Torque curves

Something SteveA said in the topic of gearing for speed, has got me thinking. He mentioned the flat torque curve of the 850 Commando engine. With my 850, the torque is brutal. Every time I have raised the overall gearing, the bike has accelerated faster. But it runs out of puff towards the ends of the long straights. I don't know if that is due to running out of gears, or going past the max of the torque curve in the face of the greater wind resistance. I don't know what the actual shape of the torque curve is, so I might be revving well beyond the max.
In Motorcycle Classics magazine, there is an article about the new 900 Triumph twin engine. It says :
'the 2019 version of the 900HT (High Torque) engine delivers 59 ft/lb at 3,800 rpm, but now more widely spread across the whole rev range. There is a claimed 18-percent greater peak power than before, with 64 horsepower on tap at 7,500 rpm versus the previous version's 54 horsepower. This power has ben obtained via a lighter crank-shaft, balance shafts and clutch, all of which allow the engine to spin up faster.'

I am interested in putting my bike on a dyno to get an idea of what the torque curve looks like. Can most dynos provide that information fairly accurately and is a flat torque curve better for racing than one with a peak ? My 850 seems to pull from whoa to go.

All dyno's measure is torque. Horsepower is calculated by multiplying torque times rpm times a constant.

acotrel said:

'the 2019 version of the 900HT (High Torque) engine delivers 59 ft/lb at 3,800 rpm, but now more widely spread across the whole rev range. There is a claimed 18-percent greater peak power than before, with 64 horsepower on tap at 7,500 rpm versus the previous version's 54 horsepower. This power has ben obtained via a lighter crank-shaft, balance shafts and clutch, all of which allow the engine to spin up faster.'

I am interested in putting my bike on a dyno to get an idea of what the torque curve looks like. Can most dynos provide that information fairly accurately and is a flat torque curve better for racing than one with a peak ? My 850 seems to pull from whoa to go.

Click to expand...

I don't think lightening things give you more power or torque so they must have done something else to the motor for that.

As htown16 says the dyno measures only torque, and they can vary in the numbers given, but, that doesn't matter because you will get a good pictoral indication of your torque curve with the revs linked to the curve.

This is my one and you can see the torque dropping from about 6000rpm, it looks bad on the graph but it only looses about 4 ftlb by the time it hits the limiter.

RPM x Torque divided by 5250 = HP
or RxT/5250=HP

If I put my bike on the dyno, does it matter what gear is engaged when measuring torque ? ( the gearbox converts torque ? ) As It understand it, torque in ft/lb is twisting force and horsepower is the time rate of doing work, so that probably means that the horsepower relative to the revs is torque - or does it ?
Thanks for the dyno graph, it sort of agrees with the picture I had in my mind. If the torque drops off at about 6000 rpm, it must deliver a slower acceleration rate if you rev beyond that and don't change up ? With my 850, I always try to change up just before 7000 rpm, but often see 7,500 rpm - when I am game enough to look. I know what 7000 rpm sounds like. I don't look at the rev counter when I am approaching the end of a straight. I just know the bike is a bit too slow there.

The fella with the 6 speed box in his sidecar has got me enthused. There is a guy about 50 Km from here who has got a dyno which is used by one of our superbike teams. That modern Triumph twin has got me interested. It is heavier than the Seeley and the steering geometry seems strange, but it must have some go - for what it is.

I was also wondering about the comment in the article in relation to lightened internals in the motor. Once the heavy crank is spinning over 6000 rpm, it does not have far to go to reach the rev limit of 7000 rpm. And if the gears are close, the step up from one gear to the next does not require much throttle response. My feeling is that the heavy crank makes the bike accelerate quicker when you race-change. If you try to wind the commando crank up from 4000 rpm to the rev limit of 7000 rpm, every time you change up - you would wait forever for the response to the throttle.
When I raced in the 70s, many of the races included Z900 Kawasakis which probably turned out 100 BHP. The guys who rode them on Winton circuit back then, probably could not use that sort of power effectively. In fact I have seen a guy on an RS125 Honda trounce a field of 600cc sports bikes. He went around the outside on every corner.

