new here

[blew]

Knowing how Honda exaggerate horsepower figures,and the xbrs released to the Press were apparently non-standard,I simply wondered whether the bog-standard 500could actually rev as high as Honda claimed.Every road test of the xbr gives a top speed of over the magic ton-is that genuine???

[guest564]

Stock mine would do 108mph, flat on the tank, which corresponded exactly with peak power in top gear on stock gearing, it had 1200 miles on the clock and the engine was in very good condition. With the Powerbronze fairing it would do 115mph with a slight crouch. It wouldn't rev higher in fifth, even with a mild cam until the cam was changed to an HRC cam and the valve springs to race springs. I think 6800rpm was about the max even with a mild tune, with the HRC cam and it would run to 8000rpm.
Years ago I created a programme that would calculate speed through the gears v engine torque, for aerodynamic drag it used a square law. Once the drag coefficient was adjusted to give the correct value derived from testing it was very accurate. If you gear too high you lose rear wheel torque and it ends up slower. The press test bikes were dynoed at 32-35bhp at the rear wheel.

[guest564]

I've been reading this topic with great interest.I have no idea what revs an xbr 500 normally pulls,since mine had a knackered piston when I bought it.I have run my xbr for a few years now with an xl 600 barrel and piston,compression ratio unknown but pretty high.It is reluctant to rev over 6000,although it will do so in the lower gears if I insist.It pulls like a train in top,and if I want it will sit effortlessly at 80 - 85 for as long as I want.Top speed is about 95-98 sitting normally,just over 6000 rpms,which it reaches without too much effort.Question for the cognoscenti-has my 600 got a lower powerband/rev range due to having a 600 piston?The 600s have a power band about 1000 rpms lower than the 500,all else being equal.Should I fit a 16 tooth gearbox sprocket?

The bigger capacity moves peak power down the rev range so you lose top end. If the rest of the intake/exhaust is stock then you won't flow any more air than the 500 so peak power is not much different. I would leave the gearing as it is and enjoy the extra grunt/flexibility.

[Propellor]

Stock mine would do 108mph, flat on the tank, which corresponded exactly with peak power in top gear on stock gearing, it had 1200 miles on the clock and the engine was in very good condition. With the Powerbronze fairing it would do 115mph with a slight crouch. It wouldn't rev higher in fifth, even with a mild cam until the cam was changed to an HRC cam and the valve springs to race springs. I think 6800rpm was about the max even with a mild tune, with the HRC cam and it would run to 8000rpm.
Years ago I created a programme that would calculate speed through the gears v engine torque, for aerodynamic drag it used a square law. Once the drag coefficient was adjusted to give the correct value derived from testing it was very accurate. If you gear too high you lose rear wheel torque and it ends up slower. The press test bikes were dynoed at 32-35bhp at the rear wheel.

What happened on the way to the back wheel! Honda claimed 44hp at the crank and we'd expect to lose 10 to 12 percent along the way.  So very high thirties. Looks like they were telling porkies. 

[Propellor]

.

[Propellor]

.

[Propellor]

I've been reading this topic with great interest.I have no idea what revs an xbr 500 normally pulls,since mine had a knackered piston when I bought it.I have run my xbr for a few years now with an xl 600 barrel and piston,compression ratio unknown but pretty high.It is reluctant to rev over 6000,although it will do so in the lower gears if I insist.It pulls like a train in top,and if I want it will sit effortlessly at 80 - 85 for as long as I want.Top speed is about 95-98 sitting normally,just over 6000 rpms,which it reaches without too much effort.Question for the cognoscenti-has my 600 got a lower powerband/rev range due to having a 600 piston?The 600s have a power band about 1000 rpms lower than the 500,all else being equal.Should I fit a 16 tooth gearbox sprocket?

The bigger capacity moves peak power down the rev range so you lose top end. If the rest of the intake/exhaust is stock then you won't flow any more air than the 500 so peak power is not much different. I would leave the gearing as it is and enjoy the extra grunt/flexibility.

Yes, but if the power remains the same but moves (say) from 7000 to 6000 rpm, surely you'd need to change the gearing or the bike would be undergeared? Assuming the gearing was right to start with, which it seems to be.

Edit: 108mph would become 92mph. It'd feel good getting there though. 

[blew]

I'm not too bothered by the mediocre performance of my xbr-I have a pair of Kawasakis to stir the blood.I bought the bike because it rekindled memories of youthful abuse of BSA singles.It has been reliable and oiltight,nothing has fallen off despite the tingles at higher revs.And it starts first or second kick,as long as you know "the routine".The kickstarter is the closest the xbr gets to a BSA single...But why go to the trouble of radial valves,complicated rocker arrangement,short stroke engine,yet producing no more power than a good British single of 30 years earlier?If Honda had given the xbr another ten horses,to equal a Goldie or a Thruxton,but with Japanese reliability,they might have produced a classic roadburner.Perhaps a standard xbr for commuting,and a sporty version for the hooligans....Imagine a GB 600 with a genuine 42 horses...

[Propellor]

I'm not too bothered by the mediocre performance of my xbr-I have a pair of Kawasakis to stir the blood.I bought the bike because it rekindled memories of youthful abuse of BSA singles.It has been reliable and oiltight,nothing has fallen off despite the tingles at higher revs.And it starts first or second kick,as long as you know "the routine".The kickstarter is the closest the xbr gets to a BSA single...But why go to the trouble of radial valves,complicated rocker arrangement,short stroke engine,yet producing no more power than a good British single of 30 years earlier?If Honda had given the xbr another ten horses,to equal a Goldie or a Thruxton,but with Japanese reliability,they might have produced a classic roadburner.Perhaps a standard xbr for commuting,and a sporty version for the hooligans....Imagine a GB 600 with a genuine 42 horses...

