(solved) (Tongsheng) Left crank arm needs to be ~3mm longer than the right <= false

xtinctionRebeller

xtinctionRebeller

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The stock crank arms for the Tongsheng TSDZ2 have flawed geometry in 3 respects:
  • The Q-factor is unusually large ~210-270mm, depending on who you ask and depending on the bottom bracket width.
  • The offsets are asymetric due to the reduction gear, which puts the midpoint of the pedals to the right of the bike frame. This shift is ~18-30mm to the right (depending on who you ask)
  • The length of 170mm is generally only suitable for non-e-bikes with tall on-road riders. The crank should be a shorter length for on-road riders shorter than 5'9", even shorter for off-road riders (who need clearance), and the motor makes the leverage of 170mm less important (and I'm not sure the leverage makes any difference at all on a geared bike in the first place anyway -- i.e. just as we know that increasing wheel size does not increase efficiency as gears render it moot). The stock size is therefore only optimum for a small minority of riders.

The first correction needed is to use a zero offset crank on the right-hand side. This reduces the Q-factor & simultaneously compensates for the right shifted center. According to Tony, the offset on the right is 100mm and the left is 70mm when the cranks are symetric and 170mm long. Since straightening the the right crank is claimed to correct both the centering and the overall Q-factor, then 30mm is the extra offset needed on the left (10 degrees).

A problem remains: if there is zero angle on the right crank arm, and the left crank arm has the same length as the right (170mm) but it's angled (thus asymetric), then the left crank arm's lowest position will be higher than the right crank arm's lowest position. Time for a picture:
Code: 
                                 /           |
                                /            |
                               /             |
          left crank arm =>   /              |   <= right crank arm
                                      ^
                                frame center

So if we do a bit of trig, the left crank arm is the hypotenus of a right triangle. To match the up and down displacement, the length of the left arm actually needs to be 172.6mm. But note that most people don't want 170mm cranks anyway, so if we make the final correction and swap in a 152mm zero offset crank on the right, then the left crank needs to be 154.3mm {152mm / cos(10 degrees)}. The closest match to market choices is 155mm.

Is my math correct? Are my inputs correct?

I realize that the body adjusts amazingly well, and that riders don't even notice a significant problem with the factory cranks, but if someone is going to do custom mods anyway they might as well get it as close to perfect as there are available parts for.

I would appreciate it if someone could revew these numbers, so I don't end up creating more awkwardness than I'm starting with.
 
Pedal circle diameter is independent from offset dimension. 170mm crank arms spin the same circle diameter regardless of offset. 170 IMO is short for most folks. If your just spinning along and never need to add much torque, 165mm could work well as long as your not needing to pedal home over hills with a heavy dead battery or crapped out motor -controller fail.
 
speedmd said:
Pedal circle diameter is independent from offset dimension. 170mm crank arms spin the same circle diameter regardless of offset.
Click to expand...

Imagine tying a pole to a pencil using a string that's 170mm, and draw a circle like this:
iu
(think of the 6.5 inches as 170mm)

Now without changing the length of the string, slide the string up the pole a bit to create an angle (simulating the angled left crank). If you draw a new circle, it will be smaller. Obviously increasing an angle in order to increase Q-factor will reduce the diameter of the circle marked by the pedal's path.

The only way for your statement to be true would be if the 170mm were not the length of the string (read: crank), but rather the length of the projection of the string on the paper (which would be the displacement of the pedal to the axle rather than the length of the arm). Are you saying that when a crank is spec'd at 170mm, it's not the measure of the crank (which may be angled) but rather the displacement between the pedal and axle?
 
Are you saying that when a crank is spec'd at 170mm, it's not the measure of the crank (which may be angled) but rather the displacement between the pedal and axle?
Click to expand...

Bingo! Correct. The angle is not adding or subtracting to length as it spins the same circle diameter regardless if angled or not. The projection is ( or should be ) the dimension the machinist should be shooting for. Without careful measurements, hard to tell what you actually have measuring the long side of the triangle.
 
