Comparing bananas and lemons - Part 2 : Torque

What happens in the hills?

While we have been harping on power all along, some of us are accustomed to grunt and scoff at high revs – which brings us to another metric – torque – To put it very simply, think of your engine connected to a pulley of one meter diameter : The amount of weight it can hoist is the torque.

Torque is the value which can tell you what maximum grade of hill a motorbike can climb – The road is essentially a rope and the rear wheel like a pulley that your bike pulls itself up on.

Most engines have the following power/torque characteristics –

• High revving four strokes – make negligible torque at the lower 1/3^rd of their RPM range, and the difference between peak power and peak torque RPM is less than 25%
• Low revving four strokes – make decent torque starting from the 1/4^th of the RPM range and the difference in peak power and torque RPMs can be up to 50%, at the expense of peak power values
• Two strokes – Rarely make good torque at low end,  and peak power and peak torque RPMs are very close – The Maico 760 is an exception here

Torque defines instantaneous acceleration based on the current gear ratio: Climbing a hill at constant speed is equivalent to accelerating steadily on a plain.

With an engine that has a narrow power band, the gears have to be shifted much more often to ensure that the engine keeps within that range. For a given gearing, a wide power band engine has an overlap of speeds across gears, allowing a climb to happen with less shifting and use of the clutch. There will come a point for a peaky engine, where close to maximum revs in a certain gear are required, but the resulting vehicle speed is too much to handle on the twisties. The low revving bikes have the option of modulating speed with the throttle in the same gear since torque does not fall off too rapidly below peak torque RPM.

Think of the shape of the torque curve, if you are riding at an RPM after the peak, the slope of the curve is downwards, that means a reduction in engine RPM makes the engine push harder – it is a negative feedback effect - and there is an equilibrium found as the bike climbs steeper or flatter slopes. If you drop below peak torque, there is now positive slope in the torque curve and speed falls off rapidly, forcing either a downshift, or wider throttle.

The ratio between peak power RPM and peak torque RPM gives a range of “cruising” speeds for each gear that are in the same proportion – My own thumper for example has 5250/3000 ratio – In practice the peak RPM is closer to 5000, so that’s a 5:3 ratio meaning in 4^th gear, I can range from 75 to 125 KPH with full acceleration potential. If I were on an extremely steep hill where 1^st gear was the only possible option, I could go from around 30 to 50 without needing to up-shift. This is what the definition of drivability is – the engine can pull away cleanly at a range of RPMs

Not to take anything away from the way two-strokes and high revvers deliver torque – They generally have this effect of ever increasing acceleration until peak, and that’s one of the highs of riding such bikes – on the plains!

What makes an engine produce power? Essentially airflow – pump enough air through the engine and you are done – the carb or fuel injector will ensure (mostly) that there is enough fuel in that air, and when all of it combusts you have glorious horse power.

The trouble is that airflow cannot easily optimized across all the RPM ranges – it’s almost as though you can change the power curve, but must preserve the area under it. So you can make it peaky at one end, or flatten it out and reduce the peak.

I do not have enough data, otherwise we could have looked at the integral of the power curve as a real metric of overall performance – An electric motor or steam engine comes to mind as the ideal maximum: Full torque at all RPMs for a motor, and torque descending from peak value to zero linearly across the rev range for a steam engine. That means for both of these, the peak power/peak torque rev range is the entire rev range, so gearing is entirely unnecessary, except for final drive!

Once again torque should be normalized according to weight, so we have a torque to weight ratio, and again one normalized based on RPM such that making peak torque at lower RPMs gives a higher metric.

