[Revol]

I've tried the actual rotor on the hub and it clears the swingarm ok but I emailed Cannondale Europe to ask advice and they said it wont fit.Is this because of the extra stress on the caliper mounting on the swing arm and not because of clearance isues with the rotor (as I know it fits) ???
Has/does any one run an 8" rotor on their Jekyll?
Anybody have any thoughts on this?

Cheers.
Revol

 

[Jayridesacove]

Even if the rotor/caliper fits, I don't think the stays were designed to take the stress of a larger rotor.

 

[ThePriceSeliger]

Why?(5 letters)

 

[cannondalejunky]

are you running lefty or headshock

edit: i need to read, your wantin it on the back

 

[BMXman]

If you put an 8" rotor on the back you run the risk of damaging the frame. The Jekyll was not designed for the loads that an 8" rotor produces. It's an xc/trail frame...not a dh/freeride frame. You either need to upgrade your frame or stick to a 6" rotor. If you still try and use the 8" rotor you are putting yourself and your frame at risk.....D

 

[Grizzle]

That does not make much sense to me, the only thing that changes when you put a larger rotor on a bike is you are applying the brake force further from the fulcrum, or the hub, which gives you more mechanical advantage. Putting an 8 inch rotor on should not affect the total amount of force generated by braking, it will still take the same amount of energy to lock up your back tire, which is the traction threshhold. Also, the force is not applied in a different place on the swingarm, so if the total force is equal with either rotor, why would it damage the swingarm? I don't have a jekyll in front of me right now, but im guessing it might be a clearance issue with the swingarm and the rotor, maybe it fits but its tighter than cannondale wants it to be?

 

[sanjuro]

That does not make much sense to me, the only thing that changes when you put a larger rotor on a bike is you are applying the brake force further from the fulcrum, or the hub, which gives you more mechanical advantage. Putting an 8 inch rotor on should not affect the total amount of force generated by braking, it will still take the same amount of energy to lock up your back tire, which is the traction threshhold. Also, the force is not applied in a different place on the swingarm, so if the total force is equal with either rotor, why would it damage the swingarm? I don't have a jekyll in front of me right now, but im guessing it might be a clearance issue with the swingarm and the rotor, maybe it fits but its tighter than cannondale wants it to be?

Well, I am not an engineer, but wouldn't installing the adaptor for an 8 inch rotor lengthen the lever which is flexing the frame? The brake placement would be farther away from the frame to accomodate a larger rotor, making the adaptor a bigger lever.

I also know no manufacturer recommends putting 8 inch rotors on XC bikes, probably for the reason I listed.

 

[Grizzle]

Well, I am not an engineer, but wouldn't installing the adaptor for an 8 inch rotor lengthen the lever which is flexing the frame? The brake placement would be farther away from the frame to accomodate a larger rotor, making the adaptor a bigger lever.

I also know no manufacturer recommends putting 8 inch rotors on XC bikes, probably for the reason I listed.

You are absolutely right about the larger rotor having more leverage. However, your tire will still take the same amount of force before it skids. Also, when the load is applied in this manner, (effort - resistance - fulcrum) the closer your resistance (brake caliper) is to the fulcrum (the hub axle), the more force you generate. The reason a larger rotor works better is because under braking, is because the further out from the center you go, the faster the rotor turns. Meaning that you have the potential to dissapate more heat energy. To illustrate my point, take two spokes. place one spoke as close to the center of the hub as you can, going across the rear swingarm. Turn the wheel by hand, and see how much force it takes to bend the spoke a little. Now take the other spoke, and place it as close to the rim as you can, and across the rear swingarm. When you turn the wheel, this spoke will take much more effort to bend because of how the lever is working. The brakes are the same way. In conclusion, it is impossible to generate more pressure with an 8 inch rotor than a 6 inch rotor if all other things are equal in the circumstances.

 

[cannondalejunky]

well i'm lookin at my jekyll right now and i mesured the clearance and unless they changed the swing arm since 2001 which is what i have, it would bairly clear it, if at all.

 

[sanjuro]

You are absolutely right about the larger rotor having more leverage. However, your tire will still take the same amount of force before it skids. Also, when the load is applied in this manner, (effort - resistance - fulcrum) the closer your resistance (brake caliper) is to the fulcrum (the hub axle), the more force you generate. The reason a larger rotor works better is because under braking, is because the further out from the center you go, the faster the rotor turns. Meaning that you have the potential to dissapate more heat energy. To illustrate my point, take two spokes. place one spoke as close to the center of the hub as you can, going across the rear swingarm. Turn the wheel by hand, and see how much force it takes to bend the spoke a little. Now take the other spoke, and place it as close to the rim as you can, and across the rear swingarm. When you turn the wheel, this spoke will take much more effort to bend because of how the lever is working. The brakes are the same way. In conclusion, it is impossible to generate more pressure with an 8 inch rotor than a 6 inch rotor if all other things are equal in the circumstances.

