Help with disc thickness? BR-MT200

[Wizzo]

Hi, I have the BR-MT200 Calipers with post mount forks with 160mm disc. I want to upgrade size disc to 203mm but on shimano site says only can go upto 180mm. The problem I have is the discs thickness at moment is 1mm, the discs on ebay are 1.9mm thick and don't know if it will go between the little gap past the pads?

 

[Chalo]

I'm willing to bet your disc rotors aren't 1mm thick, even if they're badly worn. When new, pretty much all disc rotors are between 1.8 and 2.3mm.

Your fork may have a rotor size limit too, and you should find out what it is before you make a problem for yourself.

 

[Comrade]

Your fork may have a rotor size limit too, and you should find out what it is before you make a problem for yourself.

If the stock mounting holes are used, and the wheel diameter remains the same, and the fork is designed to withstand a locked wheel, what difference does the rotor diameter make?

 

[Chalo]

Your fork may have a rotor size limit too, and you should find out what it is before you make a problem for yourself.

If the stock mounting holes are used, and the wheel diameter remains the same, and the fork is designed to withstand a locked wheel, what difference does the rotor diameter make?

The mounting points on the fork remain the same, but the lever acting on them becomes longer when you use a larger rotor. You must think the parts designers who specify these things don't know why they are doing it.

 

[amberwolf]

A thread this problem has come up in before:

https://endless-sphere.com/forums/viewtopic.php?f=3&t=40902

Another example with the same results (except my fork didn't collapse during braking, it just bent over a (max of) couple of weeks' time and I was lucky enough to notice before I couldv'e been killed by it).
https://endless-sphere.com/forums/viewtopic.php?f=3&t=99837#p1466763
before and after pics
https://endless-sphere.com/forums/viewtopic.php?f=2&t=67833&p=1466757#p1466385

 

[Comrade]

The mounting points on the fork remain the same, but the lever acting on them becomes longer when you use a larger rotor.

So if the caliper is moved 20mm straight out with an adapter, which part here experiences higher loading?

 

[Chalo]

The mounting points on the fork remain the same, but the lever acting on them becomes longer when you use a larger rotor.

So if the caliper is moved 20mm straight out with an adapter, which part here experiences higher loading?

disc-brake.JPG

"Bosses".

Considering they're made of cast aluminum or magnesium in most cases, I'll trust the designers if they specify a rotor diameter limit.

On a related note, I have a coworker with whom I was troubleshooting one of his bikes last night. He had (for jollies) put XT hydraulic brakes with 203mm rotors on his former messenger cargo bike, a Soma Wolverine with a Crust Clydesdale cargo fork. That frame has sliding dropouts, and the left rear oval slot had been pried out wider by the disc brake's bending moment, causing the wheel to sit crooked in the frame. He certainly didn't need a brake that big in the rear, and sure enough it caused him unnecessary trouble.

(This isn't his bike; I have included this photo to illustrate the brake and dropout setup.)

 

[Overclocker]

The mounting points on the fork remain the same, but the lever acting on them becomes longer when you use a larger rotor.

So if the caliper is moved 20mm straight out with an adapter, which part here experiences higher loading?

disc-brake.JPG

bosses get higher load but mostly compressive forces (see direction of disc rotation), so unlikely to be limiting factor

so rotor size limits probably have to do w/ strength of: stanchions, crown, bottom of steerer

btw i've got mt200 brakes on my acoustic bike. very light duty brakes. mounting bolts are shorter because the mounting tabs are so thin probably not good for ebikes

 

[Comrade]

That frame has sliding dropouts, and the left rear oval slot had been pried out wider by the disc brake's bending moment

So that's the part I'm confused about. Would the bending moment on those oval slots not stay exactly the same no matter where the brake pads are relative to the axle?

If you bring those oval slots further out, they will experience lower forces. If you bring them in closer, it will be higher. But those slots don't move with larger calipers.

Take a look at the picture again, but think of the forces when the wheel is fully locked up. The rotor could be 10 feet in diameter but it would not increase torque since it's defined by the radius of the tire.

