I recently read (don't recall where) that the the 2002 and up CR250 crank is significantly heavier than the crank in the 2001 and earlier engine. The person went on to say that this is a major contributor to the perceived lack of low RPM response with the 2002 engine. Any knowledge or thoughts on this?
2002+ CR250 Cranks Heavy?
- Thread starter skipn8r
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- Jun 29, 2005
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I read the same thing somewhere - greater crank inertia than the '01. Installing the '01 is supposed to make it rev quicker. Sounds reasonable - would explain the dyno numbers - engine looks good on paper, but lacks snap. Speaking of which, wouldn't it be cool to see a rpm vs time to show engine accelleration rates ? This would probably relate to how a bike "feels" when you ride it.
- Thread starter
- #4
I found the post:
http://www.allthingsmoto.com/forums/showthread.php?t=21899
I've PM'd him to find out more and he said he thought that some CR250 cranks were as much as 7 oz heavier than others but that Hot Rods, etc., use a single (lighter?) crank for several model years. I'm going to check with Wiseco and RPM about weights. BTW, the '00 - up crank part numbers appear to be:
13300-KZ3-B10 ’97 – ‘01
13300-KZ3-L20 ’02 – ‘03
13300-KSK-670 ‘04
13300-KSK-730 ’05 – ‘06
http://www.allthingsmoto.com/forums/showthread.php?t=21899
I've PM'd him to find out more and he said he thought that some CR250 cranks were as much as 7 oz heavier than others but that Hot Rods, etc., use a single (lighter?) crank for several model years. I'm going to check with Wiseco and RPM about weights. BTW, the '00 - up crank part numbers appear to be:
13300-KZ3-B10 ’97 – ‘01
13300-KZ3-L20 ’02 – ‘03
13300-KSK-670 ‘04
13300-KSK-730 ’05 – ‘06
The Weight on a crank is always ballanced on its axis. Keep in mind that crank weight works about the same way as heavier or lighter flywhees, and a lighter crank would defenetly rev faster in time but lower in rpm. heavier cranks rev higher and can keep their revs easier and again the placement of the weight relative to the axis of rotation affects reving. The Further from the axis the weight is the slower it can rev
Dino Y said:
Perhaps I didn't make myself clear.
The key is the distance of the mass from the rotational axis.
Two cranks can have the exact same weight but different moments of inertia.
Also, a lighter crank can have a greater moment of inertia than a heavier one and therefore be slower revving (both up and down).
This is why you see manufacturers like GYT-R listing their flywheel weights with a moment instead of a mass, since the moment is what is important.
:ride:
SQUIRLEYMOFO
Member
- Jun 11, 2002
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maybe I don't need to install that flywheel weight after all..........
SQUIRLEYMOFO said:
I had one on mine and it was nice and smooth on the clay but was dragging it down in the sand and softer soils. It felt very weak in those conditions and required a lot of clutch to bring it back into the meat of the power.
Much better response once it was removed.
:ride:
- Thread starter
- #10
I agree that the moment is the relevant piece of information. However, the very close physical similarity (they both use the same crank bearings - 91008-KAE-731) between the cranks coupled with the observations of a heavier crank in '02 coinciding with a perception of slow revving (classic symptom of increased moment) bears further examination. I'm not aware of any published info on Honda crank moment numbers and an accurate calculation seems difficult (??). I'd be willing to experiment with a lighter crank when it's time for the next top end...Rider 007 said:
The idea that the '02 and up engines have a higher moment of inertia would also explain the dyno results that MXA declared in a few CR250 comparison tests; they say it dynos great, but is very unresponsive at low RPM when ridden. The moment of the dyno is the dominant factor in a DynoJet arrangement, so the CR dyno figures would look similar to other bikes in the test (especially in a high transmission gear), but it would feel more sluggish when ridden (especially in the lower gears). The added moment doesn't have to be associated with the crank, but when someone posted the weight difference cranks, it seemed mighty coincidental.
skip,
I was asking about the moment since I haven't seen the cranks side-by-side. They could use the same bearings but have different web thicknesses, drilling, etc.
I can verify that the engine is not as "willing" as the YZ engine. I've ridden my 03 CR back-to-back with my 02 YZ and the difference is inarguable. The YZ feels like an open bike with a worn out top-end where the CR feels like a 125 on steroids... Barry Bonds type of 'roids! :nod:
The biggest difference is off the bottom, but this isn't news to anyone that has ridden them.
With the stock FW, clean jetting, a RAD valve and Boyesen exhaust flange it has great throttle response... it's just that the amount of torque produced is much less than my YZ. It becomes noticeable in high-load situations like sand, moist soil and hills. Here, the YZ barks and the CR groans. The FWW really amplifies the condition.
The FWW does make the spread of power feel like it will pull forever, though. :cool:
One last thing - how does the fact that Gorr and Varner agree that the porting has limitations (and I believe EG has said that the CR will never be a YZ or KTM due to the limitations of its PV design) support or undercut the "heavy crank" theory?
:ride:
I was asking about the moment since I haven't seen the cranks side-by-side. They could use the same bearings but have different web thicknesses, drilling, etc.