Let’s look at the figures:

I have no peak torque data for many of the classic bikes, only the current bikes are shown here

Japanese Bikes

Model	PTQ RPM	KGM	KGM/ton	(KGM/ton)/KRPM
Hero Honda CD100	6000	0.75	4.05	0.68
Pulsar DTSI 220	7000	1.9	8.52	1.22
Karizma ZMR	6000	1.83	7.82	1.30
CBR 250	7000	2.2	9.28	1.33
Suzuki Hayabusa	7000	13.5	42.59	6.08

Enfield’s

Model	PTQ RPM	KGM	KGM/ton	(KGM/Ton)/KRPM
Diesel bullet	2500	1.5	5.77	2.31
Std 500	3000	3.8	14.62	4.87
Std 350	2800	3.2	12.55	4.48
Lightning 535	3000	3.8	14.62	4.87
Enfield CL500	4000	4.1	15.77	3.94
ACE Fireball bullet	3000	5.7	21.92	7.31

Two Strokes (again RPM is factored by 2)

Model	PTQ RPM	KGM	KGM/ton	(KgM/Ton)/2KR
Jawa 250	4500	1.8	8.57	0.95
Yezdi Roadking	4500	2	9.52	1.06
Jawa 350	5000	3.1	12.76	1.28
Yamaha RX100	6500	0.9	5.06	0.39
Yamaha RX135	6500	1	5.62	0.43
Suzuki Shogun	7500	1	5.59	0.37
Yamaha RD350	6500	3.3	15.14	1.16
Maico 760	3600	8.2	43.16	5.99

Look at the figures! Is it a wonder that thumper fans scoff at crotch rockets?  Even the ‘busa, though having great torque, is outdone by the Fireball here.

The Fireball is off the charts, it could do great things off the road, probably has enough torque to hoist itself vertically on a rope at a good speed!

The new age 4 strokers are just not made for relaxed riding – They barely tract as well as the antiquated Jawas, while revving much higher – makes for a whiny washing machine like sound.

The CL500 needs to be ridden in a different style from the older bullets, same goes for the other twin spark and AVL engine ones which also have 4000 RPM peak torque, which is why probably many feel these are not “real” bullets.

The RD 350 is pretty decent, reaching almost to the level of the modern 4 strokers, although nowhere near the Enfield’s.

The Maico looks mighty impressive for a two-stroker, Pity only six of them were ever made.

Next let’s look at the RPM spread metric: 

We would like the ideal spread to be 100% like a choo-choo train – So let’s say the percentage of the RPM range between peak power and torque. We’d also like the difference in actual torque at those two points to be as little as possible, so the ratio between the torque values at peak power and peak torque is another thing we can multiply with the above.

Let’s look at RPM ratios ( The column titled RR ) - A steam engine would have a value of 1.0
PP - Peak power
PTQ  - Peak torque

Model	CC	PP RPM	PTQ RPM	RR	HP	KGM	PP KGM
Hero Honda CD100	100	7500	6000	0.20	7.5	0.75	0.70
Pulsar DTSI 220	220	8500	7000	0.18	21	1.9	1.72
Karizma ZMR	223	7000	6000	0.14	17.6	1.83	1.75
CBR 250	250	8500	7000	0.18	26.4	2.2	2.16
Honda CB750	750	8000	7000	0.13	67	6	5.83
Suzuki Hayabusa	1300	12000	7000	0.42	190	13.5	11.03

Model	CC	PP RPM	PTQ RPM	RR	HP	KGM	PP KGM
Diesel bullet	435	3600	2500	0.31	7.5	1.5	1.45
Std 500	500	5250	3000	0.43	22	3.8	2.92
Std 350	350	5650	2800	0.50	18	3.2	2.22
Lightning 535	535	5250	3000	0.43	26	3.8	3.45
Enfield CL500	500	5250	4000	0.24	27.2	4.1	3.61
ACE Fireball bullet	535	6000	3000	0.50	37	5.7	4.30

Model	CC	PP RPM	PTQ RPM	RR	HP	KGM	PP KGM
Jawa 250	250	4750	4500	0.05	12	1.8	1.76
Yezdi Roadking	250	4750	4500	0.05	16	2	2.35
Jawa 350	350	5250	5000	0.05	23	3.1	3.05
Yamaha RX100	100	7500	6500	0.13	11.5	0.9	1.07
Yamaha RX135	135	7500	6500	0.13	14	1	1.30
Suzuki Shogun	108	8500	7500	0.12	13.8	1	1.13
Yamaha RD350	350	6750	6500	0.04	30.5	3.3	3.15
Maico 760	760	4000	3600	0.10	43	8.2	7.49

The numbers speak for themselves – no doubt all these engines have usable torque for well below peak, but the fact remains that below that RPM the torque curve slopes upwards, so the ability to cruise is absent in that lower rev range.