 

[DanGT86]

beware of magura rotors. They are 2mm thick and most other manufacturers have been using 1.8mm rotors.

The braking stress is torque on the mount at the forks or frame. The larger the dia of the rotor the longer the lever so the more torque the frame or fork mounts are experiencing. For a given amount of brake squeeze pressure the torque on the mounts and calipers will increase as the rotor increases.

The above Statement in this color is incorrect. I'm leaving it in so the replies in this thread make sense.



If shimano says you can only run 180s than its because their caliper body cant deal with the higher load and or the radius inside the caliper body is too small to fit 203 without rubbing on the entry and exit of the caliper.

I see how there could be an argument to be made that if the brakes can lock the wheel with a small rotor than increasing the rotor size will just lock the wheel with less brake pressure.

Forks have rotor size limits due to lots of factors. The higher load from a larger rotor will impart more flexing stress on the stanchions. It will stress the fork bushings more by imparting side force on the lowers and stanchions. It also tries to twist the lower leg of the fork axially around the stanchion. Forks usually have a fork arch that connects the two lower legs and fight this twisting force. Larger diameter axles also fight this.

I have a small stanchion diameter marzocchi fork with a 9mm quick release axle from the early 2000s. I put a modern disk brake on it with a 160mm rotor and it twisted the lower so bad the tire rubs under hard braking.

MTB and dirt jump frames often have a brace running diagonally from the chianstay to the seatstay if they are designed for 203mm rotors. People have snapped their frames by the brake mount running rotors that are too large.

Like chalo suggested, the designers know what they are doing and this is why warranties are voided by running larger brakes than the mfg. recommendations.

 

[Chalo]

The rotor could be 10 feet in diameter but it would not increase torque since it's defined by the radius of the tire.

This assumes a bunch of things that aren't accounted for in real world conditions. Yes, for any given wheel diameter, bike + rider weight, wheelbase, static weight distribution, center of mass height, and tire adhesion, there will be the same average bending moment on the caliper mounts.

But average braking forces don't break stuff. Peak braking forces do. So you have to account for transient inertial forces that affect traction (imagine braking hard into a speed bump), onset force curves from the brake itself, and other things that won't come bite you unless you're way overbraked.

I'm reminded of the problem with Shimano Rollerbrakes that caused Jump bikes to be recalled at one point. They have generally predictable and unimpressive braking qualities, but they're grease packed, so a failure of lubrication can cause intense sudden grabbing. When this happened to me with my own bike, the momentary force of the brake seizing overcame my NuVinci hub's sidecover torque retention and ruined the hub. That would never have happened under normal conditions if I simply increased braking until the wheel skidded. If I had used an oversimplified model and assumed without examination that the braking torque could never exceed the skidding torque of the tire, I wouldn't have been able to understand what happened.

 

[DanGT86]

Additionally Not all states of locked rotor are equal. If a wheel is locked and skidding down a muddy trail and then the tire catches on a rock really well the instantaneous torque spike might simply overcome the locked pressure a caliper can squeeze on a 160 rotor. That same scenario on a 203mm rotor is going to be capable of generating way more torque/leverage on the mount before overcoming the pad friction. So larger rotors allow you to attain forces on the mounting points that smaller rotors are not capable of with the same exact caliper and pad material.

Its not like once the wheel is locked the forces drop to zero and stay there.

I ride trials and commonly land on just the back wheel with the brake locked. With 160mm rotors the force overcomes the friction I can achieve. The brakes make a short squeak as they give way and I land on my ass. Same scenario with the 203 and the wheel stays locked. Same friction between pad and rotor but a whole different range of forces than I can achieve with the small rotors before they give way. Both scenarios involve a locked wheel but not a skidding wheel.

 

[Comrade]

locked pressure a caliper can squeeze on a 160 rotor

That's fair. Though a mediocre cable actuated 203mm setup is probably not much stronger than a good hydraulic 160mm setup. So the rotor size guidance probably has a very healthy safety margin.

