How to measure on which gear chain is straight (non crossed)?

[JGH]

I have 12 speed ebike with Shimano drivetrain.
Chainline is 52mm (according to specs and measured).
Rear dropouts are 135mm.

I want to know which gears are best for straight chain.

So my thoughts on measuring:
* Divide dropout width by half and I will get center line: 135mm: 2 = 67.5mm. So center line is 67.5 mm inwards from dropout.
* As chainline is 52mm (aka 52mm outwards from center line), I substract 52mm from 67.5mm and I get that straightest chain is the one 15.5mm (67.5mm -52mm) inwards from the dropout. Measuring now 15.5mm from dropout I see that it is roughly 9th gear.

Why on 12 speed casette the straightest chain line is 9th gear? Why so far from the middle (6-7 gear)?

This also means that 12th gear is a bit cross chained, while 1st gear (slowest) is super cross chained?

Am I measuring incorrectly ?

 

[irie]

Align chainline using the line on the derailleur.

 

[Suns_PSD]

I have 12 speed ebike with Shimano drivetrain.
Chainline is 52mm (according to specs and measured).
Rear dropouts are 135mm.

I want to know which gears are best for straight chain.

So my thoughts on measuring:
* Divide dropout width by half and I will get center line: 135mm: 2 = 67.5mm. So center line is 67.5 mm inwards from dropout.
* As chainline is 52mm (aka 52mm outwards from center line), I substract 52mm from 67.5mm and I get that straightest chain is the one 15.5mm (67.5mm -52mm) inwards from the dropout. Measuring now 15.5mm from dropout I see that it is roughly 9th gear.

Why on 12 speed casette the straightest chain line is 9th gear? Why so far from the middle (6-7 gear)?

This also means that 12th gear is a bit cross chained, while 1st gear (slowest) is super cross chained?

Am I measuring incorrectly ?

You're math assumes the hub is dead centered with the cranks/ motor, which it may or may not be.

That said, yah ime, in that range of the upper gears is approximately where the chain operates the straightest. The good news (or bad news depending on your perspective) is that the first 3 gears are mostly useless anyways on a full power e-bike and therefore the cross chain binding isn't much of an issue in practical terms.

Also worth noting is that modern chains have a lot more friction going around tight bends, than they do being at a severe angle. The measurements on this are clear which is why all pro XC racers run heavier 12 speed transmissions and a big front chain ring so that they can have the chain further towards the dinner plates and have the chain both front and back losing less friction to having to articulate around tighter radiuses.

 

[steve_sordy]

Why not just do it by eye?
One gear either way from the exact centre will be easily noticeable.
But if I had to choose between two gears that were equally away from perfect, I would go for the bigger one as the chain usually has to transmit more torque in bigger gears.

 

[pagheca]

I may be about to say something completely wrong, but since the diameters of the different sprockets are not the same, the chain must be aligned closer to the larger-diameter sprocket than to the smaller one. The geometry of the problem is illustrated in the sketch below:

it must be clearly h1 < h2

The last formula gives you the exact figures based on the geometry of your e-mtb.

 

[steve_sordy]

I may be about to say something completely wrong, but since the diameters of the different sprockets are not the same, the chain must be aligned closer to the larger-diameter sprocket than to the smaller one. The geometry of the problem is illustrated in the sketch below:

View attachment 186740
it must be clearly h1 < h2

The last formula gives you the exact figures based on the geometry of your e-mtb.

That is what I said, only in fewer words!

 

[JGH]

You're math assumes the hub is dead centered with the cranks/ motor, which it may or may not be.

But majority of frames should be dead centered (maybe with very slight error), as otherwise frame would be crooked, misaligned, etc.

What you said about modern chain bends vs chain going tight angles is so far the only explanation.

 

[JGH]

I may be about to say something completely wrong, but since the diameters of the different sprockets are not the same, the chain must be aligned closer to the larger-diameter sprocket than to the smaller one.

My intuition would say same - on larger sprocket not engaged chain is shorter than on smaller sprocket, so bent angle is larger on larger sprocket.

