Disc brake question for the brianiacs

[robt57]

Originally Posted by oldpotatoe

Yup, small point. Those pictures I put up was from a list of '10 best small displacement motorcycles'....Lots of smaller motorcycles with a single disc front
and I even think I saw a 'modern' one with a single disc front and drum rear.

Enter the Buell lightning XB9S I had from 2006-9. 1000cc with a single rotor front brake. It had such a strong brake I would not let anyone without a lot of experience on sport bikes ride it. It should have had ABS, and I'd have let more folk take it for a spin if it had. I did more accidental nose wheelies on that thing!

In 80s I had a Eddie Lawson replica I spent a lot of money for big rotors and calipers, stainless lines, sintered pads, yada. The Buell single outboard rotor/brake was superior in every way. I can't say of racing and heat dissipation would better than the 2 13" Brembos on the EL/Kaw, but I did not race so moot for my use.


As far as MC calipers fore/aft on lower, there was probably some logic to cooling effect with them out front.

[oldpotatoe]

Originally Posted by robt57

Enter the Buell lightning XB9S I had from 2006-9. 1000cc with a single rotor front brake. It had such a strong brake I would not let anyone without a lot of experience on sport bikes ride it. It should have had ABS, and I'd have let more folk take it for a spin if it had. I did more accidental nose wheelies on that thing!

In 80s I had a Eddie Lawson replica I spent a lot of money for big rotors and calipers, stainless lines, sintered pads, yada. The Buell single outboard rotor/brake was superior in every way. I can't say of racing and heat dissipation would better than the 2 13" Brembos on the EL/Kaw, but I did not race so moot for my use.


As far as MC calipers fore/aft on lower, there was probably some logic to cooling effect with them out front.

Yowser..

[robt57]

My buell was silver, like the yellow better. ;}

The ELR came with gold disco wheels, soon had matte black DyMags.

[MikeD]

Originally Posted by Dude

Thru-axles came around for fork (and rear triangle) stiffness and strength.

I'd like to see that proved by calculation/test. A thin aluminum tube vs. a solid steel rod, clamping force provides by a low torque handle vs. an overcenter cam, smooth aluminum axle ends vs. serrated steel nuts.

[benb]

The Buell design never took off cause of issues at the racetrack apparently.

It moved the heat too close to the tire and affected tire pressures and also supposedly had warping issues due the larger diameter of the rotor being harder to keep from warping.

It would never have been an issue on the street but no one else tried it because of the heat issues.

Brembo had apparently tried the design a long time ago and rejected it.

Erik Buell was largely about showing off something unique & different. He had no choice cause he was always handicapped by the engine he had to use.

No one else decided to copy his design of putting the oil in the swingarm either. Cool feature but it added unsprung weight. Most everyone else dropped single sided swingarms for the same reason. Super cool but eventually proven to be inferior.

[batman1425]

Originally Posted by MikeD

I'd like to see that proved by calculation/test. A thin aluminum tube vs. a solid steel rod, clamping force provides by a low torque handle vs. an overcenter cam, smooth aluminum axle ends vs. serrated steel nuts.

Original TA were developed for DH MTB applications. Riders would often snap front axles, because of relatively weak QR connection combined with asynchronous compression on the left and right fork legs on large hits. The strength of this interface is completely dependent on the compression forces of the cam and friction with the dropout interfaces. A DH suspension fork is asking quite a bit of that design.

TA physically links the two ends of the fork together in shear. Compression from the bolt isn't needed beyond what is needed keep it from falling out. This physically links both ends together in shear which is a stronger connection than the compression/friction forces of QR. The result in DH applications was that with a more rigid connection, compressive forces were transmitted more equally L vs R, and riders stopped snapping axles. As a bi-product, riders also noticed greater steering precision and braking performance due to the overall reduced flex in the system.

