Mileage

Generally bike engines are designed to deliver their best at high(er) revs than cars owing to their higher power to weight ratios.

Look at it like this:

Take a person moving 50 Kg up 5 equally spaced steps.

One person will do this in 10 seconds. A second will do it in 5 seconds. Therefore the second person makes twice the POWER of the first.

Take the same 50 Kg up 5 equally spaced steps.

One person will do it in 10 seconds. A second person takes 100 Kg up the same 5 steps in the same 10 seconds. The second person in this example has twice the TORQUE of the first.

So you see torque is the ability to move weight (Or in other words produce force) over a set distance. Therefore for a car with all its extra weight to move compared to a bike a far greater emphasis on torque is highly desirable.

Where this comes into play with regards to MPG is that higher revs (Required to generate the bikes' extra power figures) will generate far greater frictional losses (These losses increase exponentially as they go up) meaning that gaining that extra power involves paying a hefty price.

This also explains why higher torque bikes such as Harleys can still provide satisfying accelaration and be pleasing to ride. Where their lack of power comes to the fore is in their inability to produce high top speeds and/or sharp accelaration when at higher speeds (80+ Mph) when the aerodynamics mentioned by JAG come increasingly into play.

Thanks JAG!

Your point about the cooling is a good one too. Also bikes tend to be far more sensitive to poor/interupted fuelling. Far more so than a car which has the extra mass to hide a slightly more jerky response to throttle input. Kev made this point in a column of his a little while back and it's an important one to bear in mind.

Therefore bike manufacturers will often use extra fuel to help smoothen throttle response at a certain engine load/speed. I suspect this is why my 2011 model 1050cc Sprint ST is less fuel efficient than my 12 year old 955cc version. Or at the very least it is one of the reasons.

A fascinating topic this one!

"Take the same 50 Kg up 5 equally spaced steps.

One person will do it in 10 seconds. A second person takes 100 Kg up the same 5 steps in the same 10 seconds. The second person in this example has twice the TORQUE of the first."

And twice the power.

Navy boy said: " Far more so than a car which has the extra mass to hide a slightly more jerky response to throttle input."

An excellent point. Easy to overlook.

Navy boy said: "A fascinating topic this one!"

It certainly is! cav has asked a good question.

Navy boy, I thought your example was a good one. And it certainly illustrates quite well how higher torque will punish components which are not up to the job. Higher torque doesn't always equate to higher power. In fact could we go so far as to say, in a naturally aspirated engine, it never does? Although higher torque AT A GIVEN SPEED is obviously going to result in more power AT THAT SPEED.

Blistering acceleration gobbles up power and therefore fuel, as Jag has said. Bikes, especially the modern incarnation of them, are acceleration devices above all else. Admittedly, they are relatively light.

Rolling resistance is a lot lower in bikes than cars.

cav asked why are bikes not more fuel efficient than cars? It begs the question which vehicle type has the potential to be the most fuel efficient. Off the top of my head, surely it must be the motorcycle? What would such a device be like? We have a stab at that already in the NC700. As the NC700 engine is half a Jazz engine, it'd be interesting to compare the respective power/weight ratios of each machine with a rider/driver and enough fuel to cover 150 miles on board.....

Pittsy wrote:

"Navy boy, I thought your example was a good one. And it certainly illustrates quite well how higher torque will punish components which are not up to the job. Higher torque doesn't always equate to higher power. In fact could we go so far as to say, in a naturally aspirated engine, it never does? Although higher torque AT A GIVEN SPEED is obviously going to result in more power AT THAT SPEED. "

Never does?

There may be one execption I can think of. The new Kawasaki 2012 ZX-14R.

Bikes with over 100 ft/lbs of torque generally are considered high torque engines.

The ZX-14R for 2012 makes up to 113 ft/lbs of torque and still manages to develop a high measured 193 RWHP.

The 193 RWHP it can deliver surprised me. I thought maybe it would develop at most about 170 to 175 RWHP.

I was way off on that one. Looks like I need a new slide rule.

Kawasaki did their homework on that one.

They even managed to improved the MPG over the older model.

NOTE: To only judge a bikes potential acceleration and power to weight ratio just based on the total weight with the rider divided by the maximum RWHP can be miss leading. As we found out in thrust curves, so much depends on gearing and how much RWHP (or as Pittsy would say POWER) the bike can deliver in that gear at that road speed.

Regards,

JAG

I'm afraid it's mostly the lousy aerodynamics.

http://www.bgsoflex.com/airdragchart.html

Bikes are much more 'draggy' than cars, so they need more power to drive them through the air. Dustbin fairings and long, pointy tails would solve it, but I don't see many customers.

