, so the driver needs to use first gear, not second, to climb the hill. With an automatic transmission, you would hear and feel the automatic transmission downshift, and run the engine faster to climb that hill.

With an electric motor, the same thing happens, but the motor gets much hotter as the RPM decreases when going uphill.The motor may self-destruct as the heat goes up and the RPM slows, under a heavy load. The power drain increases also, possibly damaging or destroying batteries or the controller. Whatever the weakest link may be in the electrical power train, it can fail if you demand more power than the system can produce, when going uphill.

On the bike, using a lower gear ratio (selected at the rear wheel or by using a smaller front chainring) will let the motor crank out enough RPM to produce more power, more speed, and have the motor running cooler. You would not need to gear down if the world was flat, and there was no air drag on the bike
 
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