Some component of these effects will translate to a rigid (non suspension) forks but the benefit from that stiffness is almost certainly much smaller, if anything, given the application. IMO - the value of TA on non suspension applications isn't for the stiffness, but rather ease of caliper alignment after wheel changes, and for the added safety against a spontaneous ejection under braking.

[Mark McM]

Originally Posted by MikeD

I'd like to see that proved by calculation/test. A thin aluminum tube vs. a solid steel rod, clamping force provides by a low torque handle vs. an overcenter cam, smooth aluminum axle ends vs. serrated steel nuts.

He's talking about MTBs, which often had telescopic suspension forks. For forks with lower sliders, the sliders are typically linked with a brace arch. But these brace arches could be flexy, allowing some independent movement of the sliders. A thru axle provides a more rigid connection than a QR axle, helping to reduce independent movement.. Inverted forks (lower legs are the internal telescoping tube) can't use a bracing arch, and the lower legs are only connected together by the axle. These forks need a very rigid axle, and QR axle is completley unsuitable.

Rear swing arm often don't as much bracing as a traditional rear triangles, and they too can benefit from the extra rigidity of a thru axle vs. a QR axle.

[MikeD]

Originally Posted by Mark McM

He's talking about MTBs, which often had telescopic suspension forks. For forks with lower sliders, the sliders are typically linked with a brace arch. But these brace arches could be flexy, allowing some independent movement of the sliders. A thru axle provides a more rigid connection than a QR axle, helping to reduce independent movement.. Inverted forks (lower legs are the internal telescoping tube) can't use a bracing arch, and the lower legs are only connected together by the axle. These forks need a very rigid axle, and QR axle is completley unsuitable.

Rear swing arm often don't as much bracing as a traditional rear triangles, and they too can benefit from the extra rigidity of a thru axle vs. a QR axle.

This video provides a different point of view. It says that thru axles aren't axles (they are really bolts), they are not in the load path, and unless tightened up with a long Allen key to at least 12 NM, they don't provide near the same clamping force as a quick release. https://www.youtube.com/watch?v=KMdsSuXGniU

[JedB]

Originally Posted by benb

The Buell design never took off cause of issues at the racetrack apparently.

For the first generation of Buell's this is correct. The overall brake wasn't good compared to the Nissin/Brembos that were OEM on the bikes Buell were competing against (Large Twins = Ducati or Honda RC51 or Aprilia). 6-piston vs. 4 piston as well. I raced an Aprilia Mille (Big twin) and the Buell's were in my category.

It moved the heat too close to the tire and affected tire pressures and also supposedly had warping issues due the larger diameter of the rotor being harder to keep from warping.

In AMA, the warping was documented on the steel brakes. To my knowledge they never had carbon discs for the Buell. There were multiple retirements in AMA competition due to this.

It would never have been an issue on the street but no one else tried it because of the heat issues.

Spot on. There's not enough heat generated in street riding.

Erik Buell was largely about showing off something unique & different. He had no choice cause he was always handicapped by the engine he had to use.

That depends. The first generation race bike Buell (1125x) had the same power plant as the Aprilia (Rotax/Bombardier 990 twin). Aprilia lit up World Superbike on that motor. The general flaws with Buell's designs were around the 1st gen rotor & the warping. Later, it became a challenge to his ego, so he didn't change mostly because he was obstinate.

I would agree that the Buells, under Harley, were sh1tboxes stem to stern if you wanted to go racing.

No one else decided to copy his design of putting the oil in the swingarm either. Cool feature but it added unsprung weight. Most everyone else dropped single sided swingarms for the same reason. Super cool but eventually proven to be inferior.

100%

The third generation of Buell (Technically EBR, post Harley divorce) were actually pretty technically sound and race competitive in Road Racing, not track stuff. My buddy Brandon Cretu raced one 2 years in a row at the Macau Grand Prix. His feedback on this 3rd gen setup, including brakes, was that they were on par with the Yamaha R6 he raced at the Isle of Man. The large single disc had been improved upon, enough for him to race Macau. My amatuer opinion, I would still take the consumer level machined Brembo or Nissin, over the Buell brakes, even on the 3rd generation.

