Disc brake question for the brianiacs

[Mark McM]

Originally Posted by oldpotatoe

OK, not winter yet and I can barely speel 'disc brake' but on bicycles, why isn't the rotor on the right side of the bike on the front?

Wouldn't that mean the calipers could be either front or back?

And more 'even' braking, opposite sides of bike..and wasn't the early disc brake ejecting wheels(maybe that would happen anyway).

Aren't some of the few motorcycles with single rotor fronts on opposite side from the rear?

Just awondering..'Mark??

I think you nailed the reason - the front brake is on the left so you can use the same caliper front and rear. The rear caliper obviously has to be on the left. Calipers also have distinct outer and inner side. The rear caliper is designed so that the cable/housing is routed to enter the top of the caliper. If you were to mount that same caliper on the front fork, then to have the cable/housing also enter the top of the caliper you either have to mount the caliper on the back of the left fork, or the front side of the right fork. Mounting the caliper to the back of the right fork would require flipping the caliper upside down, and the cable/housing would enter from the bottom. I guess it is believed (probably correctly) that the caliper is better protected when mounted on the back of the fork, so they went with mounting to the back of the left fork.

And besides, if the front caliper was on the right, then Cannondale couldn't have invented their single bladed Lefty fork - they would have had to invent a Righty fork instead.

[benb]

The bikes pictured have rotors & calipers on BOTH sides of the front wheel, any serious motorcycle with anything but "vintage correct looks" has had a dual front discs for a very long time.

A lot of engineering has gone into those forks & brakes. The bike industry piggy backs off that.

They'd never mount one caliper forward to save money by being able to use 2 identical calipers.

The whole system is likely engineered for tuned flex as well.. motorcycles have to worry about chatter under braking and managing that has become pretty sophisticated.

The last sportbike I had the front thru axle was bigger than a BB spindle on a bicycle (almost as big as the BB shell) and there were 4 bolts keeping it in that all had torque, prep, and replacement instructions. A gigantic bolt through each end of the thru axle and a pinch bolt on the bottom of each fork leg. I remember being shocked at how huge the bolts through the thru axle was and having to search for a socket large enough and the adapter for it. They were considerably larger (50%?) than what you use for a cassette lock ring or a BB/Crank tool.

Motorcycles also do not have a standard "drive side" like bicycles. The majority of them have the drive chain/belt/shaft on the riders left side and the rear brake on the right side. That makes the most sense partly because the rear brake is actuated by the right foot so it simplifies the location of the rear brake design & location. But some motorcycles have single sided swingarms and then everything is on the left side of the bike and the brake hose for the rear brake goes across the bike and then down the left side of the swing arm.

The brakes being mounted behind the fork also allows the fork to act as a shield to protect the brake system, hoses, etc.. from being hit by debris. It is also likely more aerodynamic, although there is no such thing as an "aero bike" in motorcycles really, racing regulations banned it long long ago because it was too dangerous. Street bikes have pretty much never tried to go there out of a fashion of wanting to imitate race bikes. It's kind of dumb really as doing so would result in lots of bikes likely getting massive fuel efficiency gains on the highway, but there would be tons of bikes having to have their speed restricted too as many more of them would become 200mph+ capable.

[dustyrider]

I think e-bikes will push disc brakes into the dual front rotors like in the moto world. We’re already seeing larger front rotors than then the 203mm being used in DH. Of course the bike industry will get there incrementally, so we’ll be sure to be drowning in incompatible electric parts too…

[benb]

It might be necessary.

I saw an eBike pulling a baby trailer at 35mph uphill earlier this week, it kind of blew my mind. Who knows if it was modified or what but it was FAST and was staying with traffic in a 40mph zone. It was not a road bike style eBike either.

Very likely NOT safe with bicycle brakes at that speed pulling a load.

[wallymann]

On motos early disc calipers were sometimes placed ahead of the fork legs, but then they figured out that was a poor location for caliper mass relative to steering axis...moving behind the fork minimizes that effect.