I think you missed the point. You said you didn't believe an 8 inch rotor would affect frame wear. I said a brake adaptor for an 8 inch rotor would be a bigger lever acting the frame mount.

 

[Grizzle]

I think you missed the point. You said you didn't believe an 8 inch rotor would affect frame wear. I said a brake adaptor for an 8 inch rotor would be a bigger lever acting the frame mount.

I understand your point in that it would be a bigger lever acting on the frame mount. My point was that although the lever was bigger, it could not exert any more force on the frame mount than a 6 inch rotor could because the tire will skid. In fact, an 8 inch rotor and caliper setup would place LESS load on the frame mount points, because it is further from the hub. Sorry if the previous post was confusing and technical.

 

[sanjuro]

I understand your point in that it would be a bigger lever acting on the frame mount. My point was that although the lever was bigger, it could not exert any more force on the frame mount than a 6 inch rotor could because the tire will skid. In fact, an 8 inch rotor and caliper setup would place LESS load on the frame mount points, because it is further from the hub. Sorry if the previous post was confusing and technical.

Yes I am Asian, I don't understand physics or engineering concepts. I clearly understand the concept but I don't think you do.

Keeping in mind I only have every bike manufacturer in the world on my side about 8 inch rotors and XC frames, I think you forgot one simple concept with all your technobabble: Why is this kid putting on an 8 inch rotor? So he can get more modulation on the bike path? Or go down even more steep descents at high speeds, which would put more stress on the frame.

People who use 8 inch rotors require more braking power, which is why DH brake mounts are overbuilt.

 

[Revol]

Thanks for all the input on the question.I'm thinking about an 8" rotor on the back as I've bought a matching pair off Ebay.Front will fit no problem (Lefty fork) and the rotor itself will clear the swingarm but was wondering about the calliper mount.I'm tempted but dont want to damage the swingarm.
Probably won't bother just in case.
Many thanks for all the replys though.

 

[Grizzle]

Yes I am Asian, I don't understand physics or engineering concepts. I clearly understand the concept but I don't think you do.

Keeping in mind I only have every bike manufacturer in the world on my side about 8 inch rotors and XC frames, I think you forgot one simple concept with all your technobabble: Why is this kid putting on an 8 inch rotor? So he can get more modulation on the bike path? Or go down even more steep descents at high speeds, which would put more stress on the frame.

People who use 8 inch rotors require more braking power, which is why DH brake mounts are overbuilt.

First of all, im not attacking you in any way, just trying to prove a point. I have never seen a bike manufacturer say no, you cannot put an 8 inch rotor on the rear. I have only seen them say no 8 inch rotor on the front, because the type of lever changes and can flip the axle out of the QR slots. If you read my posts carefully, you would see they are not technobabble, but they are constructed with proving a point in mind, and that i proved it quite well with physics. Personally i run an 8 inch rotor on an XC/AM bike just so my fingers dont get as tired on long decents and i have to pull less on the lever. It does not matter if he is going 100 MPH down a 30 degree incline, the rear tire will still skid at the same point as far as traction goes, making the total energy transfered into the frame LESS than a 6 inch rotor. And DH brake mounts on a rear swingarm are identical to XC brake mounts on a rear swingarm, unless they are not IS rear. How many times am i gonna have to point out that when your effort is load is applied further from the fulcrum, the less leverage it will have on your swingarm? If you are so confident, go call up one of your manufacturers and ask them. I called one of the guys at cannondale and asked him, he said it was not leverage, but the clearance they were concerned with.

 

[sanjuro]

First of all, im not attacking you in any way, just trying to prove a point. I have never seen a bike manufacturer say no, you cannot put an 8 inch rotor on the rear. I have only seen them say no 8 inch rotor on the front, because the type of lever changes and can flip the axle out of the QR slots. If you read my posts carefully, you would see they are not technobabble, but they are constructed with proving a point in mind, and that i proved it quite well with physics. Personally i run an 8 inch rotor on an XC/AM bike just so my fingers dont get as tired on long decents and i have to pull less on the lever. It does not matter if he is going 100 MPH down a 30 degree incline, the rear tire will still skid at the same point as far as traction goes, making the total energy transfered into the frame LESS than a 6 inch rotor. And DH brake mounts on a rear swingarm are identical to XC brake mounts on a rear swingarm, unless they are not IS rear. How many times am i gonna have to point out that when your effort is load is applied further from the fulcrum, the less leverage it will have on your swingarm? If you are so confident, go call up one of your manufacturers and ask them. I called one of the guys at cannondale and asked him, he said it was not leverage, but the clearance they were concerned with.