[Mark McM]

Originally Posted by MikeD

This video provides a different point of view. It says that thru axles aren't axles (they are really bolts), they are not in the load path, and unless tightened up with a long Allen key to at least 12 NM, they don't provide near the same clamping force as a quick release. https://www.youtube.com/watch?v=KMdsSuXGniU

The video doesn't really add much the conversation, because it did not consider telescopic suspension forks, and how the stiffness of the axle contributes to limiting independent suspension leg motion. The thru-axle, being larger in diameter than a QR skewer, is inherently stiffer than QR skewer. And the fact that the contact area of the thru-axle wheel/fork joint is larger also contributes to the stiffness of the joint. As long as the thru-axle clamping force is sufficient to prevent lift-off of the wheel/fork contact surfaces, more clamping force won't make the joint stiffer.

[MikeD]

Originally Posted by Mark McM

The video doesn't really add much the conversation, because it did not consider telescopic suspension forks, and how the stiffness of the axle contributes to limiting independent suspension leg motion. The thru-axle, being larger in diameter than a QR skewer, is inherently stiffer than QR skewer. And the fact that the contact area of the thru-axle wheel/fork joint is larger also contributes to the stiffness of the joint. As long as the thru-axle clamping force is sufficient to prevent lift-off of the wheel/fork contact surfaces, more clamping force won't make the joint stiffer.

I believe the video and it's the larger diameter axle of the thru axle hub that provides more stiffness, not the thru axle itself.

[Mark McM]

Originally Posted by MikeD

I believe the video and it's the larger diameter axle of the thru axle hub that provides more stiffness, not the thru axle itself.

That's kind of a distinction without difference. It is the system stiffness that matters, and the entire thru-axle system is designed around a larger diameter axle and bolt (thru-axle) than the QR system is, so the thru-axle system is inherently stiffer.

But I'm not sure the action of the QR or thru-axle works in providing stiffness is fully appreciated. It is not the bending stiffness of the QR/thru-axle that matters, it is the axial stiffness that matters. When the wheel experiences a high lateral (as is common in rough MTB conditions), the reaction is that the wheel tries to "cock", or tilt sideways in the fork, pushing upward on fork leg and downward on the other fork leg. On a fork with rigid legs, the legs resist these forces and the wheel stays straight. But on a suspension fork with telescopic legs, the "cocking" action can cause one leg to compress and the other leg to extend, allowing the wheel to tilt. This is bad for wheel tracking and bike handling, so we want to prevent the wheel from tilting due to independent fork leg travel.

The bending stiffness of the axle can help to keep the legs from extending/compressing independently. But the axle can only prevent the "cocking" action if the axle faces remain firmly in contact with the fork dropouts. The wheel radius creates a large moment arm so there can be a very large bending moment at the axle/fork contact. A typical QR axle has a relatively small axle/fork contact area. Even though you can tighten a QR to a very high force, the cocking moment can cause one side of the axle end to lift off as the wheel cocks. When this happens, the stiffness of the joint is from the stiffness of the QR as is stretches as the axle end lifts off. This is why titanium skewers were typically not recommended for suspension forks - they could be tightened just as high as steel skewer, but they had only half the stiffness, and would stretch more, and thus allow more cocking.

Several approaches were used to address the stiffness issue of the axle/dropout joint. The simplest was to use a bolt-on axle instead of QR axle. This stiffened the connection between the axle and dropout, preventing lift-off, and allowing the full axle stiffness to resist independent fork movement. Another approach was to use a larger diameter locknut on the axle end, increasing the axle/dropout contact area, which increased the force required for lift-off, and magnified the influence of QR stiffness after lift-off occurred.

