View Full Version : Quick question

Nickinoue - n/a
1-Mar-06, 10:41 AM

Is the crankshaft speed for the yamaha RD250<font color="deeppink"><font size="1"><font size="1"><font size="4">LC[/COLOR][/COLOR][/COLOR][/COLOR] Becasuse i have had conflicting infomation that it is either 8500rpm as said in the constructors guide or 10,000rpm by people who race these bikes?

Which one should i use and what ratio would be the best to get the most out of the fan considering i have a 750mm 5z fan and duct.

Thanks guys


BTW-I know this engine is rubbish

tonybroad - n/a
1-Mar-06, 12:17 PM
I had the same question with my first TZR craft and the chances are you won't get a direct answer

Bikes go to 10K because they go downhill on very long straights with the wind behind them - the fan on a craft is the perfect rev limiter - they can go a bit overspeed on certain corners but nothing dramatic if your fan is set up correctly

I worked on peak power revs which on the TZR is 9800-10K rpm depending what spec you read - my ratio is on the safe side so that even if my fan over-revs on right hand bends, i could go to just below 11,000 and not exceed max tip speed

i have no experience of the LC but know it produces peak power at 8500 rpm - considering you want peak power to your fan there is no advantage or reson to go much beyond 8500

I reckon a 750mm duct with 5Z's with tip max tip speed of 168m/s gives a fan speed of 4280 rpm (someone correct me if i'm wrong)

8500/4280 = just below 2:1 pulley ratio near as dammit, so something slightly above 2:1 would acceptable in my opinion - but then you have the question of, is it better to run close to max tip speed or does the fan perform better at lower revs/steeper pitch ?

I know you still don't have a direct answer but then i never got one - as with some elements of this sport you have to use your best judgement but remain within regulations

Hope that helps


andycollins - n/a
1-Mar-06, 06:09 PM
A couple of points here.

If you want to keep noise down, and you do, you would be better keeping tip speed closer to the 138 RPM reccommendation rather than the 168 max.

Also, you may not need 5Z blades in a 750mm duct you can fit 4Z blades in a 9 way hub in 750mm.



Nickinoue - n/a
1-Mar-06, 06:27 PM
i see what u mean andy about the 4Z's but the reasoning for using the 5Z's instead is because they are much stiffer and have less flex as the force of the airpushes back on them.

I think tony is right that i should keep the speed on the safe side and have decided to go for 20 teeth driver and 48driven. This gives me a tip speed of 137m/s if i take into account i have a 0.74m fan and the engine rpm is 8500, this should be a margin for my engine to over rev before my fan starts to break the sound barrier http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif


Paul Fitz - n/a
1-Mar-06, 11:35 PM
The standard motorcycle RD250LC gave 38Hp @ 8500 (earlier versions 35Hp)

With mild tuning they can be made to produce 43hp+ but are likely to be unreliable on hovercraft where they are run at maximum revs for long periods. They have a tendency to break cranks.

If you are stuck with using a 750 fan the best option is to use either a 750/6-9/4Z/40deg/3050rpm or


these two fans are particularly efficient and give a virtually identical performance at 35Hp absorption. They will also provide sufficient pressure and volume to manoeuvre the craft at about 2200 Fan rpm. (about 1:2.8 ratio), 6160 Engine rpm..

If you use either fan at higher speed (I wouldn't recommend taking either past 3800 rpm) efficiency drops, they produce similar cushion pressure at maximum speed, but lower thrust. Due to the poorer efficiency, at 2200rpm both will *just* provide the pressure but with a very marginal volume. You would therefore need higher revs to manoeuvre.