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post #1 of 3 (permalink) Old Apr 24th, 2011, 9:25 am Thread Starter
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Found this artical...thought I would share it.

BMW K1600GT torque comparison
Kevin Ash

(click on images)
K1600_tech_03

Considering BMWs claim of huge torque figures at low revs for the new K1600GT (full riding review here) and the GTL, which has an identical engine and drive line, its something of a surprise to find the bike demands a downshift or two to accelerate sharply when overtaking.

Its not bad, but it doesnt feel as if it has the punch of BMWs own K1300GT, so Ive compared various figures taking the overall gearing into account. What matters here is not the torque at the crankshaft but the torque at the rear wheel, as its this which is driving against the Tarmac. Both bikes have the same rear tyre outside diameter, so we can ignore that. (Note: R1200RT figures have been added at the end of this feature for comparison).

K1600GT power and torqueK1600GT power and torqueK1300GT power and torqueK1300GT power and torqueCalculating rear wheel torque is simple when you have the various gear ratio figures, as all you need to do is multiply the primary drive ratio, the gearbox ratio (Ive used top gear for both bikes) and the final drive ratio fortunately BMW provides all of these.

Ive taken torque figures at 2,500rpm and 3,500rpm for both bikes from the two graphs you can see on this page.

The figures are as follows:

K1300GT
Primary drive 1.559:1
Top gear 1.015:1
Final drive 2.82
Overall ratio 4.46
Torque @ 2,500rpm 98Nm
Torque @ 3,500rpm 113Nm

Rear wheel torque @ 2,500rpm 437Nm
Rear wheel torque @ 3,500rpm 504Nm

K1600GT
Primary drive 1.617:1
Top gear 0.788:1
Final drive 2.75
Overall ratio 3.50
Torque @ 2,500rpm 145Nm
Torque @ 3,500rpm 154Nm

Rear wheel torque @ 2,500rpm 507Nm
Rear wheel torque @ 3,500rpm 539Nm

The 1600 clearly has more rear wheel torque, particularly at 2,500rpm, but theres another factor to take into account, which is the overall weight. BMW claims 319kg for the K1600GT with 90 per cent fuel load and no panniers. Ill add another 6kg for these and 90kg for the rider, taking the total to 415kg.
The K1300GT has a claimed weight with panniers and a full tank of 288kg. Ill take 2kg off that for the 90 per cent fuel load and add the same 90kg rider, for a total of 376kg.

This produces rear wheel torque per kilogramme figures as follows:

K1300GT
@ 2,500rpm 1.16Nm/kg
@ 3,500rpm 1.34Nm/kg

K1600GT
@ 2,500rpm 1.22Nm/kg
@ 3,500rpm 1.30Nm/kg

The GTL's figures meanwhile are:
K1600GTL
@ 2,500rpm 1.15Nm/kg
@ 3,500rpm 1.23Nm/kg

So, the 1600GT has 6 per cent more torque per kilogramme at 2,500rpm (at the rear wheel) but 3 per cent less at 3,500rpm, a typical rpm figure when overtaking. This already suggests it's going to feel less muscular than the K1300GT as the speed builds at this level, while the GTL never beats the 1300 in this range, but there's more to it than that...

This still isnt the whole story sorry, there's more maths to come...

Comparing the 1300GT and 1600GT at the same rpm isnt what happens on the road. When youre riding, its road speed that counts, for example when youre following a car at 60mph (100kph), its how the bike responds in top gear at that speed which matters to how easily it overtakes, and the rpm is different on each bike.

Ill stay metric for this because the sums are much easier, and work backwards from each bike travelling at 100kph.

100kph = 27.8m/s
Rear wheel circumference is 2m so rear wheel rpm is 13.9 revs per second, which is 834rpm. This is the same for both the 1300 and the 1600.

To get engine rpm for each bike at this road speed, multiply the wheel rpm by the overall transmission ratios.

K1300GT: 4.46 x 834 = 3,719rpm @ 100kph
K1600GT: 3.50 x 834 = 2,919rpm @ 100kph

The 1300 then is revving higher at 100kph, so we must read the engine torque from the graphs at these different rpm figures:

K1300GT: 113Nm @ 3,719rpm
K1600GT: 151Nm @ 2,919rpm
The 1600 is little different, but the 1300 has gained a lot.

This leads to the following rear wheel torque figures:
K1300GT: 113 x 4.46 = 504Nm
K1600GT: 151 x 3.50 = 528Nm

And from those, the rear wheel torque to weight ratios:
K1300GT: 1.34Nm/kg @ 100kph
K1600GT: 1.27Nm/kg @ 100kph
K1600GTL: 1.21Nm/kg @ 100kph

Conclusion:
In other words, the K1300GT has 5.5 per cent more wheel torque per kg available at the back wheel than the K1600GT at 100kph (62mph), and 10.7 per cent more than the GTL. So despite the engine specs, the GT is the faster top gear accelerator at these speeds.

On top of this, further factors need to be taken into account. One is the 1600s larger frontal area, which means more drag to overcome, and as the speed builds this becomes very significant because drag increases with the square of speed. The second is the nature of the power delivery: the K1300GT has quite an aggressive throttle response compared with the six-cylinder bikes sophisticated, smoother response which softens the initial acceleration.

A bit of feel, but mostly physics...

BMW K1600GT review
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post #2 of 3 (permalink) Old Apr 24th, 2011, 12:22 pm
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Interesting stuff, especially with the K1600GTL weighing 86 pounds more than the K1300GT and losing in the passing category too. That engine has to be detuned as it is to ever see the day in a K1600R or RS configuration.

So maybe even the lowly K1200GT is even faster on a pass than the K1600GTL? Providing it doesn't chunks in the effort.


Mack
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post #3 of 3 (permalink) Old Apr 24th, 2011, 12:50 pm
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Lol. Great find KGK1200GT, things often end up differently than you expect when you 'do the math.'

Last edited by smiller; Apr 24th, 2011 at 3:17 pm.
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