Considering BMW‘s 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, it‘s something of a surprise to find the bike demands a downshift or two to accelerate sharply when overtaking.
It‘s not bad, but it doesn‘t feel as if it has the punch of BMW‘s own K1300GT, so I‘ve 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 it‘s 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).
Calculating 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 (I‘ve used top gear for both bikes) and the final drive ratio - fortunately BMW provides all of these.
I‘ve 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:
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
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 there‘s 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. I‘ll 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. I‘ll 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:
@ 2,500rpm - 1.16Nm/kg
@ 3,500rpm - 1.34Nm/kg
@ 2,500rpm - 1.22Nm/kg
@ 3,500rpm - 1.30Nm/kg
The GTL's figures meanwhile are:
@ 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 isn‘t the whole story - sorry, there's more maths to come...
Comparing the 1300GT and 1600GT at the same rpm isn‘t what happens on the road. When you‘re riding, it‘s road speed that counts, for example when you‘re following a car at 60mph (100kph), it‘s 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.
I‘ll 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
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 1600‘s 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 bike‘s sophisticated, smoother response which softens the initial acceleration.
A bit of feel, but mostly physics...
By popular request (somebody asked...), these are the comparative figures for the R1200RT (current dohc RT figures used):
Overall transmission gear ratio: 3.81 (clutch 1.737, top gear 0.848, final drive 2.62)
Engine rpm at 60mph: 3,177rpm
Engine torque at 3,177rpm: 98Nm
Rear wheel torque at 3,177rpm: 373Nm
Bike weight: (quoted dry at 229kg, est +30kg for fuel, oil etc, +90kg for rider) 349kg
R1200RT rear wheel torque per kg at 100kph (60mph): 1.06Nm
So, trade up from the RT to the 1600GT and you'll get significantly stronger top gear acceleration, as you'd hope, although do bear in mind that a twin has a naturally punchier deliver than a six, so to a degree this is disguised.
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