The physics of braking on a motorcycle (manual transmission)

[PeterO]

When trying to abruptly stop a motorcycle that is in motion, can engine braking actually have a negative effect on the rate of deceleration?

Scenario: a motorcycle is traveling down a straight highway at 100mph in 6th gear and needs to perform an emergency stop. The weather is clear and road conditions are perfect (i.e. no oil or debris on the road).

Question: will the bike stop faster if the rider pulls the clutch in AND engages the front brake? OR will the bike stop faster if the rider only engages the front brake?
(We'll say that the motorcycle's speed in 6th gear while the engine is idling is 15mph so, for the purposes of this scenario, we will consider 15mph as "stopped" because below that speed the idling will certainly produce forward force)

Fact: front brakes are most effective in slowing down a motorcycle due to weight transfer to the front wheel.
Engine braking is also effective but the force is exerted on the rear wheel only and the rate of deceleration provided is not nearly as high.

So, let's say maximum front brakes slow the motorcycle down at a rate of X.
And let's say engine braking slows the motorcycle at a rate of Y.
X > Y

So, is it possible that Y has a 'negative' impact on X and, therefore, the bike would actually stop in a shorter distance if the rider clutches IN and only uses front brakes? Or will Y (engine braking rate of deceleration) always provide additional deceleration (in 6th gear which at lower speeds does not provide a large rate of deceleration) and actually has a 'positive' impact on X and therefore will always stop the bike in a shorter distance when using both front brakes and engine braking ?

Someone please enlighten me. Initially, I thought engine braking *could* have a negative impact the rate of deceleration of the bike under heavy braking (using front brakes) but now I'm not so sure

Also please consider the difference characteristics of a sport bike vs a cruiser if that has any bearing on the response.

Thanks!

EDIT: Perhaps a better fitting scenario is:
2 riders on identical bikes accelerate to redline in 3rd gear, hold redline for 3 seconds, then perform heavy braking. Rider 1 clutches in and only uses front and rear brakes. Rider 2 leaves 3rd gear engaged and uses front and rear brakes. Assuming there is no skidding or loss of road contact with the rear tires, who stops faster?
(If it's not clear, basically, I'm trying to determine, from a physics standpoint, if adding "engine braking" can actually cause a bike to take longer to stop in certain circumstances. i.e. when braking force (from use of front and rear brakes) is greater than engine braking force. Because, of course, if a rider is only applying a little bit of brakes, engine braking will *assist* in slowing down the bike)

The greatest chance of a negative effect of the engine, is if the rate at which the engine will slow down when the throttle is closed is slower than the rate at which the front brake will slow the bike.
eg: if the engine alone (no gear selected - on the stand even) drops from 6000rpm to 2000 rpm in 3 seconds when the throttle is closed, and those rpm figures correspond to 90 mph and 30 mph you need to ask - could the brakes slow the bike from 90mph to 30mph in less than 3 seconds. If the brakes can, then you should de-clutch.
From what I hear/observe, engines on sport bikes are very responsive - when the throttle is blipped the revs rise and fall very quickly. For a cruising bike, that rev change is much slower (perhaps cruisers just don't blip the throttle much while waiting at the traffic lights). Thus you may be better to de-clutch a cruising bike - but use both front and rear brakes.

 

[TheOldFart]

It's funny someone posted a video of Reg Pridmore and y'all still missed the interesting physics :)

There's actually a bunch of physics at work with the rear brake and suspension.

Pridmore used to ride BMWs, back in the days when they exhibited a wonderful habit called "jacking" where the rear end would actually rise up when you hit the gas and the ring gear tried to climb the pinion.

So shaft vs. chain if you're trying to do all the physics, and you need to include feedback mechanisms like chain snatch and gear lash.

Second is the effect of the rear brake and the engine and chain/driveshaft on the swing arm. You're changing frame geometry, which is going to change the tendency of the bike to lift or squat, which changes braking efficiency.

I'm not a physics weenie, I just have a lot of experience on bikes. Picture a rhomboid with a hing in the middle sliding on a plane and the non-parallel sides made squishy. That's the front forks and the swing arm. (I'm ignoring BMW's telelever suspension, as well as the imbecilic no rear suspension configuration) Intuitively It seems like you're going to maximize the braking if you drive both front and rear suspension down at the same rate initially. You're accelerating the whole thing towards the ground, you're going to maximize friction.

Is that very slightly more true for spoked wheels than mags. Spoked wheels deform so you probably lengthen the tire contact patch by what? Thouandths of an inch? Or is the wheel shape overshadowed by the tire deforming? Not sure there.

If you hit both brakes and initiate that dive towards the ground, the momentum is going to try and tip the rhomboid on its leading edge.

If the front wheel does cause the rear to lift a bit and the rear brake isn't applied, it's not going to do much when it makes contact with the road again - however if you have the rear brake on while its in the air, it's going to try to accelerate the swing arm towards the ground and make contact sooner than it would if you weren't on the brake.

In other words it's not a one-time weight transfer/change in momentum, you've got a wobbly jello thing at work as bits of energy are dissipated in different places and those places change over time.

