Fuel consumption 85mph v's 90mph

[DesertStrom]

Read an interesting article by Kevin Ash in MCN recently about fuel consumption on the new Wee @ 85mph v's 90mph! (on private property of course!   )

Bit long winded but here goes............

He found that at a steady, indicated 85mph, the bike managed 53.5mpg. There was a slight tailwind & it fell by 2mpg in the opposite direction........giving a range approaching 240 miles.

After 80 miles he zeroed the computer & increased the speed to an indicated 90mph. This extra 5mph caused the mpg to plummet to 44mpg, almost 18% worse for an increased speed of 6%.......bringing down the range to 190 miles.

Here's his findings.......

The dominant factor here is the relationship between power & speed. It's quite well known   that aerodynamic drag increases in proportion with the square of the speed...........double your speed & your drag increases 4 times.

The equatiuon that matters is speed x drag = power........in other words, double your speed and you need 8 times as much power!
3bhp moped can manage 50mph, while it takes 24bhp to reach 100mph and 192bhp to reach 200mph

Back to the Wee-Strom, 90mph is a 1.0588 increase from 85mph. Cube that to find the power increase you need.........it comes out at 1.187 or an 18.7% power increase, which closely matches the additional fuel demand.

Utterly useless info    but I found it quite interesting that such a small increase in speed could drop the mpg so much!!  

[Keith Cross]

I found that article quite interesting to.  Unlike most of the rubbish that goes into his articles of late.

keith C

[frez]

The relationship between drag and speed only increases exponentially above about 50-60mph, below that it is more linear. Rolling resistance (tyres, chain, gears) can largely be ignored at higher speeds but must be considered at low speeds.

It is a lot more complex than his article suggests.

[Coval]

Surely, rolling resistance (wheel bearings, tyres, chain and gears etc.) doesn't increase with speed but remains constant.
But then, I was never good at maths and even worse at physics.

[frez]

Rolling resistance can safely be ignored at high speed because aerodynamic drag is so large by comparison, at very low speeds the rolling resistance is a larger percentage of the overall drag so can't be ignored.

It does vary by speed, see http://www.tut.fi/plastics/tyreschool/m ... 3/3_3.html although that article only discusses the wheel aspect not the losses through the chain and gears, that said, I'm not sure whether they are usually included or not.

[Keith Cross]

don't know for sure but I would have thought that losses due to chain and gears would increase, at least marginally with speed as heat build up would be quicker and less easily dispersed.  Plus surely friction foes up with speed to?
Like I say not to sure though

Keith C

[Glider65]

Which all goes to explain why My Wee suffered horrendous (and up till now) unexplained huge MPG hits at high speed two up with luggage, but my Vee doesn't.

Yay!  Another reason to like the Vee

[Upright]

Pleasure increases exponentially with road speed. We can see this in the following equation  

Speed X time= distance from wife.

ergo pleasure increase - only joking, only joking - any distance from wife is good It doesn't have to be large

I'm getting 250miles per tank

That Kms for me divided by 10 multiplied by 6 (for miles) and then divided by litres 18 and multiplied by 4.54

Anyone see anything wrong with this calculation

that's about 63mpg I'm very happy - I'm mostly commuting but there is the odd squirt. big Back box and 33/42 tyres. I'm assuming standard gearing and me 17stone geared up, standard screen top height, bark busters ,knobbly knees  can't see how to squeeze anymore out of her - maybe a long motorway only trip (engine hot etc) at 65mph with a tail wind and no major hills. But that's never going to happen. My tyre is a bit squared off I wonder if this increases mpg by a fraction.

I don't read many bike mags - I don't hold with readin' bad for you. But maybe I should

[Juvecu]

Only bike mag I think is worth reading is Motorcycle Monthly and you can read it for free online: www.motorcyclemonthly.co.uk

[Abercol]

+1 for the Vee...on the autobahns we sat at a steady 90 (GPS, not bike speed) as the leader of our group of bikes had a cruise controlled BMW. Only sped up to around 100-110 to overtake.

The Vee was always around 50mpg (with full luggage). The aforementioned K1300GT up front got 51mpg.
I liked that K1300GT, but got put off a little - the owner dropped it 3 times during our week away & its a heavy bugger to pick up (5 drops for him in one year!).

[frez]

If you rode everywhere at 40mph in top, only ever accelerating slowly, timing junctions well so you don't brake, you would get amazing mpg, but who wants to ride like that?

[Juvecu]

Even suggesting it is crazy Shuush!  

[Abercol]

If you rode everywhere at 40mph in top gear....

Hmmm, you'd have to be on a C50 stepthru (the older not so restricted ones....)

[theshed]

the same applies to cars, went to Liverpool from Birmingham the other night
all motorway driving yaris estate
going there I averaged 60 MPH and returned an average of 65MPG
a little more umph on the way back averaged 75MPH returned 43MPG
huge cost to save a few minutes but then again can't miss corrie can you

[Glider65]

I once did 300 miles in an old Landrover 108 2.3 Diesel which had just had a Feary Overdrive fitted.  It was being run in and I was told not to exceed 40mph.  5 lads and camping gear - mid Devon to Cornwall and back.

I could NOT believe it when the old hack turned in over 35mpg.  

It was, it must be said, some much needed consolation for holding up an amazing amount of traffic, with judicious use of laybys to let people pass.

[doebag]

That Kms for me divided by 10 multiplied by 6 (for miles) and then divided by litres 18 and multiplied by 4.54

Anyone see anything wrong with this calculation

A little optimistic [ about 5% out], multiply by 8 divide by 5 is better for Miles to Kilometres, but the maths is not so easy mentally.