Finally, in a pre-cursor to the thru-axle, some hub/wheels makers replaced the axle and QR skewer with what was called a "skraxle". This was basically a QR skewer that was the same diameter as an axle. Here's an example of hubs with skraxles:



Skraxle systems were much stiffer, and prevented independent leg movement better than a traditional QR skewer. As the video pointed out, for the same skewer torque, a skraxle didn't provide as much clamping force as a traditional small diameter skewer. But because the skraxle had much larger axial stiffness, it virtually eliminated axle end lift off, provided a much stiffer joint.

[MikeD]

Originally Posted by Mark McM

That's kind of a distinction without difference. It is the system stiffness that matters, and the entire thru-axle system is designed around a larger diameter axle and bolt (thru-axle) than the QR system is, so the thru-axle system is inherently stiffer.

But I'm not sure the action of the QR or thru-axle works in providing stiffness is fully appreciated. It is not the bending stiffness of the QR/thru-axle that matters, it is the axial stiffness that matters. When the wheel experiences a high lateral (as is common in rough MTB conditions), the reaction is that the wheel tries to "cock", or tilt sideways in the fork, pushing upward on fork leg and downward on the other fork leg. On a fork with rigid legs, the legs resist these forces and the wheel stays straight. But on a suspension fork with telescopic legs, the "cocking" action can cause one leg to compress and the other leg to extend, allowing the wheel to tilt. This is bad for wheel tracking and bike handling, so we want to prevent the wheel from tilting due to independent fork leg travel.

The bending stiffness of the axle can help to keep the legs from extending/compressing independently. But the axle can only prevent the "cocking" action if the axle faces remain firmly in contact with the fork dropouts. The wheel radius creates a large moment arm so there can be a very large bending moment at the axle/fork contact. A typical QR axle has a relatively small axle/fork contact area. Even though you can tighten a QR to a very high force, the cocking moment can cause one side of the axle end to lift off as the wheel cocks. When this happens, the stiffness of the joint is from the stiffness of the QR as is stretches as the axle end lifts off. This is why titanium skewers were typically not recommended for suspension forks - they could be tightened just as high as steel skewer, but they had only half the stiffness, and would stretch more, and thus allow more cocking.

Several approaches were used to address the stiffness issue of the axle/dropout joint. The simplest was to use a bolt-on axle instead of QR axle. This stiffened the connection between the axle and dropout, preventing lift-off, and allowing the full axle stiffness to resist independent fork movement. Another approach was to use a larger diameter locknut on the axle end, increasing the axle/dropout contact area, which increased the force required for lift-off, and magnified the influence of QR stiffness after lift-off occurred.

Finally, in a pre-cursor to the thru-axle, some hub/wheels makers replaced the axle and QR skewer with what was called a "skraxle". This was basically a QR skewer that was the same diameter as an axle. Here's an example of hubs with skraxles:



Skraxle systems were much stiffer, and prevented independent leg movement better than a traditional QR skewer. As the video pointed out, for the same skewer torque, a skraxle didn't provide as much clamping force as a traditional small diameter skewer. But because the skraxle had much larger axial stiffness, it virtually eliminated axle end lift off, provided a much stiffer joint.

Your saying that an aluminum tubular thru axle has more axial stiffness than a steel rod skewer? Where's your calculations?

We'll have to agree to disagree on this topic, unless you can show me calculations or tests. This independent leg action thing I think is grossly exaggerated. I've got a Rock Shox Reba fork with a skewer mounted wheel and have never detected any independent leg movement or evidence that it's occurring. However, I know that wheels are not stiff laterally.

[Mark McM]

Originally Posted by MikeD

Your saying that an aluminum tubular thru axle has more axial stiffness than a steel rod skewer? Where's your calculations?

We'll have to agree to disagree on this topic, unless you can show me calculations or tests. This independent leg action thing I think is grossly exaggerated. I've got a Rock Shox Reba fork with a skewer mounted wheel and have never detected any independent leg movement or evidence that it's occurring. However, I know that wheels are not stiff laterally.

Well, there's no reason you can't do the calculation.

