# “It’s a simple matter of weight ratios…”

This morning I left the cage at home, and decided to come to work using a two-wheeled
vehicle. Since I'm only about 2 miles from campus, I took the long way round and rode
along beautiful Lake Sammamish Blvd, into Redmond, and then up highway 520 towards
Microsoft.

I haven't been riding much the past year or so, and every time I do, I'm reminded
why I should do it more. This summer, I sold my ancient Acura Integra, and bought
a used BMW 328i. The Integra had reasonable power for a small car, but the 328 is
a lot faster, and still fun to drive. But neither accelerate like my motorcycle.

One way to measure performance - a bad way, it turns out - is to look at horsepower
to weight ratios. If you do that for the vehicles my wife and I own, you'll find the
following:

• Ford Ranger - 27 lb / hp
• Subaru Outback - 23 lb/hp
• BMW 328i - 18 lb/hp
• Honda CBR400RR - 9.6 lb/hp (the wife's bike)
• Honda VFR750 - 7 lb/hp

What does this tell us about the relative acceleration of the vehicles? Well, the
answer is "not a lot, really", for a number of reasons:

The first is that torque is the important measure, not horsepower. Torque is what
gets you accelerating. Horsepower is only a measure of the work the engine can do.
It's a good measure of top speed (assuming equal aerodynamics), but has little bearing
otherwise. So why is it the common measure? Beats me - it may be because it has "power"
in the title.

The second factor is one of rotational inertia. Big engines have a lot of rotating
mass, and when you accelerate the car, you also need to accelerate that mass. Small
engines (more correctly, engines with smaller pistons, connecting rods, cranks, etc.)
have less rotating mass, and therefore more of the torque can go into accelerating
the vehicle. This is one reason engines with more cylinders perform better (but not
the only one).

Power curves have a huge difference on acceleration. Race engines are tuned to have
lots of torque in a very narrow rev band, and they make lots of power there, but not
a lot elsewhere. Their peak power may be higher, but the peakier an engine, the more
work it is to extract the power in a useful manner (which, strangely, equates to both
"more fun" when you want to work at it, and simply "more work" the rest of the time).

Finally, the rev range of the engine has a huge impact on the amount of acceleration
it can produce. If you take two engines, one which makes 30 lb ft of torque at 3000
RPM and one that makes 25 lb ft at 6000 RPM, which one is likely to produce more acceleration?
The whole answer lies in gearing - you can take the second engine, gear it down by
50%, and have an engine that produces (ignoring losses) 50 lb ft at 3000 RPM. So,
small engines are better because you can spin them faster, which (within reason) gives
you a free lunch, so to speak, in the power realm.

So how do you really compare vehicles? Well, one obvious way is "at the dragstrip",
which isn't really a bad measurement, delta the differences between dragstrip use
and real use, and the launch characteristics. (If you're curious, the BMW does 15.3
sec @ 83MPH, and the VFR does 11.4 sec @ 112 MPH). But a nicer way is to create what's
called a thrust graph. For each gear, you take the torque graph, scale it based upon
the various gear ratios and the tire size, to finally arrive at a measurement of pounds
of thrust at the rear wheel vs road speed. You then overlay the graphs for each gear,
and come up with an overall "thrust graph". If you compare the graphs for two vehicles,
you'll get a decent idea of how they stack up.

There are only a couple of motorcycle mags that do this, and I've never seen it in
a car mag.

Somehow I got from a quick entry about my ride to a dissertation on comparing engines.
Not quite sure how that happened...

1. Jim Argeropoulos says:

You know most of the good car rags use a dynamometer. So why don’t they put that little extra bit of knowledge in? I guess no one has asked for it.

2. Eric says:

It’s not built into the dyno software. Given today’s computers, it would be easy to do so, but dyno software isn’t really cutting edge stuff, and adding this probably wouldn’t sell more dynos for them, so it would probably be a wash.

You can do it with a bit of software – you have to take the raw dyno data and put in all the gearing and tire information, and out comes what you need. I did it in Excel once, but it was decidedly not pretty.

I think the real reason is that it would be a lot of work, and most of their readers only look at HP numbers (and perhaps 0-60 or quarter mile times).

3. Werner says:

Personally I find that in daily life I do not encounter many drag-strip situations :-). I have yet to ride a bike that could not outgun any car at the stop lightlight.

I do think torque is important in riding, but more the torque you get when you are already doing 2000 rpm. For example when you are already doing 50 mph uphill and want to bypass these cars, how much power do you have to quickly accelerate to 85 or 90 mph? It was one of the major decision criteria when I bought my last bike, a BMW R1200C, It’ll give you over 70 lbs at 3K and consitently over 65K anywhere between 2.5K and 4.5K. Basically whenever I accelerate whether it is doing 10 mph or 70 mph the bike immediately responds with enormous power. (Yes I am absolutely in love with that feeling, writing about makes me want to go outside and just take it for a spin).

4. Eric says:

Werner,

I agree totally with you, and I think for most riding the dragstrip times are less important than roll-on times, which occur at much lower RPMs. If you had a thrust graph, you could see how much thrust you’re getting at a specific speed in top gear, and base your comparisons on that. IIRC, the R series engines only spin to about 6000 RPM, so an engine that made half the torque but reved to 12000 could be geared identically to yours, and produce similar results.

This is not to put down BMW bikes or riders. Some of my best friends ride BMWs, and it’s not like my VFR is the fastest or lightest thing around. The Yamaha R1, for example, has 130 hp pushing only 445 pounds of bike. With me on that, that would give 130 hp for only 620 pounds, for only about 4 lb/hp.

5. Ian Griffiths says:

Of course performance is not just about straight line speed.

It’s true that in a drag race, it’s safe bet that most bikes will beat most cars. (Unless you’re only going up to 15mph where, in my experience, the tendency of bikes to wheelie if you dump all the available torque into the rear wheel puts them at a serious disadvantage to a RWD or AWD car, where the limiting factor is usually grip, rather than the ability to keep all the wheels in contact with the ground under acceleration. But it’s an advantage that usually vanishes after only a few yards.)

However, driving is usually most fun when corners are involved. It’s true that the majority of car drivers don’t actually push very close to the limit, and most competent bikers will probably be able to corner faster than the average car driver. But if we’re talking about assessing a vehicle’s performance, then you’ve got to consider what happens when you do push to the limits. And the fact is that cars have a substantial advantage over bikes when cornering. This advantage can make up for an awful lot in straight line acceleration on any moderately interesting race track.

(Of course being British, my idea of an interesting racetrack is one which is mostly corners – we’ve never really gone in for those bizarre oval tracks over here. It might be a different story on that kind of track.)

6. Paul Melnikov says:

I do not understand how you arrived at your ratios… The way you have it is upside down! The more lb per HP is actually worse! I think you meant to get it HP/LB. In this case the more is better!

For example Honda CBR600F4i has a HP of roughly 100 (give or take) and dry weight of 370 pounds. That would give you 0.27HP/pound.

Ford ranger (V6) has 148HP. In order to match the HP/weight ratio, it should be around 550lbs ðŸ™‚ In reality, using a low 3000lbs, you get a "wonderful" 0.05HP/lb.

Those rates will go down a bit if we add passenger weight, but still…

An average car, I would say, is about 3500 (Acura TL). With 270HP, it gives you a whooping 0.077HP/#. Even Dodge Viper, sporting 500HP weighs 3380#. That results in "only" 0.148. So, Honda CBR600 is basically has a twice better ratio of HP/lb than Viper!

So, I think bikes rule big time when it comes to HP/weight ratios… I do agree with all other points in your article though.