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KX450F Lugging vs Revving

Post by 4Strokes » Wed May 25, 2016 4:53 pm

Topic: To Lug a 4-stroke vs High rev. (KX450F)
Author: wera276
Posted: 04/22/2006 6:20:17 PM

This may sound stupid. But i suppose ya never know unless you ask. I am a good mechanic and have wrenched, raced and done just about everything you can to a sportbike. Im fairly new to dirt. Rode a 2000CR250 from last Sept to this spring and now have a new KX450F. My question is......is either worst than the other? High rev or lugging a 4stroke. My buddy seems to think high rev (but still below the rev limiter) is going to be worst on the motor, valves, piston, etc. But lugging the motor seems to really make it chug also. Any input on this? My stock gearing is that, i rev a bit high while in 1st but lug a bit when in second (i trail ride). So im thinking of +2 on the rear to utilize 2nd gear more of the time...if not nearly all the time, but with out so much lug. Any input?

Reply by ThumperTDC on 04/22/2006 11:08:45 PM
To me I would say high reving it would be worse on the engine, simply because everything is moving much faster and there for making it easier for something to let loose, or go wrong.

The gearing change might be a good thing, but one thing that might help would to look into getting a flywheel weight. Since you trail ride, this will help keep the engine from not stalling as easy when lugging the engine. The only problem I have seen with changing the gearing is it changes the power of the bike. I once tried running a one tooth smaller front sprocket on my bike and 5 minutes later i took it off because it totally changed the power of the bike. I had to shift sooner and flat out hated it.

I know gearing is a cheaper fix, but if it were me, I would try the flywheel weight first and then see if i needed gearing changes. Flywheel weights run about $100 or so. A buddy of mine installed one on his KX250 which he only trail rides with, and said it made a huge difference. He said it smoothed out the hit of the powerband, helped him when he would lug the engine, and made the bike easier to ride on the trails.

Reply by wera276 on 04/23/2006 07:00:33 AM
Really....hmmm. Who makes the weights? Would this also give the bike more engine braking?

The bike does engine brake a LOT compared to my 2 stroke, but i still use my back brake quite a lot....prob more than i need to. A little more engine braking would be nice.

Reply by ThumperTDC on 04/23/2006 08:13:35 AM
Not sure if it will add more engine braking or not. Steahly off road is one company that comes to mind for flywheel weights, not sure if IMS makes them as well or not. You can get the weights in different sizes as well. You could give them a call and chat with them to see what they would think might be good for your bike and riding conditions.

Reply by maximo on 05/11/2006 2:57:38 PM
for that type of bike (4 stroke mxer) it is worse to lug the engine. Its not built to lug around in the bush, its ment to rev high like the 2 stroke mxers....if you want a bike that you can lug, buy a 4 stroke trail bike, like an XR.

Reply by 4StroKen on 05/12/2006 11:34:43 AM
Originally posted by ThumperTDC
To me I would say high reving it would be worse on the engine, simply because everything is moving much faster and there for making it easier for something to let loose, or go wrong.
I disagree. My thoughts are.. With the engine reving, the inertia is there and all parts are moving smoothly. Yes things will wear but that would be normal. With the engine lugging there would be more strain on connecting rod, bearings and transmision. Now...over reving is a whole other thing.

Reply by maximo on 05/12/2006 12:04:55 PM
yea, i would agree. lugging a bike puts alot of strain on engine parts.

Reply by tjs on 07/03/2006 07:18:01 AM
My Kx450 is really hard to ride. It don't lug at all it will die. my old yz 250 2 stoke lugged better than this bike. I have been told to change the fly wheel weights butI can't find no one that makes them. checked with all the dealers here in Seattle area. So if you find the company can you leave the name. thanks

Reply by Admin on 08/07/2006 10:21:24 AM
Originally posted by tjs
My Kx450 is really hard to ride. It don't lug at all it will die. my old yz 250 2 stoke lugged better than this bike. I have been told to change the fly wheel weights butI can't find no one that makes them. checked with all the dealers here in Seattle area. So if you find the leave a company name. thanks
Steahly, SFB, Falicon, Trail Tech, Zip Ty are just a few companies that produce flywheel weights.

Reply by tjs on 08/07/2006 4:58:30 PM
Well I found where I can get a flywheel weight from Kawasaki the weight is 110. 400.00$ Funny most dealers don't even know about this. tried all the flywheel shops all told me the same we don't have any yet.

Reply by Jlutty650L on 08/15/2006 5:37:40 PM
I am not telling anyone how to ride. I am a novice on two wheels. But I have been building big 4 strokes for a long enough time to know that they like the middle of the tach just as much as the top. Every engine has a sweet spot. On that one, you should be shifting by 6000 RPM (hopefully before) to keep from over revving and to drop back from the meaty part of the horsepower peak to the meat of the torque peak when shifting up. That is why four strokes are so nice compared to the manic idle or twisted (no in between sometimes) nature of a two stroke. 4 strokes do have torque at 3200 to 4800 RPM and the smaller ones have even more horsepower than torque at 4500 to 6000. That's why I say run it where it is happy- 4500- ish it should have a little ways to go down or up depending on the course. On the long flat parts (roads, etc.) keep it in fourth so you have enough on- demand spool- up to get out of trouble without being caught shifting in a corner. Gear selection on a known course should be easy to do with this in mind. This may sound crazy, but you can always slow down to stay in a given gear where the motor is neither "lugging" nor "screaming" and have a more relaxed ride. Just remember,"If it burns brighter, it burns faster". But that's no fun.

Reply by Chillwabbitt on 10/30/2006 06:36:02 AM
OK I own a KX450F very good bike in MX and offroad events however for offroad you will have to do some work to the bike like the flywheel weight: I added 15% off the current weight on(don't go to heavy or you will snap the crank!) I have also just added a 3.6 Gal tank for the longer rides, as well as gearing the bike to 14/47 looking at going 44 at the rear for longer legs. As for lugging the bike, it's not a good idea rather stay in the MID/HIGH RPM range as this bike is designed to go flat out at all times. Hence the hectic maintenance schedule - stick to it or pay the price.

Reply by DIY650L on 10/30/2006 6:18:07 PM
Originally posted by wera276
Would this also give the bike more engine braking?
Adding weight to the flywheel will not change the engine braking itself, but you will feel less of an effect because there is more rotating mass that needs to be slowed down. Same as loading up a pickup truck, with the same pedal force, it will take more distance for the heavier truck to stop because there is more mass that needs to be slowed down. PS: If anybody has trouble sleeping, I could go into the math behind this or send you one of my old textbooks.

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