Larger chainring for Radster Trail?

[Biplaneguy]

I've only put around 50 miles on my new Radster Trail so far, but I'm thinking the chainring is a bit too small. It has a 50t chainring paired with an 11/34 rear cluster, and I find I' rarely using anything other than gear 7 or 8 on the road, 6 or occasionally 5 when offroad. When going over 20mph on the road I'm pedaling way too fast for comfort while not pushing very hard, so a larger chainring would seem to be the answer (and I can just downshift farther when necessary).

But how much larger is reasonable? 52. 54, 56? I haven't looked closely at the available clearance but at a quick glance it looks like there's room. Obviously if I go much larger I'd need to add chain links.

And what brand and where to purchase? I'd need larger chain guards (is that the correct term?) for both sides of the chainring as well.

 

[Jeremy McCreary]

But how much larger is reasonable? 52. 54, 56? I haven't looked closely at the available clearance but at a quick glance it looks like there's room. Obviously if I go much larger I'd need to add chain links.

One approach: Note your ground speed when you spin out in top gear on a descent — say, at 30 mph. If you'd like it 20% faster (36 mph), then you need a chainring 20% larger — if it'll fit. (Others can help you there.)

But bear in mind that all gear ratios will then increase by 20% — including the ones you use for climbing. If the gear you need to climb your steepest plausible hill is already your lowest or next lowest, a 20% chainring increase might be too much even if you had the room.

 

[Biplaneguy]

I don't necessarily want to go faster; I just want a reduced cadence at, say, the bike's current 28 max pedal assist speed. As it is, I've never even used gears 1-4 even going up steep hills (it'd be different without pedal assist, of course) so I think I'll have plenty of low end with a larger front sprocket. Maybe not 20% - that'd be a 60 tooth - but something more than it is now.

 

[Jeremy McCreary]

 

I don't necessarily want to go faster; I just want a reduced cadence at, say, the bike's current 28 max pedal assist speed.

So measure your cadence at 28 mph in top gear. If you think 90% of that would be comfortable to pedal, then try a chainring 1/0.90 = 1.11 times larger, clearance permitting. May take more than 1 iteration to really dial it in.

Have your existing chainring bolt pattern (usually 4 or 5 bolts) and BCD measurement in hand before you shop. Wolf Tooth aluminum alloy chainrings are high quality at middle-tier prices. The one on my 38 lb, 35 Nm mid-drive shows no sign of wear at 1,900 miles, and I do a lot of climbing. Race Face is another middle-tier option, but no personal experience there.

On a hub-drive (less chain loading), an inexpensive alloy chainring like a Deckas from Amazon might be just fine. No sign of wear on the Deckas on my 70 lb, 65 Nm hub-drive at 2,100 miles, same hills.

 

[PedalUma]

@Biplaneguy, A long time ago, before I knew better, I put a 58-T ring on a bike. I thought more would be better. Watch a couple of videos on How to Pedal Like a Pro. Maintaining a high cadence with light pedal pressure is good for the bike and good for you. A larger chainring will not allow for the use of the lower gears. The lower gears will act as a derailleur so the chain drops off the front ring. Sometimes it is best to find out things for ourselves. Just hearing 'Don't touch that hot stove,' is not enough.

 

[Jeremy McCreary]

As it is, I've never even used gears 1-4 even going up steep hills (it'd be different without pedal assist, of course) so I think I'll have plenty of low end with a larger front sprocket.

One last consideration: Powering up hills at high assist cuts into battery range. Might want to hold onto some low-end gearing for times when you need to stretch your battery with significant hills or headwinds or soft surfaces ahead.

Then you'll be able to take on more of the work needed to get home in relative comfort. Especially important on a heavy ebike that's hard to pedal without the motor.

 

[Biplaneguy]

Maintaining a high cadence with light pedal pressure is good for the bike and good for you. A larger chainring will not allow for the use of the lower gears. The lower gears will act as a derailleur

Can you elaborate on that? I'ma an engineer so I understand gear ratios, torque, and power, etc., I've designed a lot of geared systems, but I know a bit less about the actual practice of bicycle gear design.

If, say, I have a 10% larger chainring, then I would be pedaling slower but with more pedal force... with the same chain tension, chain speed, and torque on the rear sprocket. Or, downshifting to a 10% larger gear on the rear cluster would provide the same torque and rpm at the wheel, with the same pedal force and cadence. Though I'm not clear on just how the torque sensor is constructed and how it measures the torque, which would affect things. Most of the time I'm using PAS 2 or 3 out of 5, or less if I'm going downhill.

When I'm going fast, 25-28 mph in top gear, I'm pedaling uncomfortably fast with very little pedal force, and the bike overruns my pedaling, then if I speed up my pedaling to catch up I get a clunk as the freewheel re-engages. That doesn't seem good for the bike.

I'm also not clear on what you mean by "The lower gears will act as a derailleur so the chain drops off the front ring"?

 

[PedalUma]

Can you elaborate on that? I'ma an engineer so I understand gear ratios, torque, and power, etc., I've designed a lot of geared systems, but I know a bit less about the actual practice of bicycle gear design.

If, say, I have a 10% larger chainring, then I would be pedaling slower but with more pedal force... with the same chain tension, chain speed, and torque on the rear sprocket. Or, downshifting to a 10% larger gear on the rear cluster would provide the same torque and rpm at the wheel, with the same pedal force and cadence. Though I'm not clear on just how the torque sensor is constructed and how it measures the torque, which would affect things. Most of the time I'm using PAS 2 or 3 out of 5, or less if I'm going downhill.

When I'm going fast, 25-28 mph in top gear, I'm pedaling uncomfortably fast with very little pedal force, and the bike overruns my pedaling, then if I speed up my pedaling to catch up I get a clunk as the freewheel re-engages. That doesn't seem good for the bike.