[Likes2ridefar]

Originally Posted by benb

The bikes pictured have rotors & calipers on BOTH sides of the front wheel, any serious motorcycle with anything but "vintage correct looks" has had a dual front discs for a very long time.

This isn’t correct. It’s based on the engine output and bike requirements. I don’t know if there is a rule but in my bike days I generally saw bikes with less than 600cc to have a single rotor up front and those above have dual.

[Dude]

When a caliper is placed behind a leg, while braking, the caliper is supported by the fork leg. As in, the force of wheel rotatation pushes the caliper towards the leg making it much stronger. Placing the caliper in front places a lot of load on the screws holding the caliper to the fork leg and is a weaker design.

As to why it’s on the left side…the same reason why qr skewer levers were on the left. So it matches the back.

[oldpotatoe]

Originally Posted by Likes2ridefar

This isn’t correct. It’s based on the engine output and bike requirements. I don’t know if there is a rule but in my bike days I generally saw bikes with less than 600cc to have a single rotor up front and those above have dual.

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.

[Mark McM]

Originally Posted by Dude

When a caliper is placed behind a leg, while braking, the caliper is supported by the fork leg. As in, the force of wheel rotatation pushes the caliper towards the leg making it much stronger. Placing the caliper in front places a lot of load on the screws holding the caliper to the fork leg and is a weaker design.

This explanation doesn't pass the sniff test. The first disk brake calipers were typically mounted with transverse through-bolts (such as with ISO mounts). The braking forces were completely carried by the bolts in shear, regardless of the location/orientation of the caliper. The caliper location/orientation with post mounts load the mounting bolts primarily in shear also. Only with flat mounts (a recent development) do the calipers get "pushed into the leg" (so it is likely that the flat mount design was influenced by caliper location, rather than the other way around).

Also consider that the fork dropouts/axle experience the same forces as the caliper. When the caliper is mounted to the back of the fork, the dropouts are loaded in their weakest direction (straight down), and that with QR axles, the dropout/axle joint is even weaker. This strongly suggests that the direction of forces on the fork was not a strong consideration in locating the calipers on the back.

[Mikej]

The angle of the drop out would have to be in line with the rotational circumferance of the disc to pop out, otherwise it just pushes against the inside bottom of drop out, therefore keeping the wheel in place. Also, the force would have to overcome the rider and bike velocity to lift enough to allow ejection. Think about it. The front drop out would need to be pointed to the rear with a nds position. Anybody ever ssee one actually pop out?

[Dude]

I think the fact that they are designed this way currently (caliper on back) demonstrates that the direction of forces on the fork IS a design consideration.

Thru-axles came around for fork (and rear triangle) stiffness and strength. Added benefit of a stronger system is that it can handle more/different loads removing constraints that allows designers/engineers to innovate - for example, moving the caliper behind a fork leg (or on top of a chainstay).

Not sure about motorcycles but bike design prioritizes weight highly, removing extra bulk for shear loads seems like a win.

Additionally, calipers transfer shearing force to a fork leg torsionally. Flat mount reduces that over post mount.

Originally Posted by Mark McM

This explanation doesn't pass the sniff test. The first disk brake calipers were typically mounted with transverse through-bolts (such as with ISO mounts). The braking forces were completely carried by the bolts in shear, regardless of the location/orientation of the caliper. The caliper location/orientation with post mounts load the mounting bolts primarily in shear also. Only with flat mounts (a recent development) do the calipers get "pushed into the leg" (so it is likely that the flat mount design was influenced by caliper location, rather than the other way around).

Also consider that the fork dropouts/axle experience the same forces as the caliper. When the caliper is mounted to the back of the fork, the dropouts are loaded in their weakest direction (straight down), and that with QR axles, the dropout/axle joint is even weaker. This strongly suggests that the direction of forces on the fork was not a strong consideration in locating the calipers on the back.

[unterhausen]

Caliper on the front looks weird and that's enough for the industry not to do it. There is very little engineering done on bicycle designs. I doubt they have even tested this at all. Draw a free body diagram that somehow proves your thesis, it's not hard. But it's not going to show that the forces are different other than the one driving the axle into the dropout for front mounted calipers. That force is supported by the fork somehow in either situation. I'm sure the rider's weight dominates it.