In the old days of dirt bikes we used different riding techniques for full-floating rear brakes vs. non-floating ones. By dragging the brake pedal even with the throttle on you could get the rear suspension to compress partway, which would actually make you faster through the whoopdies. It kept the rear wheel in contact with the ground longer by preventing the springs from extending all the way, so it didn't try to do a little launch at the top of each bump. By changing geometry, you could go faster with the brake on than you could with it off.

There's another trick that may or may not work with modern street bikes (probably not with a steering damper) Just wiggle the front wheel a bit. Setting up a side-to-side motion can improving the braking. It definitely does with dirt bikes in mud, it forces the knobs to make contact with the sides of the tire as well as the center.

Now to really mess things up, consider the physics of single-sided vs. double sided swing arms

MIT Physics Demo -- Bicycle Wheel Gyroscope



:)

I'm a fan of physics, mainly because it shows up like the Spanish Inquisition, when you least expect it.

I was riding my R1150GSA in MT during the spring and got caught in a very heavy slush/snow squall. I was approaching a steep uphill with curves and I wanted to make sure while I was still getting reasonable traction because there was a solid 3-4" of snow and slush on the road, so I goosed the throttle a tiny bit to see if I could break the rear loose at all. It did, just a little. So I went back to a neutral throttle so I wouldn't slide. It returned...and then the wheel/driveshaft fed back and it started to go a little to the side...and back...and to the side...and back...and... It actually set up very slowly oscillating feedback loop that caused it to go sideways harder and harder, eventually (after ~0.1 miles) resulting in me sliding down the road and a very long way through the grass on the side of the road. No throttle, no brakes, just drive line slack and wheel rotation and rear suspension combining to put me on my butt.

Oh, and the short answer? Sorta like the value of helmets, don't ask physics weenies, ask the Moto GP guys. If there's a way of stopping faster on pavement, they'll know it and probably have tried every other combination before settling on it.

Never been on a modern sport bike but personally I'd used both brakes, scoot my butt as far back as I could, sit up tall and put my knees out to catch as much wind as possible. Every little bit helps in a panic stop.

 

[sysprog]

Complete analysis of this is not simple. A simpler model is of braking a train. Application of engine braking and load car braking increases the heat dissipation of the overall system compared to disengagement of the engine and using engine car wheel brakes alone and results in shorter stopping distance. Formula I racing makes extensive use of engine braking. That's the purpose of heel & toe and double declutching.

Motorcycles require techniques that trains and cars do not require, and modern bikes and most race cars make use of a synchromesh, which means single declutching is sufficient, but a skilled rider can brake faster if he uses engine braking and rear wheel braking along with front wheel braking than he can with front wheel braking alone, including into a turn.

 

[velocity_boy]

No real motorcyclists would be caught dead on a motor scooter with manual transmission. LOL

I ride a Honda Shadow 1100. One down and four up, baby.

The way a transmission SHOULD work!

Cheers.

 

[cosmik debris]

No real motorcyclists would be caught dead on a motor scooter with manual transmission. LOL

I ride a Honda Shadow 1100. One down and four up, baby.

The way a transmission SHOULD work!

Cheers.

And which side is the gear lever? Most motor bikes are not manual they are pedial, if that's a word.

 

[berkeman]

One down and four up, baby.

I prefer the inverted racetrack shift pattern...

 

[cosmik debris]

I prefer the inverted racetrack shift pattern...

I'm too old to readjust I think. All my early motorbikes had that race shift pattern but the gear lever was on the right side. Since the International agreement in the late 60's early 70's I have only had the standard down for down, up for up, on the left bikes. At one stage in the late 60's I had one of each, that was quite interesting, but context made it easier, one bike was a motocross bike and the other a road bike. One bike I had had neutral at the bottom, not recommended!

 

[Mark44]

No real motorcyclists would be caught dead on a motor scooter with manual transmission.

As opposed to automatic transmission? Or are you distinguishing between hand-shift transmission vs. foot-shift transmission?

Most motor bikes are not manual they are pedial, if that's a word.

I don't think pedial is a word, but pedal is, and it can be either a noun or a verb.

 

[velocity_boy]

As opposed to automatic transmission? Or are you distinguishing between hand-shift transmission vs. foot-shift transmission?

I don't think pedial is a word, but pedal is, and it can be either a noun or a verb.

Oops. Sorry. I messed up. Meant to say that no true biker would tide a bike with automatic tranny.

 

[Mark44]

Oops. Sorry. I messed up. Meant to say that no true biker would tide a bike with automatic tranny.

That's sort of what I thought you meant, although I don't know if there are any that actually shift on their own. The very first bike I rode, Honda 55, back in about 1963, had an automatic clutch (i.e., a centrifugal clutch). There was no clutch lever, but there was a gearshift lever. You just let off the gas when you wanted to shift.

If there are any bikes with transmissions that shift by themselves, they have to be pretty rare.

 

[velocity_boy]

Do any of you guys know if engine braking is bad for your engine? Every mechanic I've asked says no. But I cannot help but wonder. Sure it saves over time on brake pads but who cares? Pads are cheap. I just wonder if continual engine braking is tough on valves or rings or seals?