I can verify that the engine is not as "willing" as the YZ engine. I've ridden my 03 CR back-to-back with my 02 YZ and the difference is inarguable. The YZ feels like an open bike with a worn out top-end where the CR feels like a 125 on steroids... Barry Bonds type of 'roids! :nod:
The biggest difference is off the bottom, but this isn't news to anyone that has ridden them.
With the stock FW, clean jetting, a RAD valve and Boyesen exhaust flange it has great throttle response... it's just that the amount of torque produced is much less than my YZ. It becomes noticeable in high-load situations like sand, moist soil and hills. Here, the YZ barks and the CR groans. The FWW really amplifies the condition.
The FWW does make the spread of power feel like it will pull forever, though. :cool:
One last thing - how does the fact that Gorr and Varner agree that the porting has limitations (and I believe EG has said that the CR will never be a YZ or KTM due to the limitations of its PV design) support or undercut the "heavy crank" theory?
:ride:
My buddy called up Varner before he got his engine work and asked him if a ported CR would still not be able to match a stock YZ like EG mentioned....Varner was like "What?? It can be made faster than a YZ".
Now I have ridden the Varner motor and I cannot see how any stock motor could match a Varner CR...It is hands down the fastest 250 I've ridden...
We are supposed to finally get together again tomorrow and test the two motors(mine is modded by EG) back to back...And we're going to sample each motor with the SST pipe and shorty silencer...
- Thread starter
- #14
I''ve ridden my CR back to back with my friends '05 YZ250. Where the stock CR seems lacking is in the responsiveness down low. As said above, when the CR comes on, it is at least as strong as the YZ and seems to be stronger. Now that I have my Boyesen ported cylinder, my CR is silly fast compared to the YZ. But the YZ still seems more responsive down low.Rider 007 said:
The original observer of the weight difference said that an older (lighter) crank will bolt into the later engines. He went on to say that the heavier cranks are better for oval tracks and the lighter crank is better for MX; hence, he has presumably observed a performance difference. BTW, if the bearings are the same, and the cranks interchange and one is lighter; it is reasonable to presume a priori that the lighter crank may have a lower MoI.
That being said, I'm not claiming that a lighter crank will solve the observed low RPM CR "problem". I am saying that given two otherwise equivalent engines, the one with the lower MoI will feel (and will be) more responsive to the throttle. It's at least worth a try....
john3_16 said:
Not the first time I've heard of a fast modded CR (not that a stock one is slow) but have never heard of anyone getting the breadth, flexibility and ease of use of the YZ power out of it. Is that the case here?
Does it still run on pump gas or does it now require race gas?
skipn8r said:
I missed this... is it in another thread? I'd like to learn more.
skipn8r said:
Not disagreeing, I've just seen other bikes where the only thing changed from one model year to another was the distribution of mass on the crank. Same ends, same bearings.The overall weight was also the same but they moved it further out for greater inertia.
Just not wanting to ignore the chance of this type of scenario.
skipn8r said:
I agree, I'd like to ride one and see what it's like. If you do this, please let us know what you think.
Makes me wonder where the "sweet" spot is for the inertia. I've ridden bikes with light and lightened cranks that turned over very quickly but seemed weak (and would stall) under a load due to the lack of inertia. Also, they were more difficult to ride with my limited skills due to the sudden swings in RPM.
My 03 YZ450 has demonstrated this tendency (to a lesser extent). It revs quickly and pulls hard but was easily stalled by sudden loads. A GYT-R FWW and Rekluse fixed this for off-road. :nod:
Thanks guys, good stuff. :cool:
:ride:
- Thread starter
- #16
Partly in a PM response I got from the original poster as well as other inputs I've read:Rider 007 said:
"You can call Rick Peterson at RPM or drop him an email RPMSBIGBORE.Com I believe. He would probably have the exact weights. more like 7 oz on some,,the 2000-2001 if I recall has a different part number than the ones from earlier, though I think that Wiseco and HotRods flock shoots them with one number. I have a 2000 if you want to trade (the heavier one is nicer for flattrack engines) that is low time."
The bike runs on race gas.....It's not like a YZ with an easy to use and flexible powerband...Maybe that's what Eric meant by saying a CR can't be made into a YZ...
The Varner bike is not a YZ....No the Varner bike is just flat out ferrocious and pulls so hard it's rediculous...It's definitely a pro motor and has a nasty top end bark...I got off that bike wondering how my buddy raced that bike with the throttle to the stops because it was just too much motor....Coming out of corners in 3rd gear and wick the throttle open and the bike is revving like it's burning nitro... Definitely stronger than any stock bike out there.
SQUIRLEYMOFO
Member
- Jun 11, 2002
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Now that's what I'm looking for whether I need it or not!john3_16 said:
- Jun 29, 2005
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Be patient - I am currently running a PC cylinder in my CR, I'd say ferocious is a good description. I have an Erric Gorr cylinder coming my way - I'll let you now how they compare. My current cylinder has decent bottom, but gets with the program in the midrange and revs to the moon. You really notice the power in 3rd and up - it flat halls azzzzzzzz ! You'd better have it pointed in the right direction befor you pull the trigger - man do I love 2 strokes !!!!
