How fast - on 50cc?!

[xwhatsit]

After listening to the sound of a Honda RC110 (see Who needs porn?), a 50cc GP bike from 1962, I decided to read up a bit more about this 50cc GP class. Wikipedia, as usual, has the goods (http://en.wikipedia.org/wiki/50cc).

Anyhow, what fascinated me was this;

By the end of the season, with the addition of rudimentary streamlining and the increase of carburettor size to 17 mm, the 9 hp four-speed two-stroke bikes could top over 85 mph.^[4]

What the. Apparently eventually Honda's four-stroke RC110 made 10hp, more than the two-strokes, but that's not the story here.

85mph is more than 135kph. How is it, that I'm struggling to hit 140kph in a 26hp machine, and these 9hp things were almost getting there? Even bikes like the GPX250, with fairings, and substantially more horsepower are only hitting about 160kph.

I'm going to rule out streamlining as a reason; surely modern production bikes have better streamlining than the `rudimentary' streamlining on these GP bikes in the 60s. Weight is a factor here; obviously these things weighed next to nothing, with a 50cc engine and very lightweight frames. Skinny tyres, too. But all that doesn't seem to add up to more than doubling the horsepower. Does it?

[gamgee]

probably a combination of the gearing, and making an engine which is made to perform at extremely high revs

[McJim]

How much torque were they generating?

[marty]

maybe they actually knew how to ride?

[thehollowmen]

At one time suzuki (?) had a 50cc 2stroke racer with a narrow powerband and 17 speed gearbox. They didn't have the restrictions then that we do now

I'll try and look it up.

A derestricted RS50 will do over 70 mph (112 km/h)

Then again my DL650 doesn't go much faster than 180km/h

Edit - nine speed ones here: http://en.wikipedia.org/wiki/50_cc

This still wasn’t enough to compensate for the machines relative lack of power and three weeks later at the Isle of Man TT, nine gears were fitted and the rev limit increased to 17,000 rpm with output now up to around 10 hp.[8] Sometime during the season, the machines designation was changed to RC111, but Honda’s records are unclear as to what precise change in the development this signified or when it was used.[9]

In contrast, the Suzuki team were committed to two-stroke technology and their single cylinder RM62 machine featured rotary valve induction and an 8-speed transmission and produced about 10 hp at 12,000 rpm

[James Deuce]

Quite simply it comes down to power to weight, jockey-like riders, and superb technique.

A good RG50 will do 100km/hr

[Kickaha]

They were also very light and ridden by midgets, there are guys racing in world GP at the moment who have a family history dating back to those bikes

The Suzuki RK67,50cc GP bike was a twin cylinder,revved to 17,500, had 17 gears and would supposedly do 170km

NZer Hugh Anderson won world 50cc championships in 63/64 on Suzukis

[James Deuce]

NZer Hugh Anderson...

Speaking of midgets.

[nudemetalz]

My wee RG-50 I had a number of years ago would go to 120km/h (indicated), however only on a really long straight and also only if one of the 4 other RG-50-shod riders didn't put his foot over the exhaust to choke it !!!!!!!!

[Kickaha]

There's 50cc GP bikes on here
http://www.suzukicycles.org/history/...960-1967.shtml
you'll see the 50cc triple RP68 that they built,the rules were changed to restrict them to 1 cylinder before it was raced


Some more about them
http://www.juen.com/racing50cc/

I've seen the one Hugh Anderson won his championship on in real life back in the late 1970s at a display in King Edward barracks in Chch

[Bass]

I'm going to rule out streamlining as a reason; surely modern production bikes have better streamlining than the `rudimentary' streamlining on these GP bikes in the 60s.

Tain't necessarily so!
Also, some of these guys would ride lying flat on the seat, feet out behind and changing gear with their left hand.

[James Deuce]

Actually bike aerodynamics in the '50s and '60s were much better than now, at least in regard to top speed, and the Dustbin or Porpoise fairing allowed 50-60HP 500cc machines to get close to 200mph.

The problem with motorcycle aerodynamics is that bikes roll about an axis and the moment they move off vertical the aerodynamic properties of a fairing that works well from a top speed perspective, change drastically. The Dustbin fairings were banned because there were fatalities that may have been caused by those fairings generating lift at high lean angles and speeds and literally lifting the wheels off the ground.

Motorcycle aerodynamics are still very poorly understood. What works well at one track may be a liability elsewhere. Rossi demanded a bigger fairing for the first RCV211V, but had to return to the small fairing because of the massively negative effect on turn-in the bigger fairing had.

HRC have changed the design of the rear hugger on the RC212V throughout this year. Looking at the shape after the initial photos were published for the RC212V I theorised that the shape of the hugger was developed to meld aerodynamics to rear suspension action and to create a vortex generator to deny a draft to a following motorcycle. I think it probably had a deleterious effect on top speed as the little pointed spoiler disappeared from the hugger. A flat topped hugger with spoilers on each side at the rear, angled at roughly 45 degrees to horizontal has appeared since, possibly to improve lateral grip in high speed corners.

[Coyote]

maybe they actually knew how to ride?

Knowing how to ride doesn't increase top speed in a straight line. Any chimp can wind the throttle up

[xwhatsit]

Interesting to hear about the aerodynamics. It's funny how much difference that can make to a bike -- I know that first hand; my bike will do 130-135kph in a normal riding position (albeit with clipons), however if I lie flat on the tank the speed instantly begins to climb, slowly. Yep, I can see why it'd be a very tricky job doing aerodynamics on a bike. In cage racing there's issues even with the tiny roll angle they have when cornering fast sweepers (leaning the other way, though), so 45 degrees or so is going to be a headache.

Do the gears make that much difference? I would've thought they'd let it get to the top speed much faster, but would they actually add much?

I had no idea weight played such an important role in straight-line speed. Acceleration, cornering speeds, but sheer top speed? Interesting.

[Timber020]

Knowing how to ride doesn't increase top speed in a straight line. Any chimp can wind the throttle up

With some bikes, especially ones that require some nursing, you do need to know how to ride to get the best out of them, just winding the throttle up isnt always enough.

My 2 RG's have been good for over 100. The one I have currently got will do 120kmph if I get a draft or downhill for the gap in powerband between 5th and 6th! Once at that speed and in the band it will hold it. New piston and rings might fix that issue or another tooth on the rear sprocket. The other was lower geared and would do 110. Bloody good fun having to work them so hard. 82 gear changes between karori and island bay! (and thats with a clear run!)