I was always a bit surprised that TA didn't take over immediately on inexpensive bikes. The way lawyer lips had grown over the years made QR skewers essentially useless anyway.

[Dude]

Originally Posted by unterhausen

Caliper on the front looks weird and that's enough for the industry not to do it. There is very little engineering done on bicycle designs. I doubt they have even tested this at all.

Wrong. First, the industry does weird looking sh*t all the time. Second, the big manufacturers (and probably the small ones but I don't have first hand experience with them) absolutely test this stuff.

[benb]

Originally Posted by Likes2ridefar

This isn’t correct. It’s based on the engine output and bike requirements. I don’t know if there is a rule but in my bike days I generally saw bikes with less than 600cc to have a single rotor up front and those above have dual.

It varies more with destination.. the US is heavily biased towards "small bikes are for beginners, then upgrade to big bikes to be a real rider."

So the US models of a lot of the 250cc-400cc bikes often have the single disc, lower quality suspension, etc.. to cost save for beginners. For a long time we got older versions of the bikes that didn't have the upgrades to fuel injection, etc... that were for sale in other markets because they thought the US wouldn't pay the higher prices for a smaller bike.

In foreign markets where they have a different mentality about small bikes the bike will often come with dual discs in the front. Traditionally when a small bike did get shipped here with dual discs, better suspension, etc.. it becomes a cult classic.

Bigger bikes in the US almost always have racetrack quality brakes and the only thing they need for racing is safety wiring. Whether you need them has more to do with whether you go to the racetrack and whether or not you are skilled. A skilled rider on a 250-400cc bike will need/desire them on the track, a less skilled rider on a 1000cc bike on the street won't need them. The skilled rider on the smaller bike will be exiting corners with much higher speeds and ends up braking harder on corner entry.

It's really nice that they do this IMO. It makes track days so affordable compared to sports cars. The bikes have almost zero extra maintenance expense.

Road bike brakes have always been weird IMO. Most races they're fine, but there are plenty of mountain descents they've always been sketchy but the mentality has always been weight saving above making the brakes 100% safe for all road conditions.

[Mark McM]

Originally Posted by Dude

I think the fact that they are designed this way currently (caliper on back) demonstrates that the direction of forces on the fork IS a design consideration.

It might have been considered, but I don't think it was dominating factor. The forces just aren't so great that a slightly larger bolt or slightly thicker mounting flange wouldn't accommodate them (heck, a rim brake caliper is held on with only a single 5mm bolt in bending). And as mentioned, the weak link is the quick release skewer resisting the wheel ejection forces, but that wasn't enough to prevent behind the fork caliper mounting.

Originally Posted by Dude

Thru-axles came around for fork (and rear triangle) stiffness and strength. Added benefit of a stronger system is that it can handle more/different loads removing constraints that allows designers/engineers to innovate - for example, moving the caliper behind a fork leg (or on top of a chainstay).

I agree that thru-axles have many advantages for disc brakes. But behind the fork caliper mounting was standardized before thru-axles became standard for disc brakes. There were a lot of disc brakes on QR frames and forks.

Originally Posted by Dude

Not sure about motorcycles but bike design prioritizes weight highly, removing extra bulk for shear loads seems like a win.

Like was mentioned earlier, the first disc calipers mounted with bolts that were in shear regardless of the caliper mounting orientation, so that wasn't a factor in deciding the front vs. rear of fork. Mounting to the back of the fork has other advantages as well, such as better protecting the caliper and hose. Remember, disc brakes were first widely used on MTBs, where front mounted calipers and hoses are more prone to getting hit by branches and debris or hitting rocks if the bike is laid down. Also mentioned earlier is that mounting the caliper to the back of the fork puts it closer to the steering axis, so its weight affects steering forces less.

Originally Posted by Dude

Additionally, calipers transfer shearing force to a fork leg torsionally. Flat mount reduces that over post mount.

Sure. But flat mount didn't come about until long after it became standard to mount the caliper to the rear of the fork, so that wasn't a deciding factor in caliper position.