As mentioned, just the larger diameter contact area of thru-axle hubs is enough to increase "cocking" stiffness, but since you asked, let's look at the axial stiffnesses of an aluminum thru-axle vs. a steel QR skewer. I measured some of my QRs and thru-axles, and also measured drop-out widths ona a thru-axle fork and a QR suspension fork, and used these values below.

The axial stiffness of a column or shaft is: K = E x A / L

where: K is the stiffness, E is the elastic modulus of the material, A is the cross sectional area, and L is the length.

Chrome-Moly steel has an elastic modulus of about 200 GPa, and 7075 aluminum has an eastic modulus of about 70 GPA.

The cross sectional area of a tube is: A = pi x [ D^2 - d^2 ] / 4
where A is the area, D is the outer diameter, and d is the inner diameter (for a solid shaft, d = 0).

My steel QR skewer has D = 5mm and d = 0mm, so A = pi x( 5mm)^2 / 4 = 19.6 mm^2. My aluminum thru-axle has a D 12mm and d - 6mm, so A = pi [ (12mm)^2 - (6mm)^2 ) / 4 = 84.8 mm^2.

Both forks use 100mm spacing between dropouts. My QR dropouts are 7mm thick, so the free length of the QR skewer is 100mm + 2 x 7mm = 114mm. On the thru-axle for, the thickness of the dropout where the head of the thru-axle sits is 6mm, and the thru-axle threads directly into the other dropout, so the free length of the thru-axle is 100mm + 6mm = 106mm.

With these numbers, we can calculate the axial stiffnesses of each shaft:

QR: K = (200 GPa) * (19.6 mm^2) / 114mm = 34,400 N/mm
Thru-axle: K = (70 GPa) * ( 84.8 mm^2) / 106mm = 56,000 N/mm

Of course many suspension forks use 15mm thru-axles, which will be quite a bit stiffer than 12mm thru-axles.

[MikeD]

Originally Posted by Mark McM

Well, there's no reason you can't do the calculation.

As mentioned, just the larger diameter contact area of thru-axle hubs is enough to increase "cocking" stiffness, but since you asked, let's look at the axial stiffnesses of an aluminum thru-axle vs. a steel QR skewer. I measured some of my QRs and thru-axles, and also measured drop-out widths ona a thru-axle fork and a QR suspension fork, and used these values below.

The axial stiffness of a column or shaft is: K = E x A / L

where: K is the stiffness, E is the elastic modulus of the material, A is the cross sectional area, and L is the length.

Chrome-Moly steel has an elastic modulus of about 200 GPa, and 7075 aluminum has an eastic modulus of about 70 GPA.

The cross sectional area of a tube is: A = pi x [ D^2 - d^2 ] / 4
where A is the area, D is the outer diameter, and d is the inner diameter (for a solid shaft, d = 0).

My steel QR skewer has D = 5mm and d = 0mm, so A = pi x( 5mm)^2 / 4 = 19.6 mm^2. My aluminum thru-axle has a D 12mm and d - 6mm, so A = pi [ (12mm)^2 - (6mm)^2 ) / 4 = 84.8 mm^2.

Both forks use 100mm spacing between dropouts. My QR dropouts are 7mm thick, so the free length of the QR skewer is 100mm + 2 x 7mm = 114mm. On the thru-axle for, the thickness of the dropout where the head of the thru-axle sits is 6mm, and the thru-axle threads directly into the other dropout, so the free length of the thru-axle is 100mm + 6mm = 106mm.

With these numbers, we can calculate the axial stiffnesses of each shaft:

QR: K = (200 GPa) * (19.6 mm^2) / 114mm = 34,400 N/mm
Thru-axle:K = (70 GPa) * ( 84.8 mm^2) / 106mm = 56,000 N/mm

Of course many suspension forks use 15mm thru-axles, which will be quite a bit stiffer than 12mm thru-axles.

Very good, thanks Mark.