Good question. I remember reading a review by John Robinson when the xbr came out. He said it made extremely good bmep.

Kevin camerson's book classic motorcycle race engines covers a lot in this regard. Especially the two valve ducati 's contrasted with Keith duckworths four valves. Swirl versus tumble. There's more to it than meets the eye!

[guest564]

Yes, but if the power remains the same but moves (say) from 7000 to 6000 rpm, surely you'd need to change the gearing or the bike would be undergeared? Assuming the gearing was right to start with, which it seems to be.
Edit: 108mph would become 92mph. It'd feel good getting there though. 

Once you raise the gearing the torque at the rear wheel goes down and it ends up slower.

I wrote my programme based on the original programme in John Robinson's book, which now appears to sell for a stupid price.
http://www.amazon.co.uk/Motorcycle-Tuning-Four-Stroke-Robinson/dp/0600333655
I added better graphing and torque through the gears. One day I will sit down and rewrite that programme.

[guest564]

What happened on the way to the back wheel! Honda claimed 44hp at the crank and we'd expect to lose 10 to 12 percent along the way.  So very high thirties. Looks like they were telling porkies. 

That was par for the course back then, the XL600LM was rated at 44bhp but on one magazine test dynoed it at 27bhp at the rear wheel.

[blew]

The original CB 750 was a good example of wishful thinking regarding bhp.The first 750 could manage 125 or thereabouts.The final CB750 K7 struggled to reach 110.Yet according to Honda they both produced equal power-67bhp.Honda simply could not admit that the 750 had been gradually detuned and strangled.

[Propellor]

Yes, but if the power remains the same but moves (say) from 7000 to 6000 rpm, surely you'd need to change the gearing or the bike would be undergeared? Assuming the gearing was right to start with, which it seems to be.
Edit: 108mph would become 92mph. It'd feel good getting there though. 

Once you raise the gearing the torque at the rear wheel goes down and it ends up slower.

I wrote my programme based on the original programme in John Robinson's book, which now appears to sell for a stupid price.
http://www.amazon.co.uk/Motorcycle-Tuning-Four-Stroke-Robinson/dp/0600333655
I added better graphing and torque through the gears. One day I will sit down and rewrite that programme.

You're right...... If you took a standard xbr, or any bike, and raised the gearing you'd lower the rear wheel torque. But........

We're talking here about a different bike than the standard xbr. One where the engine power and torque will have been shifted 1000 rpm down the Rev range. So, if you want to keep the rear wheel rpm matched to peak power engine rpm you'll need a different overall ratio. True, if you want to raise the rear wheel torque, then lower the gearing. But to attempt to keep the maximum power allied to the same road speed on an engine which makes that max power much lower you'll need a different overall ratio. Higher gearing.

Do the maths. Assuming the same max power (which the two examples pretty much are) and assuming the same road speed (which we can, because we already know that the power we have will get us there), then we're saying that the final rpm, the rear wheel rpm, will stay the same. Using the power formula and cross multiplying you will compute the same rear wheel torque. You have to! The only two variables are both the same value.

So, to achieve the same rear wheel rpm but with an engine which is giving that same power at a different rpm we will need a different ratio.

Would you not agree?




Edit: if we had full power and torque curves then even better. We can plot rear wheel torque curves for each gear and compare to stock xbr. We can also see how the power tails off after peak power rpm. Maybe that would influence our overall ratio choice. But the stock xbr is a known quantity and so is valuable as a datum point.

[guest564]

Do the maths. Assuming the same max power (which the two examples pretty much are) and assuming the same road speed (which we can, because we already know that the power we have will get us there), then we're saying that the final rpm, the rear wheel rpm, will stay the same. Using the power formula and cross multiplying you will compute the same rear wheel torque. You have to! The only two variables are both the same value.

But now if you raise the gearing 10% then you have 10% less torque at the rear wheel. You may be able to go a little higher but, in my experience, nothing kills the top speed of an XBR faster than getting the gearing too high. If you want to go faster then you need a cam that pushes peak power back up the rev range.

[Propellor]

Do the maths. Assuming the same max power (which the two examples pretty much are) and assuming the same road speed (which we can, because we already know that the power we have will get us there), then we're saying that the final rpm, the rear wheel rpm, will stay the same. Using the power formula and cross multiplying you will compute the same rear wheel torque. You have to! The only two variables are both the same value.

But now if you raise the gearing 10% then you have 10% less torque at the rear wheel. You may be able to go a little higher but, in my experience, nothing kills the top speed of an XBR faster than getting the gearing too high. If you want to go faster then you need a cam that pushes peak power back up the rev range.

I would say that if you want to go faster you need more power (or less drag or both). It wouldn't matter (as a design principle) what engine rpm you got your extra power, that's the job of the transmission.

Regarding raising the gearing. Yes, I fully agree that raising the gearing of a given machine will lower the rear wheel torque. But in the context of questions asked in this thread, what we are doing is keeping the engine power the same but moving it 1000rpm lower. To put it another way we are raising the engine torque. So to raise the gearing in this instance,by the percentage difference in the engine rpm, won't lower the rear wheel torque. It will keep it the same as the std xbr.