I'm also not clear on what you mean by "The lower gears will act as a derailleur so the chain drops off the front ring"?

Watch a video on How to Ride Like a Pro. I went the direction you are considering and that is how I know. You might just need to try it for yourself. I encourage experimentation. The lower gears in the rear will shift the chain right off a too large chainring. A torque sensor is just a strain-gauge that works with millivolts of impedance. But torque sensor bikes also use cadence. To make it smooth and feel natural they typically give more assistance at higher cadence. If not you would do a wheelie off the line from a dead stop. The most assistance is most often at 70 plus pedal revolutions per minute. With some bikes everything can be programed, with most that is all locked to be optimal.

 

[Jeremy McCreary]

I'ma an engineer so I understand gear ratios, torque, and power, etc., I've designed a lot of geared systems, but I know a bit less about the actual practice of bicycle gear design.

Excellent! We could use your expertise. You might enjoy Wilson & Schmidt, 2020, Bicycling Science, 4th ed.

 

[Biplaneguy]

Watch a video on How to Ride Like a Pro. I went the direction you are considering and that is how I know. You might just need to try it for yourself. I encourage experimentation. The lower gears in the rear will shift the chain right off a too large chainring. A torque sensor is just a strain-gauge that works with millivolts of impedance. But torque sensor bikes also use cadence. To make it smooth and feel natural they typically give more assistance at higher cadence. If not you would do a wheelie off the line from a dead stop. The most assistance is most often at 70 plus pedal revolutions per minute. With some bikes everything can be programed, with most that is all locked to be optimal.

Not quite sure what you mean by "shift the chain right off a too large chainring"?

Where does the strain gauge measure the deflection? Is it inside the hub or at the crank? A true torque sensor at the crank would have to be rotating but I see no evidence of sliprings, unless it's just measuring radial load on the crank bearings and converting that to a torque number? I observed that it also uses cadence as the motor doesn't kick in if I'm just pushing on the pedal with the bike not moving. I'd love to know what the motor program algorithm is but no doubt it's proprietary. Whatever it is, Rad did a good job as it feels very natural.

But I'm already questioning the idea of a larger sprocket. I'd still like to pedal less furiously when going fast, but this evening I took a ride on a somewhat rougher trail behind my house, and for the first time shifted all the way down to first gear on some steep sections. They were steeper than I really want to ride (how do you find the limits without getting there?), but I can see that maybe I really do want the low gear ratios. But I gotta learn the bike, and figure out how I like to ride first.

 

[PedalUma]

What I have found is the more a person rides, the more they use a high cadence with a smooth pedaling technique that looks effortless like a swan paddling. There is something about the physics of a large chainring that makes it like a straight chainline, and to have big problems with cross chaining. I don't know how this works, but I have seen it. Someone may not be able to fully explain lightning either. It could be that the pressure on the first tooth to contact the chain is much less because the overall force is more distributed over more teeth or something like that.

 

[Jeremy McCreary]

Where does the strain gauge measure the deflection? Is it inside the hub or at the crank?

Some take the deflection at the crank, some at the rear hub (at least on unassisted road bikes), and some (like the IDbike TMM4 torque sensor on my torque-sensing hub-drive) at the rear dropout on the drive side. The last basically measures dropout distortion due to chain tension as a proxy for rider torque. Actually works quite well.

Specialized says that their mid-drive motors use internal sensors to measure true cadence and rider power directly, with real-time rider torque presumably calculated from that data stream in firmware. Since the main determinant of mechanical motor power in their mid-drive PAS is rider power, I think of it as a power-sensing PAS.

I observed that it also uses cadence as the motor doesn't kick in if I'm just pushing on the pedal with the bike not moving.

Most so-called "cadence sensors" do nothing more than return a yes-no answer on crank or cassette rotation. For safety reasons, all ebikes need this capability. Some higher-end ebikes like the Specialized mid-drives also measure true cadence in RPM and report it on their displays.

Custom display page on my Specialized Vado SL mid-drive. SPORT is the current assist level.

The TMM4 torque sensor on my hub-drive includes a yes-no cadence sensor watching the cassette rather than the crank.

I'd love to know what the motor program algorithm is but no doubt it's proprietary.

Me, too. Most ebikes are total black boxes in this regard. The Specialized mid-drive PAS they call "Turbo" is by far the most openly documented I've seen. For details, see the long-winded intro to my interactive graphical model of this PAS:

Visualizing and exploring the power-sensing PAS in Specialized mid-drives — an interactive graphical model

For our more math-minded Specialized mid-drive riders, here's an interactive graphical model of your bike's tunable power-sensing pedal assist system -- hereafter, just "PAS". The model doesn't cover complex PAS responses to transient events like starting out or approaching PAS cut-off speed...

forums.electricbikereview.com

But I'm already questioning the idea of a larger sprocket. I'd still like to pedal less furiously when going fast, but this evening I took a ride on a somewhat rougher trail behind my house, and for the first time shifted all the way down to first gear on some steep sections. They were steeper than I really want to ride (how do you find the limits without getting there?), but I can see that maybe I really do want the low gear ratios. But I gotta learn the bike, and figure out how I like to ride first.

Totally agree. To best play the gearing trade-offs, you really need to develop a feel for what you're up against WRT terrain, bike weight and responsiveness, and the motor and rider power you can bring to bear. Sample representive hills and trails first, then decide.

 

[PedalUma]

The download of the dial it all in yourself motor/display I like best are found here, this is the good stuff:

T154 E-bike Centre Control Panel - To7motor

This is a T 154 E-bike Centre Control Panel with a TFT colorful screen for ebike drive system , transparent silicon rubber button with LED back-light

to7motor.com