 

[Hillhater]

.......
The braking stress is torque on the mount at the forks or frame. The larger the dia of the rotor the longer the lever so the more torque the frame or fork mounts are experiencing. For a given amount of brake squeeze pressure the torque on the mounts and calipers will increase as the rotor increases.

Oh dear !.... you seem to be forgetting basic mechanics and Torque reaction...
As someone else mentioned , the torque loading is that generated by the tyre diameter and rate of retardation,..it is independent of disc size
Brake squeeze pressure is pretty much irrelevant . What the rider is concerned about is “retardation” and for any particular retardation effect , the torque load on the disc, caliper, and fork etc, will be the same irrespective of disc diameter.
So, with larger discs the FORCE loading on the caliper will REDUCE because the “torque arm”. ( distance from the axle to the caliper is greater)
But since the original fork mounting points are being used, and the same braking TORQUE is being generated, the those fork lugs see the same FORCE loading with any size disc.

 

[DanGT86]

Oh dear !.... you seem to be forgetting basic mechanics and Torque reaction...
As someone else mentioned , the torque loading is that generated by the tyre diameter and rate of retardation,..it is independent of disc size
Brake squeeze pressure is pretty much irrelevant . What the rider is concerned about is “retardation” and for any particular retardation effect , the torque load on the disc, caliper, and fork etc, will be the same irrespective of disc diameter.
So, with larger discs the FORCE loading on the caliper will REDUCE because the “torque arm”. ( distance from the axle to the caliper is greater)
But since the original fork mounting points are being used, and the same braking TORQUE is being generated, the those fork lugs see the same FORCE loading with any size disc.

I would argue that I'm not forgetting. I never knew it in the first place.

So this is super interesting to me. Let me see if I follow. So for a given amount of retardation the larger radius of the rotor is balanced by the reduced force to do the same job? This seems pretty logical.

But what about the fact that the clamping force of the pads on the rotor is not infinite? Some amount of torque will overpower the pads on the 160mm rotor. If you apply the same amount of force on the brake lever with the 203mm rotor wont it take more torque than the 160mm to spin it?

I'm picturing the pads as a mechanical overload device. If there is a limit to how hard they can grip the rotor then the larger radius you apply them at the more torque you can resist right?

If I can only pull on a wrench with 100lbs of force and I have a 1ft long wrench I can apply 100 ft/lbs. If I put the same 100lbs on a 2ft wrench isnt that 200 ft/lbs?

 

[Hillhater]

.
But what about the fact that the clamping force of the pads on the rotor is not infinite? Some amount of torque will overpower the pads on the 160mm rotor. If you apply the same amount of force on the brake lever with the 203mm rotor wont it take more torque than the 160mm to spin it?

Sure, all other things being the same, the bigger rotor can produce more braking torque.
BUT.. a half decent hydro set up (4 piston), with good pads, on a 160 rotor, will put you over the bars on a hard surface .!

 

[DanGT86]

This makes more sense now. Thanks for the physics lesson.

To recap what I think I have learned:
Basically if I put a torque wrench on the disk flange and then apply torque to the wheel it doesnt matter at what radius I'm holding the wrench. The reading will be the same.

However, if I'm not strong enough to keep my grip on the wrench at 160mm but I am strong enough 203 then I can achieve a higher torque at 203 then I ever could at 160.

So this seems to answer the questions of why manufacturers have rotor size limits. A large rotor can put more force into the fork or frame than a smaller rotor.

 

[Hillhater]

So this seems to answer the questions of why manufacturers have rotor size limits. A large rotor can put more force into the fork or frame than a smaller rotor.

Yes,.. In theory, but the maximum torque the rotor will experience is limited by the tyre grip. So if the 160 rotor can lock up the f wheel on a high grip surface, there is nothing to gain from the bigger rotor, and since you dont want to lock up the front if possible, there is even less justification.
But the bigger rotor may give you more more progressive feel, depending on the system and pads.

 

[Chalo]

But the bigger rotor may give you more more progressive feel, depending on the system and pads.

Also less heat fade, and less likelihood of boiling the hydraulic fluid or melting plastic components of the brake. Not that posers ever do those things.