Larger chainring for Radster Trail?

[BlackHand]

Can you clarify what cadence feels too fast for you? I think at 28mph your cadence is prob still below 75rpm? Everybody's natural range is a bit different, but you may find that your upper limit increases as you start cycling more often.

It's been noted a couple times, but I would recommend you give yourself more time -at least a month or two - before making changes to the bike. At only 50 miles your body and mind haven't really acclimated to it yet.

When/if you do make changes, another possibility would be to put longer cranks on. 175mm cranks should be roughly like adding 1-2T to the chainring. That does have implications for rider fit and ground clearance to consider of course.

 

[Biplaneguy]

Interesting. What quantites are being corrected with these factors? Power transmission efficiency? Optimal chain tension?

The amount of torque that can be transmitted without excessive wear or outright failure. Typically a sprocket manufacturer (at least in the industrial world) will publish tables of the allowable power their sprockets can handle at various rpm. A larger sprocket can handle more power than a smaller one just because of its larger size (more torque for the same tension), but when you get too small the load per tooth increases beyond what the sprocket can handle, so there is a fractional correction factor for smaller size sprockets, based on the number of teeth in contact with the chain. A chain or belt wrap of less than 180° also reduces the number of teeth in contact.

Can you clarify what cadence feels too fast for you? I think at 28mph your cadence is prob still below 75rpm? Everybody's natural range is a bit different, but you may find that your upper limit increases as you start cycling more often.

It's been noted a couple times, but I would recommend you give yourself more time -at least a month or two - before making changes to the bike. At only 50 miles your body and mind haven't really acclimated to it yet.

It's 71 rpm at the crank with 50/11 gearing at 28 mph. Perhaps that's not too fast as all I have to compare it to is a non electric bike where I was probably going slower. But as I said I'm not planning to change anything right now; I'm just exploring options and learning.

When/if you do make changes, another possibility would be to put longer cranks on. 175mm cranks should be roughly like adding 1-2T to the chainring. That does have implications for rider fit and ground clearance to consider of course.

That wouldn't change the cadence at a given speed. What it would do is increase the distance the rider's feet move, and decrease the pedal force... not what I want.

 

[PedalUma]

I’m picturing this. It sounds like if the chainring is too large/tall then, the angle from the top of the low gear to the top of the chain ring changes upward. This would mean that there are less teeth engaged on the lower gears. It shouldn’t matter much on the lower part of the smallest cog because the chain still goes immediately through the derailleur.

Here is a dramatic increase in the angle of the chain, but you’ll notice how high the chain goes towards the chainring. 120t!

View attachment 192648

I think that is part of the problem. A minor cross chain can be handled by similarly sized cogs. But on a large one the force is also pulling down as it also pulls sideways. Also the distance to the first tooth touching the chainring is reduced and is further to the rear. This all leads to derailment.

 

[Tom@WashDC]

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.

use this:

Ritzelrechner - Graphical Gear Calculator for Bicycles

The bicycle gear calculator displays graphically the gearing of derailleur gears and gear hubs. It can be used to compare different setups to find the optimal gearing for all kinds of bicycles.

www.gear-calculator.com

 

[Gionnirocket]

Gear calculators are a great place to start but they don't take into account ones physical abilities
I've come to the conclusion that what looks good on paper doesn't always match up with what my chicken legs are comfortable with.

 

[Jeremy McCreary]

use this:

Ritzelrechner - Graphical Gear Calculator for Bicycles

The bicycle gear calculator displays graphically the gearing of derailleur gears and gear hubs. It can be used to compare different setups to find the optimal gearing for all kinds of bicycles.

www.gear-calculator.com

View attachment 192653

My favorite online gear calculator! A masterpiece of graphical data presentation. My own Google Sheets spreadsheet is still my go-to, but I like to enter results here to see what they look like.

 

[Jeremy McCreary]

It's 71 rpm at the crank with 50/11 gearing at 28 mph. Perhaps that's not too fast as all I have to compare it to is a non electric bike where I was probably going slower.

Agree with @BlackHand that your preferred cadence will likely creep upward with time.

A real-time cadence readout can hasten that. Just glance at it now and then and make a conscious effort to pedal a little faster when it dips below a certain RPM, changing gears or assist or both as needed.

Added a real $20 Bluetooth cadence sensor (not just a yes-no crank rotation detector) to my hub-drive just for that purpose. The RideWithGPS app running on the phone on my handlebar provides the readout.

I "spin out" (stop producing useful torque) at around 100 RPM now — mainly due to loss of the necessary coordination. When that happens, a conscious effort to smooth out my stroke can often buy me another 5 RPM on a top-gear descent.

Wilson & Schmidt cover the science
on rider power vs. cadence. The human power vs. cadence curve resembles an upside-down parabola. Turns out that preferred (self-selected) cadence is just a little short of the peak in most riders. With training, the peak tends to rise and shift to higher cadence.

The parabolic shape of the curve implies an optimal gear-shifting strategy: As you accelerate, pedal just past your preferred cadence, then shift to drop cadence just below, then repeat as needed. Works for me.

 

[PedalUma]

100 RPM now

As you accelerate, pedal just past your preferred cadence, then shift to drop cadence just below, then repeat as needed.

The Vado has a motor cutout at 100.
As for the second part quoted, think of how an accelerating motor cycle sounds when shifting gears. And down shift when slowing, approaching curves and hills.

 

[Jeremy McCreary]

The Vado has a motor cutout at 100.

Maybe on the regular Vado, but not on my SL 1 5.0. If it has a motor cutout, I have yet to encounter it. That puts any possible cutout above 105 121 rpm.

 

[Gionnirocket]

That's one setting I like about the Bafang UART motors. You can set a max cadence for each PAS and as you approach it, it will gradually taper motor assistance so you don't spin out and keeps you from having to change gears. Great for exercise and when you don't necessarily want to go faster. I use it in the lower PAS effectively.

 

[PedalUma]

Maybe on the regular Vado, but not on my SL 1 5.0. If it has a motor cutout, I have yet to encounter it. That puts any possible cutout above 105 rpm.

On many bikes the cutout is abrupt. Velotric comes to mind. With the Vado SL it tapers off a bit over 100, so it is not like warm shower, then sudden freezing cold shower, just when there are suds in your hair.

 

[Jeremy McCreary]

On many bikes the cutout is abrupt. Velotric comes to mind. With the Vado SL it tapers off a bit over 100, so it is not like warm shower, then sudden freezing cold shower, just when there are suds in your hair.

Sorry, just rechecked it on my SL 5.0: No detectable motor cutoff through 121 RPM.

 

[Gionnirocket]

I just checked... The cut out on the SL 5.0 is at 122 RPM

 

[Tom@WashDC]

On another note, I cannot believe that Rad is still using a Freewheel on this or any of their bikes anymore.