Large cassette torque on QR

I recently changed cassette from an 11-36t SLX HG81 to a SunRace CSMX3 11-40t and on my first proper ride my rear brake locked up twice and I had to reseat the wheel in the dropouts.

I was wondering if anyone had experienced torque from a large (11-40t) cassette pulling a QR mounted wheel out of the drop outs? I've heard of something similar happening under disc braking on the front.

My other concern is that the derailleur is not clearing the cassette quick enough when I dump several gears. I'm running 1x10 with a 30t upfront, using a SRAM S500 10spd bar-end shifter to move an X9 2.1 mech. I can shift a lot of gears quite fast. Would tightening the B-screw help here? The chain appears to be sized OK, there was plenty of clearance for the cage on the large sprocket when I had it in the stand and no noise when it's running.

Wouldn't you be getting maximum torque in the 11 rather than the 40?

Hmm, I think I have a gap in my knowledge here.

I have always believed that lower gears gave increased torque at low rpm.

I'm googling around but it's not becoming much clearer...

EDIT: https://www.engr.ncsu.edu/mes/media/pdf/gears

For the same given RPM, you'd need more torque on a harder gear, surely?

Think short lever, long lever, it is much easier to generate more force/torque with a longer lever.
Hence you will generate more torque on the back wheel using the largest/longest sprocket.

https://cyclingtips.com/2009/06/power-vs-torque/

In the general context, torque is the (cross) product of the force applied and the moment arm. What this mean is if you have a large force applied far away from the pivot point, you have a large torque.

In an automobile, pushing on the gas a certain amount at a certain RPM will produce the same torque. By changing the moment arm, or the distance from the axis of rotation, you can change the force that is applied to the teeth (or the chain if you pretend the car’s transmission is like a bicycle, which is a fine assumption if you’re just curious and not serious about engineering cars) and if you change the moment arm going to the axle, you can change the torque applied to the axle. You can also change the force going from the tire to the road by changing the diameter of the tire, but that’s not really something you can change while you’re driving…

So, to recap, the engine outputs some torque. That torque can be scaled up or down to get more or less torque at the axle and thus the wheel. More torque means more force between to wheel and ground which means more acceleration (F = ma). Now the question is how does the torque get scaled up or down?

If you have a small gear at the engine and a big gear at the axle, then the moment arm at the engine is smaller than that at the axle while the force applied on the teeth is the same (due to newton’s third law). Since torque is force times moment arm, changing from a smaller moment arm to a bigger moment arm will increase torque, so a small gear at the engine turning a big gear at the axle will increase torque, but now consider that one rotation of the motor will result in less rotations of the axle because the axle’s gear is bigger. This is the tradeoff between torque and speed.

In first gear, essentially the gear at the engine is smallest and the gear at the axle is largest, to produce the most torque at the axle with the same gas flow into the engine.

In reality, the transmission in a car is more complicated to reduce the chance of failure, increase transmission lifetime, decrease size, etc. Real cars usually use some sort of planetary gear scheme, but the idea is the same where each “mode” or “gear” will have an equivalent gear sizing ratio between the engine and axle. First gear will have the smallest ratio meaning small engine gear to large axle gear. For the same engine output, you get more force at the wheels for greater acceleration.

The reason we don’t always drive in first gear is because of the speed tradeoff. The engine can only turn so fast, and the faster the wheels turn, the faster the engine has to turn. Upping the gear ratio allows the wheels to turn faster with the same engine speed, but you can’t accelerate as quickly or go up as steep hills.

Pretty sure the leverage on the rear axle will be greater in the 40t, torque = force x distance...shouldn't be anywhere near enough to move the QR though. Is your skewer working properly?

Thanks @hippy that's a great explanation but I still have trouble with all the physics terms due to no working knowledge of them. I got a B in Physics at school too...

@croft You're right about the input torque but not the output torque

@Danners I'm using a DT Swiss RWS skewer and you can get it pretty tight. It's not cam-operated though, which might mean it's not as clampy.

Maybe try another skewer then if you can borrow one.

The shifting problem might just be that the Sunrace cassette doesn't have such sophisticated ramps to enable fast shifts...I have no experience with them though so am just pointlessly pontificating.

B-screw should be adjusted so that the top jockey is as close to the bottom of the biggest sprocket as possible for the best shifting.

Are the bearings ok? Does the freehub spin freely? How slack is the chain when in the small sprocket?

@Danners Good call. I've probably got one...

@hippy That's interesting about the B-screw. I've always given myself a "sensible" gap. I'll reassess when I have it in the stand again.

All other things are as they were before I changed cassette. I had expected to have to add a chain link due to the increased size of the large sprocket but the mech appeared to be positioned fine without it. It being a 1x10 the ratio on the small sprocket is unchanged and again the running position of the mech appears fine.

Thanks for all your replies, they're very helpful in thinking it all through

In the general context...

Who is this and what have you done with Tstr?

Oh and I received a recommendation from a bike mechanic pal for the SunRace cassette. I've fitted a fair few of their lower end cassettes to others' bikes thinking that they were just cheapo ones that would do.

After poor shifting performance with the HG81 (again not a high end cassette) I thought I would give it a go. It wasn't that cheap but it's definitely quieter in operation than the one it replaced...

...plus it came in black.