The gearbox is a torque multiplier so there would be more torque at the back wheel in the lower gears but that doesn't change the real torque of the engine or its curve, the dynos have some algorithms used by the software that take the rate a known weight is accelerated and decide what the numbers given are going to be, and as they don't know the primary reduction or the internal friction of the gearbox ect, i think the numbers are estimates. I would guess that the visual curve you are after would be the same shape

Again that doesn't really matter as the dyno is a development tool so when you start with a dyno you need to use the same one each time for any meaningful info.

The heavy crank will take longer to spin up but when spinning it will have more kinetic energy to release so you probably feel a bigger kick in the pants from a gear change with a heavy crank but it has probably took longer to need the gear change than a lighter crank would. you don't get anything for nothing.

all the above is me thinking out loud so I would check my statements.

The Commandos flat torque curve is surely a wonderful thing !

Interesting comments by Al, when I first bought my Commando it had good torque, and pulled well, right from tickover. But then it ‘ran out of puff’ noticeably at around 5,000 but in my case this was due to it having a single 36mm Mikuni carb!

A bike that pulls well low down with its torque, then runs out of puff higher up, is probably not producing enough bhp to keep the acceleration progressing as desired.

It’s being ‘held back’ somewhere / somehow.

Look at the fuel curve of the above trace, and how it changes in relationship to the curves above, it is typical. So yes, it is held back, as the revs increase and it leans out slightly the power goes up, torque drops, if you could keep the if it could 'breathe more in ' the torque curve would roll later You can see the same below.

At large throttle openings, the motor tends to lose vacuum, so the main jets are larger. But for a torquey motor, the inlet port diameters are usually smaller than for a top end motor. With a Commando motor, you have pretty much a fixed rev limit, so a top end motor defeats itself. At the ends of the straights, you come up against the greatest wind resistance, with full open throttle and the engine revving beyond peak torque in the Commando motor. So the motor stops pulling hard enough to go faster. So what it means is that on a race circuit, you have to go faster everywhere else and certain circuits favour good handling Commandos.

If you think about it, it is not so difficult to outride somebody who is struggling with a 100 BHP superbike around a fairly tight race circuit. On Winton some of the guys ride around bouncing off the ripple strips. My bike goes nowhere near the ripple strips. I just sit there like a dummy and let it do it's own thing, while risking brain fade. I hope that with the 6 speed box, I will have enough gears to keep the bike pulling at the ends of the straights, while still accelerating hard everywhere else.

Al here is a torque curve for an 850 Commando with a RH10 head, standard Vv's, standard manifolds 32 mm Premier carbs, JS2 cam and a 2into 1 exhaust
Its my race bike , although I have since changed the head to a FullAuto item with longer inlet tracts and 34 mm smoothbore Amals Mk 2.
It was sampled in 4th gear , it is a 5 speed, the reason being it gives a longer sample time instead of running it lower gears and unless you need to measure top speed, no need to use 5th gear
the crossover for the torque/HP curves is right where is should be at 5250 rpm
Regards Mike

acotrel said:

If I put my bike on the dyno, does it matter what gear is engaged when measuring torque ? ( the gearbox converts torque ? ) As It understand it, torque in ft/lb is twisting force and horsepower is the time rate of doing work, so that probably means that the horsepower relative to the revs is torque - or does it ?
Thanks for the dyno graph, it sort of agrees with the picture I had in my mind. If the torque drops off at about 6000 rpm, it must deliver a slower acceleration rate if you rev beyond that and don't change up ? With my 850, I always try to change up just before 7000 rpm, but often see 7,500 rpm - when I am game enough to look. I know what 7000 rpm sounds like. I don't look at the rev counter when I am approaching the end of a straight. I just know the bike is a bit too slow there.

