How high/fast does an airhead rev

pete.keys

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As an esoteric exercise I am interested in knowing how fast an airhead revs. Not cause I want to go faster, just because I'd like to know the limitations. Well actually I would like to go fast - every now and again :-)

My G/S has loads of power and drives straight up to its flat out speed around 105 but then doesn't get any faster. It gets there so fast and powerfully that I presume it doesn't go faster is because at this speed the engine just won't rev higher due to constraints:
- valves can't operate any faster
- no more fuel can get in

Are these the constraints? What others are there?

To go faster then:
- stronger springs & less valve train inertia
- bigger valves

Interesting I calculated that at 105mph the bike would be at 7109rpm which doesn't sound particularly fast?

Calculation I did was


Radius measured from ground to centre of wheel 318mm

Circumference = 1998.312

number of turns for a mile.....

1mile =1609.344 meters

Therefore the wheel needs to turn 1609.344 / 1.998312 times = 805.352 times

at 80mph, travels 80 x 805.352 = 64428.137

so now divide by 60 to get per minute = 1073.80

final drive ration is 37/11 os final drive input revs = 3611.88

Top gear ratio is 1.5 so engine revs should be 5417

Ok so at 100mph revs are 6771

and full speed 105mph 7109rpm
 
Wind resistance will stop it gaining more speed easily, lots of power needed for every MPH gained after a certian point, whereas a narrow set of bars and lowering the bike will be cheap by comparison and as effective.

Shep
 
Shep said:
Wind resistance will stop it gaining more speed easily, lots of power needed for every MPH gained after a certian point, whereas a narrow set of bars and lowering the bike will be cheap by comparison and as effective.

Shep
Click to expand...

Wind resistance is important - but my bike will run at 105ish mph into a head wind or tail wind - so this isn't the restraining factor on my bike. I guess its got the HP to go faster, it something else that's stopping it
 
'Engine power required' is nominally related to speed cubed (ok, it's a simplification but good enough for this case).

So to double the speed of a vehicle we need 8 times the engine power.

That's why you run out of puff :)
 
pete.keys said:
Calculation I did was


Radius measured from ground to centre of wheel 318mm

Circumference = 1998.312

number of turns for a mile.....

1mile =1609.344 meters

Therefore the wheel needs to turn 1609.344 / 1.998312 times = 805.352 times

at 80mph, travels 80 x 805.352 = 64428.137

so now divide by 60 to get per minute = 1073.80

final drive ration is 37/11 os final drive input revs = 3611.88

Top gear ratio is 1.5 so engine revs should be 5417

Ok so at 100mph revs are 6771

and full speed 105mph 7109rpm
Click to expand...


My head hurts....:(
 
pete.keys said:
As an esoteric exercise I am interested in knowing how fast an airhead revs. Not cause I want to go faster, just because I'd like to know the limitations. Well actually I would like to go fast - every now and again :-)

My G/S has loads of power and drives straight up to its flat out speed around 105 but then doesn't get any faster. It gets there so fast and powerfully that I presume it doesn't go faster is because at this speed the engine just won't rev higher due to constraints:
- valves can't operate any faster
- no more fuel can get in

Are these the constraints? What others are there?

To go faster then:
- stronger springs & less valve train inertia
- bigger valves

Interesting I calculated that at 105mph the bike would be at 7109rpm which doesn't sound particularly fast?

Calculation I did was


Radius measured from ground to centre of wheel 318mm

Circumference = 1998.312

number of turns for a mile.....

1mile =1609.344 meters

Therefore the wheel needs to turn 1609.344 / 1.998312 times = 805.352 times

at 80mph, travels 80 x 805.352 = 64428.137

so now divide by 60 to get per minute = 1073.80

final drive ration is 37/11 os final drive input revs = 3611.88

Top gear ratio is 1.5 so engine revs should be 5417

Ok so at 100mph revs are 6771

and full speed 105mph 7109rpm
Click to expand...

This seems to ignore any differential of the tyre diameter due to centrifugal forces once it's spinning hard (could be 1/2 difference), very poor science IMO:rob:D:augie
 
Proff said:
All I know is that @ 7,250 rpm my R90s is doing a genuine [mira checked] 126 mph....

hope that helps :D
Click to expand...

hmmm if you have an 18" wheel and 32/11 rear drive and standard final ratio you should have been doing 7379rpm - sure your rev counter is accurate? ):augie

Eitherway I am interested in why you bike can pull higher rpm than mine?
 
Dell Orto carbs and mildly tuned to cash in on the Daytona victories in the 70's :D
Plus 11/33 final drive1973/dec74......... 11/32, 1975 on models

My '75 Orange one revs to 7500 through gears but tops out at 7250 in 5th... plus has the high ratio final drive..:drool
My '74 Smoke grey one revs to 7500, but not tried top speed yet on this one..:rob
Most '75 and onward 90s's [eleccy foot only] will only rev to 7200.....as the cam was changed...[ not a lot of people know that :augie] both mine of course, have the early beefy cam fitted :D
 
My VFR is just clearing its throat at 180 km/hr and it has brakes and handling to match ---------.

I read somewhere that the problem with high revs on an airhead was crankcase rigidity - start getting a few revs on it and you need a bit more piston / head clearance that you would ideally want as squish clearance.
Over 2 mm was suggested, so if you have a decent carbon build up on a squish band motor you may be getting close to touching at high revs.
Push rod flexing is reputed to become a problem too, as the angle of the followers is less than optimum.

No problems on a race engine , but probably reason for caution on a 40 year old bike, particularly as the brakes and handling are , err, not up to modern standards.
 
The bike is flat out! If you want to go faster change it or spend a small fortune on tuning, larger valves,different cams, different carbs, different exhaust system, different ratios, better brakes, gas flowing etc..... You will still have an old bike with old technology.

I assume you don't want more speed for road use as your bike is already capable of exceeding the speed limit by 35mph. Your theoretical maths don't totally stack up off a rolling road. Drag has a major effect above 30mph.
 
+1 - why ?
Going to cost a lot of £££'s for much more power - .
I'll be selling my Mikuni Carbs soon if you're interested ? !!

Quote "Its just every now and again when those oilheads and sports bikes get onto the straight bits and get away ..."

You'll catch the sports bikes at the Petrol Stations, ...

My previous airhead RS mono was pretty fast - around 117 on the speedo no problem, but that had a loverly fairing on it, and very narrow bars.

Conclusion - horses ( read brake horses !) for courses .
But I'd still love one of Ritchie's bikes - :augie:D
 
If it does the same speed into a head wind as with a following wind you are running out of revs.

You need to change the final drive ratio:)

Of course that will mean that you have less effective engine breaking to assist the pipe cleaner forks in their effort to retard an overweight vintage technology machine.

You may want to consider better forks:D
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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