Dean15, A very interesting read. Thanks for the link.

Ok, what if we apply a limit? Say 70 mph. What then? The drag effect has only just started to play a significant role at this speed. Would you agree? Surely then the other aspects of a bike which are an attribute will kick in. The low rolling resistance and light weight. Using less power to accelerate up to that speed limit in a reasonable time. A moderately powered, unfaired 125 will reach 70 mph. What speed would a 10hp car reach? 50mph? How long would it take to get there?

If we raise the speed limit to 150 mph will the tables have turned? An unfaired 125hp bike should reach this speed. What hp car would we need?

Interesting stuff.

Nicely put Navy Boy. And I'm not technical, but I do have one minor nit pick in the overall good statement that you made:

"One person will do this in 10 seconds. A second will do it in 5 seconds. Therefore the second person makes twice the POWER of the first."
... in my mind, they actually produce the same power (torque), because the weight they are moving is a constant. However the second person can reproduce the (same) power, twice as quickly. In engine terms, they have the same torque (power), but the second person revs higher to reproduce the same work (torque) in half the time, therefore the second person produces (an extrapolated calculation of torque) more bhp.

The analogy I always think of, is a boxing champion. How hard he can hit me is his torque value. How quickly he can reproduce hits of the same power is his horsepower.

For heavy objects such as cruiser-tourers or cars, torque is needed to starve off the boredom of otherwise lethargic acceleration. But this is usually at the expense of power (which facilitates higher speed), which is an acceptable trade off, when not breaking lap records or when you're warm and dry on a wintery day.

Light vehicles such as sports-bikes and kit cars, benefit from higher revving, high horsepower engines, at the expense of torque. They don't need much initial grunt, to get their light weight moving, but plenty of hp to keep on reproducing that power, giving a good compromise of sprightly acceleration versus higher top speeds.

In the main bikes may not be as streamlined as a car, but then again their frontal area (even with rider prone) is significantly smaller than that of a car, even a very small one.

I don't actually believe aerodynamics are that much of a factor at normal (below 100 mph) speeds. If you look at the Kawasaki GPX1000RX for instance. That was the first bike to cleanly break the 150 mph barrier. It was actually quite aerodynamic, fully faired, and not too big of a distant silhouette to today's hyper-bikes. But to make modern bikes go a modest 20 or so mph faster than the ancient Kwack, modern bikes have had to lose weight and gain massive power, just to get that extra few (in direct relation) mph. The GPZ was about 115 hp (crank, claimed) versus a modern equivalent, such as a K1300S at 175 hp. I.e. A not too dissimilar silhouette motorcycle had to increase it's power by around 33% (60 hp) just to increase it's top speed by about a further 12.5%. And to get the BMW a further 20-30 mph faster (200+) would probably require it to increase it's horsepower by maybe 66-100%. You can't just make an already slippery motorcycle smaller or 'considerably' more aerodynamic, the incremental increases have predominantly come from power gains IMPO.

I believe that the real difference between cars and motorcycles is that most motorcyclists will sacrifice 10 mpg for a 3 second 0-60 mph figure, at a retail purchase price which is 50-70% cheaper than a mainstream car - which will typically accelerate three times slower. If you made a mainstream car accelerate like most motorcycles (apart from having a very low top speed) it's fuel consumption would really suffer to typical motorcycle levels (or more) too.

Some vehicles can perform well in both accelerative and economy disciplines, so they needn't be mutually exclusive of each other. A good example is Honda's NC700X, itself ironically, half a Honda Jazz (car) engine. But others, such as the F800ST also offer amazing fuel economy, with Porsche type 0-illegal speeds acceleration.

With fuel prices rising globally, none more so than in England it would seem, economy is definitely more of an important issue nowadays, than it might have been to the average rider, even a couple of years back.

A final note, my Triumph eXplorer (even when I frash-eeeiittt! :-D) is 'much' more frugal than my SUV which only averages 16-17 mpg, which is normal for it - apparently! My Triumph has 10k service intervals and my SUV needs one every 3,750. Perhaps I should ride more? ;-D

CS

Your boxer analogy is a very good one and is spot on. If you want a real example of what we're referring to just take a look at the H-D range. My XR revs to around 7k revs and makes (At the crank) 90 Bhp. The V-Rod which is roughly the same size of engine (1202cc vs 1250cc) revs to around 9 k revs and makes some 120+ Bhp. I can't remember the torque figures off the top of my head but they are pretty similar.