The fella with the 6 speed box in his sidecar has got me enthused. There is a guy about 50 Km from here who has got a dyno which is used by one of our superbike teams. That modern Triumph twin has got me interested. It is heavier than the Seeley and the steering geometry seems strange, but it must have some go - for what it is.

Click to expand...

torque is in ft-lbs, not ft divided by lbs. Maybe U know, but many people have a block when writing it. Just remember it is force on a lever: 58 lbs at 1 ft radius is 58 ft-lbs. ( slightly pedantic rant over!)

Brooking 850 said:

Al here is a torque curve for an 850 Commando with a RH10 head, standard Vv's, standard manifolds 32 mm Premier carbs, JS2 cam and a 2into 1 exhaust
Its my race bike , although I have since changed the head to a FullAuto item with longer inlet tracts and 34 mm smoothbore Amals Mk 2.
It was sampled in 4th gear , it is a 5 speed, the reason being it gives a longer sample time instead of running it lower gears and unless you need to measure top speed, no need to use 5th gear
the crossover for the torque/HP curves is right where is should be at 5250 rpm
Regards Mike

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Nice curves.

Should make its way around the track fairly briskly that Mike !

Just get it on the dyno and you’ll learn a lot.
Things to bear in mind though: I struggled to find a dyno operator in Australia who would let me into the room to work on the bike. Most of them said “ leave it with us and we’ll sort it out”.
Call them first and explain that you need to do the jetting changes etc. the place I use in Brisbane are used to classic bike racers and understand that we know the intricacies of our bikes.

Are the NZ classic race bikes running alcohol fuel?


Glen

Brooking 850 said:

the crossover for the torque/HP curves is right where is should be at 5250 rpm

Click to expand...

The crossover point HAS to be at 5250 because of the formula used to convert torque to HP. If ever the torque curve and HP curve do NOT cross at 5250, a non-standard conversion that can't be used for comparison with others has been used.

pommie john said:

Just get it on the dyno and you’ll learn a lot.
Things to bear in mind though: I struggled to find a dyno operator in Australia who would let me into the room to work on the bike. Most of them said “ leave it with us and we’ll sort it out”.
Call them first and explain that you need to do the jetting changes etc. the place I use in Brisbane are used to classic bike racers and understand that we know the intricacies of our bikes.

Click to expand...

I once had a 20 year old kid offer to rebuild my race motor for me. He was lucky I did not strangle him. I would not lot any one of those morons put any sort of spanner to my bike. If it gets re- jetted or the timings adjusted - I do it - NOT them. If it was a modern computerised bike, I might think a bit differently. But would you trust renovation of the Mona Lisa to a house-painter ? All I need is one set of graphs as the bike stands in it's current state. If they cannot do that, they can get stuffed. If I get a set of numbers, then if I get it performing better, I will take it back and have it measured again.

A lot of things in life are based upon speculation by 'experts'. With an old race bike, nothing beats experience. I am not blowing my own trumpet, because there have been far better guys than myself at building fast bikes. But often the theories do not work in practice. For example, the theory about testing oxygen in the exhaust gas to get the jetting right. It has to be different for every type of fuel you use. Do the dyno guys tune your bike for their fuel or yours' ? In Australia, use of methanol as fuel is common in two historic racing classes. Phil Irving wrote 'if you rum methanol rich, you still get good power'. So the guys all run methanol rich. When I jet for methanol, I lower the needles until I get the cough, then raise them one notch . My mate raises the needles until he gets 8 stroking, then lowers them one notch - it is safer, but slower. There is a significant difference in performance. If I wanted to use the dyno to jet for methanol using an oxygen meter, I would probably have to correlate the oxygen content of the exhaust with best performance - before I put the bike on the dyno, to calibrate the oxygen meter. In effect I would be working backwards. What works is what is important, NOT the numbers. The shape of the torque curve is of more interest than the actual values, unless you are looking for improvement, even then the calibration is usually relative, not absolute.