But my measurements with calipers and not super accurate "by eye" measurement show the opposite.

Most reasonable sounds what @Suns_PSD
said:
* On powerful eBikes low gears does not have much use
* Chain bend is more forgiving on modern chains.

Also maybe I should have said initially - the mentioned ebike is not eMTB bike, even if it has eMTB motor. Maybe it is "tuned" for faster speeds than steep climbing (~0.75, ~0.85 ratio gears) and these climbing gears are more for show.

 

[JGH]

Also a quick observation on my other bikes:
* City bike looks to have straightest chain towards higher gears
* Old school 3 x 9 gravel bike also looks to have straightest chain using middle front chainring towards higher gears too.

This is opposite what AI LLMs suggested (straightest on mid casette gear).

Maybe MTBs are different.

 

[bmwpowere36m3]

I have 12 speed ebike with Shimano drivetrain.
Chainline is 52mm (according to specs and measured).
Rear dropouts are 135mm.

I want to know which gears are best for straight chain.

So my thoughts on measuring:
* Divide dropout width by half and I will get center line: 135mm: 2 = 67.5mm. So center line is 67.5 mm inwards from dropout.
* As chainline is 52mm (aka 52mm outwards from center line), I substract 52mm from 67.5mm and I get that straightest chain is the one 15.5mm (67.5mm -52mm) inwards from the dropout. Measuring now 15.5mm from dropout I see that it is roughly 9th gear.

Why on 12 speed casette the straightest chain line is 9th gear? Why so far from the middle (6-7 gear)?

This also means that 12th gear is a bit cross chained, while 1st gear (slowest) is super cross chained?

Am I measuring incorrectly ?

52 mm CL is typical for modern 142/148 mm spacing, whereas 135 is older QR spacing before 12 spd. So yea, its not ideal CL...

 

[pagheca]

My intuition would say same - on larger sprocket not engaged chain is shorter than on smaller sprocket, so bent angle is larger on larger sprocket.

But my measurements with calipers and not super accurate "by eye" measurement show the opposite.

Most reasonable sounds what @Suns_PSD
said:
* On powerful eBikes low gears does not have much use
* Chain bend is more forgiving on modern chains.

Also maybe I should have said initially - the mentioned ebike is not eMTB bike, even if it has eMTB motor. Maybe it is "tuned" for faster speeds than steep climbing (~0.75, ~0.85 ratio gears) and these climbing gears are more for show.

to be honest, although mathematically correct, I am not convinced either that this is the issue. My point is that the distance between the front derailleur is much larger than the difference between the radius of the two sprockets (r2-r1). Therefore, h1/h2 ~ 1.

 

[Welly]

Most EMTBs I have worked on have a 155 driveline. The reason for this, is to lessen wear on the smaller sprockets. Most chain slip issues I see are caused by excessive wear on the 3 smallest gears. Here in the UK you can buy just the bottom three SRAM gears.

 

[irie]

Most EMTBs I have worked on have a 155 driveline. The reason for this, is to lessen wear on the smaller sprockets. Most chain slip issues I see are caused by excessive wear on the 3 smallest gears. Here in the UK you can buy just the bottom three SRAM gears.

Same with Shimano Y1X498020 (10t and 12t) I think?

 

[bmwpowere36m3]

Most EMTBs I have worked on have a 155 driveline. The reason for this, is to lessen wear on the smaller sprockets. Most chain slip issues I see are caused by excessive wear on the 3 smallest gears. Here in the UK you can buy just the bottom three SRAM gears.

What is a 155 driveline?

Do you mean 157 super boost hub spacing? Because predominately today MTBs are 148 (boost) and some are 157 (superboost) hub spacing. Regarding CL, 52/55 is the norm with boost and 57 with super boost.

 

[Welly]

What is a 155 driveline?

Do you mean 157 super boost hub spacing? Because predominately today MTBs are 148 (boost) and some are 157 (superboost) hub spacing. Regarding CL, 52/55 is the norm with boost and 57 with super boost.

Sorry typo I meant 55 CL not 155.