sprockets & Derbi GPR

Tony Sherriff /

-----Original Message-----

From: don-ohio <simon@mopedarmy.com>

Date: Friday, September 14, 2001 11:15 PM

Subject: Re: Sprocket change [1:15557:15561]

>This message was sent from: General Moped Discussion.




> Hi,Tony! If this is roughly the same bike Scott Huvler showed me the


>day,it has 12,000 rpm to play with.To increase power with such awesome rpm

>would require only a minor sprocket change,also due to the 6 speed tranny

>range.The bike I saw developed 14 h.p. with a 50cc engine.Be careful not to

>over-rev man!




>Sent using Phorum software version 3.2.11 http://phorum.org

Many Thanks for the reply Don-Ohio,

Probably is the same bike, as specs you gave are about right. My aim is

to be able to use the full range of the gearboxas at present 5th & 6th are

realy only fun gears! The bike runs at about 60 kph at 5000 rpm in top & as

you say goes to 12000 (actually 11000 to red line, but who's counting!).

I've noted some critisism from members that this is not a true moped, to

some extent I agree as it is really a scaled down superbike, but in some

countries it is legally classed as a moped (50cc version, not the 75cc

GPR100 version). However I argue that it is alot safer in some ways due to

it's greathandling & braking capabilities. As with any moped or bike you

have to respect the bike & road conditions.

Safe riding, TONY.


Re: sprockets &amp; Derbi GPR

Yeah,Tony! I wish it WAS a moped in Ohio! But I'd probably ride it so slow the wheel bearings would seize up.LOL.Let me know how a sprocket change affects it,O.K.?Be safe,man!

Re: sprockets &amp; Derbi GPR

I wish I had your problems. Can you tell me if your bike engine has a flapper valve. That is, a mechanism for blocking the intake port when the fuel/air mixture is pressured into the cylinder?

Re: sprockets &amp; Derbi GPR

Ron Brown /


That would be a "reed" valve.


Re: sprockets &amp; Derbi GPR

Tony Sherriff /

Hey Oldtimer,

Yes, fuel is drawn into the crankcase through a reed block consisting of

six reeds (two sets of three carbon fiber reeds) as the piston rises a vacumm is formed in the crankcase and fuel / air mix is drawn into the crankcase through the reeds which open under vacumm, as the piston decends the fuel/air mix is presurised in the crankcase which closes the reed valves against their sealing surface and transfers the pressurised fuel /air /oil mix to the cylinder via transfer ports in the block. Oil is introduced into the fuel / air mix (by way of high

pressure injection pump driven via gear drive taken from the crank.) between the carb and reed block by way of a small spray valve in the inlet tract. Hope this explains the process.

All the best, TONY


Re: sprockets &amp; Derbi GPR

Thanks for the explanation. This is how a 50 cc can develop a lot of horsepower. The typical moped has no reed (flapper) valve and depends on the resonance of the intake piping. The charge to the cylinder is nowhere near what you get when you have a reed valve. I have this notion about designing a reed valve for my ped and seeing what happens. Do you think this is reasonable?

Re: sprockets &amp; Derbi GPR

Tony Sherriff /

Hey oldtimer,

I think it would be quite difficult to do as there would be alot of flow calculations that you would have to get just right. I am not sure what make of engine you would want to convert but crankcase volume is critical for this system to work properly. Too small a volume will not give enough draw through the carb, likewise to large a volume would allow the mixture to compress in the case reducing the efficiency of the cylinder charge. also timing for the introduction of fuel to the cylinder (and exhaust is achieved by the positioning of the ports in the cylinder, again this positioning is critical. If your engine is of the disk valve type then the timing of fuel inlet is determined by how the disk is keyed to the crank. I do not believe adding a reed valve to this would work as you must still use the disc valve to retain correct fuel inlet timing.There other types of layouts that use a reed valve that leads directly to the cylinder & does not rely on crank case compression but again uses ports in the cylinder that are uncovered by the piston at precise times in relation to crank angle - again fuel inlet timing, this type of engine rely's heavily on the scaviging effect of the exhaust stroke - again by way of precisely positioned ports in the cylinder. So to sum it up it would be very difficult to convert a non reed valve engine, But then again we have put men on the moon!!!

All the best, TONY.

reed valves


a reed valve by itself is not the performance improver

What a reedvalve does is allows an engine designer to build more radical port timing into the cylinder without having to worry about the intake charge blowing back through the carb so much.

And there is nothing that critical about crankcase volumes or timings that make the installation of a reed valve so tricky.

If you simply used an average sized reed valve, in the average location... it would work fine.

Doing the aluminum welding and fabricating would be the hard part of putting a reed valve on a 'piston port' cylinder.

But the true performance increase would only come with altered cylinder port timing and higher compression and a correctly designed expansion chamber (exhaust pipe) and carburetor size..

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