The Hayabusa impresses again, with almost 50% of the rev range spread out between PTQ and PP!

The Enfield’s are all superlative when compared to the commuter bikes, and the Std 350 is still the king in terms of tractability! The fireball does as good on account of spreading the rev range upwards to 6000 revs

The CL500 (and all AVL and UCE engined bullets), have opted for a more rev happy engine, sacrificing that thumpy nature, by tuning it towards a later peak torque and better higher end breathing – most likely with higher valve overlap.

Here’s a simple test to determine true peak torque RPM on a bike : On a straight road, accelerate well past the estimated PTQ RPM, and then slowly back off the throttle -  the RPM keeps dropping until there’s a point where the speed of the bike stabilizes. If you go below the actual PTQ RPM, the bike will slow rapidly and ask for more throttle to maintain speed. So essentially find the slowest “cruising” RPM – you can feel the top of the torque curve.

It’s not quite that you cannot cruise at any given speed on any given bike, it’s just that if you are on the downward sloping part of the torque curve, the engine governs itself due to a negative feedback effect – if it encounters more load, the revs drop and there is more torque, and vice versa. This happens without any throttle change. Having as large a range as possible where this happens makes the ride “relaxed”.

Let’s now incorporate the ratios between the two torque values and see what we get - we get the ratio between the torque values at peak torque and peak power and multiply it by the previous RPM ratio value.

Model	Torque metric
Hero Honda CD100	0.19
Pulsar DTSI 220	0.16
Karizma ZMR	0.14
CBR 250	0.17
Honda CB750	0.12
Suzuki Hayabusa	0.34

Model	Torque metric
Diesel bullet	0.30
Std 500	0.33
Std 350	0.35
Lightning 535	0.39
Enfield CL500	0.21
ACE Fireball bullet	0.38

Model	Torque metric
Jawa 250	0.05
Yezdi Roadking	0.06
Jawa 350	0.05
Yamaha RX100	0.16
Yamaha RX135	0.17
Suzuki Shogun	0.13
Yamaha RD350	0.04
Maico 760	0.09

The metric here is

(KGM@PP / KGM@PTQ) * ((PP – PTQ) / PP)

Meaning the RPM spread times the ratio between the torques – sort of describes how flat the curve is and how wide the “plateau” is.

All said and done, the Hayabusa is more or less like an Enfield in this metric! Eminently rideable.

The UCE bullet will do fine as long as you ride at revs, and not in “Uncle” mode.

I’m a bit surprised that the L535 tops this metric – I’m sure both the CL 500 and the Fireball have a torque curve well above it in most areas, but it’s all about the plateau shape…

Among the two strokes, the RX’s are pretty awesome, figuring higher than the DTSI and the ZMR – Who said two strokes are always twitchy and peaky?

The whole point is unless you are racing and have umpteen gears, and are willing to shift every time your speed increases by 10% to 15%, having extremely peaky power curves does not make for easy riding.

I’ve seen with my own eyes a few days back, ZMRs, DTSIs, CBRs spinning wheels and stopping on an off road track. They have to be revved high in 1^st to get the torque to climb, but that needs 80% or more or so of the 1^st gear top speed. That is surely not a slow enough wheel speed to start off up a slippery slope smoothly, so the wheels spin hopelessly and much use of the clutch is necessary.

I shudder to think how a two stroke would have fared!

My own beast had enough low end to chug up along that slope at 1200 to 2000 RPM, barely above idle in 1^st gear, at almost fully closed throttle.

Now let's move on to everyone's favorite yarn - speed