Hmm. I never thought about it, but I am checking with my engineer friend about the physics.

I was thinking about true XC frames and could they hand the stress. Also the Specialized Enduro had a recall a few years ago when the 8 inch rotors they had on front broke the fork.

Anyway, good advice.

 

[Palilla]

Grizzle,

I think you are confusing yourself with physics. You are leaving out the most basic principles. All braking systems are based off of one simple equation: F=ma.

Putting an 8 inch rotor on should not affect the total amount of force generated by braking, it will still take the same amount of energy to lock up your back tire, which is the traction threshhold. Also, the force is not applied in a different place on the swingarm, so if the total force is equal with either rotor, why would it damage the swingarm? I don't have a jekyll in front of me right now, but im guessing it might be a clearance issue with the swingarm and the rotor, maybe it fits but its tighter than cannondale wants it to be?

Yes, putting an 8inch rotor will effect the total amount of force generated to your brake mounts, which you proved later on in your posts. Remember the "a" in F=ma, faster acceleration at the caliper in comparison with a 6 inch. True, force is not applied on a different place on the swingarm, but a longer radius (from the mount point to the caliper) produces a larger moment of force at the mounting bracket. The moment of force is a quantity that represents the magnitude of force applied to a rotational system at a distance from the axis of rotation. The concept of the moment arm, this characteristic distance, is key to the operation of the lever, pulley, gear, and most other simple machines capable of generating mechanical advantage.

So yes, it could damage the swingarm at the mounting bracket.

However, your tire will still take the same amount of force before it skids. Also, when the load is applied in this manner, (effort - resistance - fulcrum) the closer your resistance (brake caliper) is to the fulcrum (the hub axle), the more force you generate. The reason a larger rotor works better is because under braking, is because the further out from the center you go, the faster the rotor turns. To illustrate my point, take two spokes. place one spoke as close to the center of the hub as you can, going across the rear swingarm. Turn the wheel by hand, and see how much force it takes to bend the spoke a little. Now take the other spoke, and place it as close to the rim as you can, and across the rear swingarm. When you turn the wheel, this spoke will take much more effort to bend because of how the lever is working. The brakes are the same way. In conclusion, it is impossible to generate more pressure with an 8 inch rotor than a 6 inch rotor if all other things are equal in the circumstances.

Now your example here is a good start, but you are completely missing the point. So your hand is on the tire, and spoke is near the hub right, that's about 12 inches or so, then move the spoke by the rim and you shorten your length to a couple of inches. The spoke represnts the disc brake mount, your hand the force of the the spinning rotor exerts back to the frame. Which way was easier to bend? By the hub, your distance was longer, producing more force, same would be true with a longer distance from the caliper to the mounting tab.

I understand your point in that it would be a bigger lever acting on the frame mount. My point was that although the lever was bigger, it could not exert any more force on the frame mount than a 6 inch rotor could because the tire will skid. In fact, an 8 inch rotor and caliper setup would place LESS load on the frame mount points, because it is further from the hub. Sorry if the previous post was confusing and technical.

Wrong, back to distance again. A longer distance, more force on the mount. "a" for accerlartion is greater with a larger rotor, in return the force is higher. Even if the tire is locked, and the rider is sliding down a hill, the caliper is putting more stress on the disc brake mount with the 1 inch greater radius.

It does not matter if he is going 100 MPH down a 30 degree incline, the rear tire will still skid at the same point as far as traction goes, making the total energy transfered into the frame LESS than a 6 inch rotor. And DH brake mounts on a rear swingarm are identical to XC brake mounts on a rear swingarm, unless they are not IS rear. How many times am i gonna have to point out that when your effort is load is applied further from the fulcrum, the less leverage it will have on your swingarm? If you are so confident, go call up one of your manufacturers and ask them. I called one of the guys at cannondale and asked him, he said it was not leverage, but the clearance they were concerned with.

I can't believe you actually believe this. It does matter if he's going 100mph and on a 30 degree incline. Back to F=ma. The force it takes to even begin to slow the wheel down is greatly increased, especially at steeper angles...remember gravity :devil:

The farther the force applied to the rotor is from the brake mounts, the larger the moment will be at the mount, so technically this could cause a failure. Now ISO brake mounts can be different on different frames, beefed up ones are put on DH frames all the time. They are standardized in the fact that the mounting holes and position are the same, not the material, welds, etc.

 

[sanjuro]

Thanks for your response. I had also considered Cannondale's advice. They might think the Jekyll rear triangle is strong enough to handle an 8 inch rotor, if there was clearance.