In other words my XR's engine is far more 'Car like' and the V-Rod's is much more of a typical 'Bike' engine.

At 70 Mph a typical bike (Shall we say a Triumph Street Triple for example) is starting to hit the aerodynamic 'Wall' to some extent. Just measure its fuel consumption at 60 vs 70 however the 70 vs 80 Mph difference will really start to make its presence felt. Also this varies from bike to bike. A Daytona 675 would of course have the ability to slice through the air more easily thanks to a greater aerodynamic package.

More and more bike manufacturers are starting to quote fuel consumption figures for their bikes now. I see that Triumph quote 3 separate figures as in the car world with a 56 Mph, 75 Mph and a combined figure. Naturally the 56 Mph figures are quite impressive but the differences between 56 and 75 Mph, as a percentage, differs noticeably between different models.

Interesting and not just down to aerodynamics either as gearing plays a large role here too. My XR is geared so that it is at its torque peak (In theory where the engine is giving you the biggest effort for least throttle/effort) in top gear it is doing around 65 Mph. This has always struck me as being pretty much spot on as a higher speed would mean tha one would start to lose out aerodynamically.

However look at my old Triumph TT600 for a contrasting example. It made its max torque at aorund 10500 Rpm which equated to - Well let's just say that I found out one evening along a quiet stretch of dual carriageway!! In other words the gearing wasn't real roads relevant. I guess that is no great surprise given that it was designed to hurtle around a track but it does demonstrate the extent of the shift in priorities manufacturers need to make if fuel consumption is to become more of a big deal for them.

Pittsy, thanks.

I think the bike/car difference we are all missing here is that a big bike runs at small throttle openings nearly all the time. This causes pumping losses in the inlet tract. It also means that the effective compression ratio is much less than the geometric compression ratio.

Imagine a one litre bike at 60mph. Its throttles are very restricted, causing low efficiency. A one litre car at 60 is running closer to its max. efficiency.

A 125 bike is better, which brings me neatly back to my question on another thread; why do we buy such big bikes? Do we want to break the law systematically, or do we like running big engines at small throttle openings and thereby halving their compression ratios? Either way, we use a lot of fuel.

Diesel engines are a solution. They have no throttle valve at all, & run with excess air at all times. That's one reason why they're economical, and it's also the reason they're so compatible with turbochargers.

Every so often a mag runs a story called 'why don't we have turbo bikes?' They mostly miss the true reason; bikes run most of the time on part throttle. Not much point in pumping air with a turbo, then choking it through a throttle valve.

Aero engines do spend long periods at (or near)full throttle and they benefit from turbos.

Forget that idea. Utter rubbish!

Variable stroke but no compression. Must try harder!

Here's the correct link. I hope Kev doesn't mind me posting a link to the TELEGRAPH on here. No slight intended Kev.

http://www.telegraph.co.uk/motoring/motorbikes/9088717/Track-T800-CDi-re...

It makes for an interesting read all told.

Cost of performance

We all generally agree how a motorcycle performs is important.

So how much does performance cost?

I often use a cost per RWHP to set a basic comparison scale of about $100 CAN dollars per RWHP.

So a 150 RWHP motorcycle would cost about $15,000.00 CAN which is about right.

For that you get a modern 4 cylinder engine with good brakes and handling and often advanced ABS, traction control and even power modes.

If the bike costs more than $100 per RWHP, what are you getting to justify the extra cost and is it worth it to you.

Based on this scale a BMW K1600 would cost about $14,000.00 and a Harley Davidson about $7000.00.

The Kawasaki ZX-14R at 193 RWHP would cost about $19,000.00 (Canadian list price $17,000).

The Yamaha VMAX at 169 RWHP would cost about $17,000.00 (Canadian list price $23,000.00).

Honda VFR1200 at 143 RWHP would cost about $14,300.00 (Canadian list price about $16,500.00)

Regards,

JAG

The TREK does have its limitations, a number of them in fact, but it does put a very different spin on the whole Diesel bike question.

I too am in the fortunate position of being able to have a car for the practical stuff and not have to worry about using a bike for more mundane commuting etc. The nearest thing I came to a 'Practical' bike was the Honda Silverwing scooter I owned a few years back. Howevre its fuel consumption (Between 10-11 miles per lire or 45-50 Mpg) didn't make it any cheaper to run than a car.

So we are back to the original question - Why do bikes not get better Mpg? I just hope that Honda's NC700 series of machines will get the other manufacturers to start looking on Mpg as something that they can sell. only when market forces dictate it will Mpg have greater resource thrown at it.