Thanks!

 

[berkeman]

Do any of you guys know if engine braking is bad for your engine? Every mechanic I've asked says no. But I cannot help but wonder. Sure it saves over time on brake pads but who cares? Pads are cheap. I just wonder if continual engine braking is tough on valves or rings or seals?

Thanks!

IMO, engine braking plus both brakes should be the strategy for smooth riding, including moderate performance riding, and up to racetrack level riding (modulo the back tire off the ground thing I mentioned earlier in the thread).

If you listen to the 2nd Reg Pridmore CLASS video that I posted, you will hear how much Reg uses engine braking as part of his super smooth ( and super fast) riding. He emphasizes smoothness in transitions from throttle to brakes and back to keep the suspension transitions smooth, which keeps your tires in the best contact with the pavement for maximum traction. If you slam on the brakes, you skid them which loses traction and loses you braking power. If you learn to be smooth transitioning into hard braking to let the suspension smoothly (and still quickly) settle, that gives you the best traction for slowing the bike.

I once asked Reg if it was okay to downshift if you were at redline at the end of a straight, since that would over-rev the engine past redline. He answered yes, that since it was not under power going over redline, the brief transition over redline under engine braking was okay. That's an extreme example to answer your question, but maybe it helps some. If you are just using engine braking at normal RPM settings (like 3/4 of redline), it should be pretty gentle on your engine, IMO.

 

[velocity_boy]

IMO, engine braking plus both brakes should be the strategy for smooth riding, including moderate performance riding, and up to racetrack level riding (modulo the back tire off the ground thing I mentioned earlier in the thread).

If you listen to the 2nd Reg Pridmore CLASS video that I posted, you will hear how much Reg uses engine braking as part of his super smooth ( and super fast) riding. He emphasizes smoothness in transitions from throttle to brakes and back to keep the suspension transitions smooth, which keeps your tires in the best contact with the pavement for maximum traction. If you slam on the brakes, you skid them which loses traction and loses you braking power. If you learn to be smooth transitioning into hard braking to let the suspension smoothly (and still quickly) settle, that gives you the best traction for slowing the bike.

I once asked Reg if it was okay to downshift if you were at redline at the end of a straight, since that would over-rev the engine past redline. He answered yes, that since it was not under power going over redline, the brief transition over redline under engine braking was okay. That's an extreme example to answer your question, but maybe it helps some. If you are just using engine braking at normal RPM settings (like 3/4 of redline), it should be pretty gentle on your engine, IMO.

Thank you! That's all good info to know. I can now practice my smooth deceleration with no worries!

Thanks.

Ride on!

 

[cosmik debris]

IMO, engine braking plus both brakes should be the strategy for smooth riding, including moderate performance riding, and up to racetrack level riding (modulo the back tire off the ground thing I mentioned earlier in the thread).

Yes I agree, it should be part of the strategy and professional riders spend quite a bit of time getting the engine braking to their liking. I prefer low engine braking because with today's ride-by-wire throttles it helps stop the on-off nature of the throttle in turns where you may need to be vey steady.

I once asked Reg if it was okay to downshift if you were at redline at the end of a straight, since that would over-rev the engine past redline. He answered yes, that since it was not under power going over redline, the brief transition over redline under engine braking was okay. That's an extreme example to answer your question, but maybe it helps some. If you are just using engine braking at normal RPM settings (like 3/4 of redline), it should be pretty gentle on your engine, IMO.

A lot of modern machines have slipper clutches which make this technique even more usable.

Cheers

 

[sysprog]

And which side is the gear lever? Most motor bikes are not manual they are pedial, if that's a word.

Yes, pedial is a word, derived from Greek πεδίον (pedion) = metatarsus (literally "foot thing") via truncation of "on" (nonal ending) and appendation of "al" (Latin adjectival ending). It has well-recognized meanings precisely as you intuited -- of or relating to the foot, or of or relating to a foot-operated device, or pedal, and takes the adverbial form "pedially", as in "a pedal is a pedially-operated device". The word "pedial" is more perspicuously an adjective than "pedal" is -- "pedial" does not require special context to be interpreted as an adjective. Although the word "pedal" originated as an adjective (it has adjectival ending "al"), just as some other words commonly used as nouns did, it quickly took on its nonal and verbal usages, while the adjectival usage waned into obscurity. Using "pedial" to distinguish from "manual" regarding motorcycle gearshift lever operation is lexico-structurally elegant.

As opposed to automatic transmission? Or are you distinguishing between hand-shift transmission vs. foot-shift transmission?

I read it as the latter.

I don't think pedial is a word, but pedal is, and it can be either a noun or a verb.

The word "pedial" is a rather obscure word, but it's been around for a long time, and it's apt for the usage cosmik debris made of it. You're right that it can be a noun or a verb, along with it being an adjective, having the adverbial form "pedally". I prefer "pedial" and "pedially" for adj. and adv. purposes, because of the strong entrenchment of "pedal" for noun and verb.