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-   -   HP VS Weight VS max load (http://www.horizonsunlimited.com/hubb/which-bike/hp-vs-weight-vs-max-39468)

HendiKaf 12 Dec 2008 22:17

HP VS Weight VS max load
 
I was looking at some of my past trip and was surprised to see that with the years the bikes have become smaller in CC and weight and the max load is actually higher. I was comparing bikes and for example if you look at a DL650 with 67 HP and a R1150GS with 85HP and a120 pound extra you get a fairly close max load and both should cruise easily a 70M/H loaded, I know that the BMW will have way more torque but still its interesting, I was in India and did a 7000 miles trip on a Royal Enfield 2 up loaded and the engine had a small 22 HP ratio . Even my old BMW had only 50HP and still I manage to take it loaded like a mule across Africa.
I think when considering a travel bike its important to compare these factor.

AliBaba 13 Dec 2008 11:56

IMHO max power doesn’t mean much for touring. Max power is achieved on high RPM, which we barely use. (Typically around 7500-8000 rpm on a twin).
Max power is usable on the pub, not when touring.

What I think matter is how the engine works at cruising speed, one way to look at this is to look at the torque-chart. Normally you would find that maximum torque is at rpms above your cruising speed.
Acceleration, weight and torque are directly linked. If you double the torque and the weight on a bike you will still have the same acceleration (in theory, and it’s not far off as long as you compare reasonable speeds).

If we compare the DL650 and F800GS we will find that they both weigh 222kg (at least that what they found when they weighed them in a magazine).
If we say that typical cruising speed (4000 RPM) we will see:

DL650: 52Nm (tourque/weight=0,23Nm/kg), at 4000 rpm
F800GS: 68Nm (tourque/weight=0,31Nm/kg) , at 4000 rpm


That means that if the bike has the same gearing you will have 30% more torque available on the 800.
This difference gets a bit smaller through the rev-range, but not much.
If you look on acceleration (60-100 km/h top gear) you will see

DL650: 6.0 Sec
F800GS: 4.7 Sec

The 650 is 28% (ca 30%) slower then the 800 so the theory seems to work, okay?

We have not seen on the weight yet, because the bikes have the same weight so it doesn’t matter. But if we introduce a heavier bike 1200GS, we will see:


Weight: 235 kg
Tourque: 92Nm (tourque/weight=0,39Nm/kg) , at 4000 rpm


Okay, so the 1200 have 25,8% higher tourque/weight then the 800. This is without driver and luggage, so we add 120 kg.

F800: 68Nm/(222kg+120kg)=0,20Nm/kg
R1200: 92Nm/(235kg+120kg)=0,26Nm/kg

The difference in torque/weight increases to 30%, which is almost 20% gain in favor of the 1200.
This shows us the bike with high torque suffers much less from the added weight then an engine with lower torque.

If you repeat the above with a 650-single you will find that it suffers a lot from the added weight because it has very low torque at cruising speeds.


Funny enough the max load is identically on the two bikes, which means that you are allowed to carry more luggage on the F800GS.
On the other hand I’ve ridden both bikes and I would rather prefer a 150 kg load on the 1200 then on the 800.

Does this matter in the real world?
As stated before on this site it’s possible to tour the world with any kind of bike. It’s also possible to live your life in seclusion. What fit’s you it’s up to you.

A bike with high torque will normally have a bigger engine (twin) and a sturdier frame then a bike with lower torque. It will also carry it’s load better, both in the aspect of engine-wise and frame-wise. The drawback is that it has a higher weight, but the weight is only a problem if you can’t handle it. You can learn to handle the weight or you can drive a lighter bike, it’s all up to you.

This is just some thoughts, a lot of other factors are important. It depends on you and your trip, well mostly you because you can go everywhere on any bike.


Note:
All numbers above are meant as indicators to show the physics. Calculations are made around 4000 rpm and the gearing should be more or less the same.

HendiKaf 13 Dec 2008 12:56

HP VS Weight VS max load
 
Great post Alibaba, I agree with you that torque is way more important but I was just noticing that the new bike are lighter and more powerfull which surprise me a bit. Cruising at 70 on a new bike is now done a lower RPM than before on some of the new bike and once you work on gearing you can get a better setting for travelling. Regardingthe new BMW 800 seems to be a great design and if the reliability is good my guess is you will see many on the road or off road do that matter.

AliBaba 13 Dec 2008 13:54

Yes, the power/weight-ratio has increased.

I have not done any research about this, but the new F800GS is heavier then the R80G/S (and R80/100GS dependent on model) which is up to 27 years older. The new Tenere is heavy, is it lighter then the old one?
But it’s true for lots of bikes.

Maximum power has increased during the years, torque have also increased but not that much. (There are exceptions here!)
It’s difficult to make an engine with both good torque in the cruising range and high maximum power. My theory is that the focus on maximum power leads to less low end torque then if we had start focusing different.

Dodger 13 Dec 2008 19:35

Horses for courses
 
Some interesting points have been mentioned here .

When you look at torque ,you should also look at where in the rev range the bike produces maximum torque. Ideally you should operate the engine at an rpm that is slightly above the max torque produced .
Then as the bike comes under increased load ,as in going up a hill ,the engine will have a torque "back up" and climb the hill easier .Sometimes called lugging ability this is easier to demonstrate on a graph .

It's very difficult to compare bikes because they produce the max torque at different rpms -
these days 4000rpm is not very fast .
The 650 VStrom produces it's power higher up the rev range -it revs to 10,500 and it's not a very fast bike by modern standards.
Smaller engined modern bikes can deliver the goods by increasing the revs to compensate for the lesser torque ,modern materials inside the engines and improved engineering tolerances mean that they can do this reliably .

I have an XS650 Yam that revs to 7500 ,if you held it at 7000rpm for any length of time it would self destruct .A 650 Vstrom will do 7000 rpm all day .

Frames are better designed these days ,forks are fatter, swingarms are beefier, longer and are usually monoshock ,engines have emission equipment and exhausts have cats and extensive baffling ,all this adds weight .
But usually it means that the frames are capable of handling a higher load -if you want to load them to the maximum .

Weight is the enemy , if you can travel light you will get better performance, better fuel consumption and your bike will be a lot easier to handle .It's not much fun trying to lift a heavy loaded bike on a 45 degree rockstrewn downslope on your own .If you never travel off tarmac ,it may not be a problem that you'll have to face .

So in many ways the modern bikes like the Versys , VStrom 650 and the F800 GS are a very good thing ,they will give the performance of the big bikes for less money and less weight ,you'll just have to get used to revving them a bit higher .


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