CR250 Cranks

[Gerry MacNutt]

So I need to start building a motor and need the following info if possible on 05-07 CR250 cranks

1) With an OEM crank sent to be trued/ rebalanced, are the cranks set to vibe in the low RPM range or set to vibe above max RPM?

2) What shop or person really knows how to do a kart CR250 crank on the west coast?

3) Has anyone had any success using Wiseco or Hotrod cranks kits?

[Ian Harrison]

Hi Gerry

On the standard 05-07 crank (with the Genuine Honda '02 Piston Kit), there is a fairly bad vibration period from about 10,900 to 11,200.

Really you've got to think about the rev range you are going to use all the time. So from say 9,000 to 11,500 rpm you don't want a bad vibration period. It dosen't really matter too much if it shakes your teeth out at 1,700rpm

Altering the balance factor alters the size and direction of the forces, many of which can be absorbed by the frame/mounts. Of course a bike frame is very stiff in the vertical plane, whereas a kart frame is softer, so the ideal balance factor is different for each application. Even frame to frame is different.

As a for instance, I can install an engine with a standard crank in one make of chasis and you will hardly feel the vibration, but put it in another and it can cause the chain to oscillate so seriously that the motor can't pull through the vibration period in top gear and the driver can get friction burns from the vibration transferred to the seat.

I know of 2 companies in the USA:

1. Falicon (Florida) http://www.faliconcranks.com/index.html
2. Crankworks (Arizona) http://www.crankworks.com/

Both can adjust the balance factor of your crank by drilling and/or press fitting tungsten (mallory) weights. Tell them what you want and the mass of the piston/little end assembly and they can adjust the balance factor accordingly. As to how good they are I couldn't comment as I haven't used them myself. But I'm sure you would gauge that in a conversation.

Here's the force diagrams for a CR250 crank at a combined balance factor of 76%.


Of course the above diagrams show you the various forces, but it's very difficult to know what that will feel like, so there is a bit of experimentation involved. With a balance shafted engine it's dead easy. You want 50% balance factor and that 50% fully counterbalanced by the counter rotating shaft.

Wiseco and Hot Rod Cranks and rods are manufactured in Taiwan (ROC). Make of that what you will, but you won't find them in a Viper Racing motor. Certainly I've measured their brand new CR250 crankshaft assemblies on occassion found them out of tolerance even for Motocross use.

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Gerry MacNutt]

Ian, thanks for the great info as I've always been a bike guy this kart stuff is is forcing me to unlearn a lot of my past motor knowledge. So now adding in chassis to crank me a serious amount of trial and error. My only experience to date is 02 motor with 02 crank but I do want to run a 05-07 crank.

When you do yours do you do the flywheel too? or is Honda close on flywheels? mines an 01

My concern is for the near future I'm running an old chrome moly frame and stress cracking is a huge concern.

[James Carmen]

I have had Crankworks in Arizona balance cranks for me. Good plan on using the 05 crank. They have done CR250's, TRX450R's stock and strokers and a CR500 crank for us. They have a target RPM to balance each crank for, I have never asked for something different. I have used Hot Rods cranks with no problems, the Hot Rods bearings are better than stock, just my opinion. They need to know the top end weight in grams (piston, rings, bearings, clips and pin). Depending what brand piston, they might have the weight on file. If using Wiseco, call them they can tell you. I have never had to send the flywheel. Crankworks will do you a great job and fast turnaround. Sometimes they might have the crank you need in stock, saves time and shipping cost from you to them.
_________________
James Carmen
jc_speedworks@hotmail.com