Thanks! This simplifies my effort in getting a proper kit ordered. I'm glad I can order different brands and angles on the cranks and expect a match as long as the spec is the same length.
 
speedmd said:
If your just spinning along and never need to add much torque, 165mm could work well as long as your not needing to pedal home over hills with a heavy dead battery or crapped out motor -controller fail.
Click to expand...

This is complete nonsense! I ride a 50 pound fat bike just fine with 152mm cranks, with no battery, no assist! Rides just like you'd expect it to, until you're trying to climb uphill on a trail from a stop, that's where the extra leverage of longer cranks comes in handy... For everything else, even taking off from a stop on flat ground, the shorter cranks work beautifully.
 
This is complete nonsense! I ride a 50 pound fat bike just fine with 152mm crank
Click to expand...

I agree. Complete nonsense! :lol:

Seriously, I get that shorter folks could need shorter cranks and even kid size cranks. Also with mid to low BB height and extra wide crank setups needing to get pedals away from hitting most everything. One good pedal strike could ruin your day.

you're trying to climb uphill on a trail from a stop, that's where the extra leverage of longer cranks comes in handy.
Click to expand...
Kind of proves the point. No? You don't mention your height or more important your leg length. Getting to the point where you have no lower gear option onboard, you are walking once you run out of torque on the steep. Just math my friend. Certainly the kid cranks will work, but what is best for you depends on what fits you and your riding best.

Crankrec.jpg
 
Careful how you mince my words, what I was trying to highlight was the fact I have no problem enjoying that 50 pound fatbike even without assist, with those "kid cranks". Ditch the graph too, if you're using an ebike or modern groupsets etc (although sadly, you won't regularly find short crank options in a hollowtech for example, not yet.. Not that it matters in a world where mid-drives rule).

Also, I take back my assertion that a longer crank has any value for hill climbing from a stop: if it's an issue, then the bike has too large of a chainring for the intended task.

I don't use my fat bike for steep trail climbing since it has a 50T ring on a 1x11, it's geared for winter roads and trails year-round... Even with that bigger chainring still a joy to ride un-assisted with short cranks... This is on my daily driver by the way.

There is certainly some truth in using longer cranks if you're above average in height, but what I'm really getting at here: try shorter cranks, if you're on an ebike. Also try shorter cranks than recommended it you're using high end components and drivetrain... You'll be surprised.

(if you haven't given shorter-than-recommended cranks a fair test on a good assisted bike, or aren't willing to, end of conversation)
 
No issue Deafcat. Some trends make sense, and many are just bogus. As I mentioned before, if you loose your assist miles from home, you may find value in pedaling a more appropriate setup for both conditions. Ask me how I know. :oops:

As far as the graph goes, most of the studies of crank lengths only used seated pedaling. That boxes you in to a single position with some significant constraints. If your not flexible and wish to ride in more aero riding position, crank arms longer than chart is showing is certainly not a good way to go. Also if your shorter in the legs. That will put your knees up close to your chest if your belly is not in the way. :lol: It also may put too much strain on the knees at the top of the stroke. Certainly, going shorter will allow you to spin faster easier.

On my old classic with 165mm arms, I could spin hours seemingly effortlessly. My PR's on the bigger climbs are all with 175mm arms. Many of them I am stuck in my lowest gears for most of the duration and out of the saddle every time I fall off pace. Just no comparison when standing on the peds.
 
I've ridden that particular bike without assist over a hundred km, hill climbs included... It's how I gear down on the weekends to ride with less experienced riders and friends who don't have assist. Sometimes I even ride my pure acoustic bike, but it's not quite as nice.

No argument on the road cycling importance of bike fitment (keeping your knees away from optimal lung capacity, etc). That's a different scenario and if we're talking assisted road bike, a fresh set of design challenges... Assisted pure road configs are rare since most folks only add assist to commuter-equipped configs when it's for serious pavement use.
 
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