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Old 03-25-2013, 10:53 AM   #1 (permalink)
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125cc 2 stroke cr125r

I have been doing some research on this site as well as many others. I have been an auto mechanic for over ten years now as well. I know my way around chevys to ferraris. That is my background.

I purchased a (1991 honda cr125r frame and 6spd transmission 1999 jug and upper engine bored out from a 124cc to a 125cc) 125cc 2 stroke dirtbike for modifications for an mpg to and from work bike and off road dirtbike. The plans are to modify the plastics to handle the 70-80mph cruising (the obvious mechanical stuff i have already done sprockets etc). The plastics I have been looking at are for the rs125r honda. I will be removing the plastics for when the bike is used as a dirt bike (mostly trails). another consideration is the street legal version, a nsr125.

i guess i can't post images yet...

aside from that- does anyone have any suggestions? a quick mspaint draw up of anything? pictures? I don't want to do a huge shroud deal like done in the sticky here. screw that. I am not as hard core about it as you fellas on the economy side. I come from the racing word, we do care about emissions and mpg because the less you pit stop the farther ahead you can be. Although it isn't nearly as important to us as it is to the distance mpg guys around here. my biggest concern is i don't want the little light weight bike to lift up at higher speeds. I will eventually be taking it to the track at BIR in MN.

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Old 03-25-2013, 02:40 PM   #2 (permalink)
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Wow...sounds like a very cool project - something I would love to hammer on in the twisties. There's nothing quite as much fun as a 2-stroke when it "hits the pipe" and switches on.

As far as economy goes....hmmmm.

The CR125R 6-speed transmission has very close ratios, so not good at all for economy.

Also, remember that this engine produces power like crazy (~40 HP) in a very limited rev range. In the power band, it probably has a better BSFC than expected, but outside of the powerband, the BSFC is absolutely horrific. 40 HP is too much for moderate acceleration on a light bike and you will have to use most of the 40 HP in the "high BSFC zone" to extract the most energy from your fuel. This will mean frequent pulsing (for very brief periods because of all that power), combined with even more EOC. The good news is that bump starting will be easy because of the easy turnover of the 2-stroke.

EDIT: Duh...I just reread your last paragraph. You don't care much about mpg, so disregard all I have just feverishly typed!
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Old 03-25-2013, 02:50 PM   #3 (permalink)
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Agree.... 2 stroke engines in dirt bikes are designed more for pegging the throttle and power shifting which is great for racing and track use. Not so much for fuel economy... It will work on the road with different tires and sprockets though there is no question. I have a 1990 RM250 that I have taken on a 35MPH street in off road form. At 35MPH the power valve remained closed so there was a little bit of surging which wasn't bad but it was annoying.

Note the road was owned by the off road park and does allow unlicensed vehicles to travel on it to get to the other parking lot. No laws were broken.

I imagine if I put a smaller rear sprocket it would correct the surging but I didn't buy it to be on the street


Be prepared to spend some time and money converting it to be street legal. You will need a DOT approved headlight, tail light, turn signals, license plate mount and light, speedo, odo, kill switch and tires. You will also need to think about sprockets, key and if it isn't equipped already you will need to get a stator that has a second circuit for charging a battery to run all the street junk.
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Old 03-25-2013, 03:27 PM   #4 (permalink)
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conversions in mn are easy. we dont have emission tests and i don't have to bring in the bike to be checked. i have a check list i need to go over. the fee is 10 bucks. i will probably have 5-600 in the street conversion. that is including street tires and a separate set of rims. i hate mounting those damn bike tires. fun-i got a buddy who has a tire machine so- we will see if that will work fine.

it was either the 5 spd or the 6 spd. i went with the extra gear. 6th is pretty much od anyway for highway use. i am probably onyl going to make 40hp at the crank with the mods to the motor as is. to make 40whp i would need more work to it- not much but still. these are already pretty built as is. road bikes suck compared to dirtbikes. road bikes are way too heavy. who the hell does 200mph anywhere? if it can handle 130mph its cool with me for a bike. my v8 datsun should do 224mph when all done at bonneville flats. People have put the old s30 z cars in windtunnels. so i have the benefit there already.

its the aerodynamics of the plastics i was curious about. any help there fellas? or do you guys think the plastics from a rs125r or a nrs125 would be good enough? the goal is 130mph pegged. i would like to cruise at 70mph and if i have the sprocket setup for 130 i should be fine at 70mph without sucking 10 gallons a second.
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Old 03-25-2013, 03:50 PM   #5 (permalink)
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The only thing that, based on gut feel and suppported by 0 empirical data, I think would help aero-wise is a front windshield/faring to help re-direct air flow. The other plastics, again IMO, are more likely to increase (rather than decrease) drag.
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Old 03-25-2013, 04:37 PM   #6 (permalink)
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Quote:
Originally Posted by Beau View Post
The only thing that, based on gut feel and suppported by 0 empirical data, I think would help aero-wise is a front windshield/faring to help re-direct air flow. The other plastics, again IMO, are more likely to increase (rather than decrease) drag.
pictures? and what do you mean by the the other plastics? which other plastics? the stock dirt bike ones you mean?
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Old 03-25-2013, 04:49 PM   #7 (permalink)
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Quote:
Originally Posted by faytmorgan View Post
pictures? and what do you mean by the the other plastics? which other plastics? the stock dirt bike ones you mean?
Yes; espescially the stock radiator shroud - they are designed to catch air, not slip through it.
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Old 03-25-2013, 06:02 PM   #8 (permalink)
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The dirt bike front fender is an aerodynamic nightmare also. The front fender and the radiator shrouds are the two aero offenders with the rider himself being by far the worst. Probably most of the "lift" is caused by the wind resistance on the rider way high up on the bike/rider combination, effectively trying to push the light bike over backwards.

I second a handle bar mount windshield if for no other reason than to make the ride more comfortable. I doubt it will make much of an aerodynamic improvement. If you want temporary-mount aerodynamics you're pretty much relying on your fabrication skills learned in mech school. This way you can go as conservative/extreme as will suit your taste. Note that the roadracing fairings are designed for handlebars far lower than yours.

A second set of shorter fork springs and a matching shorter shock swap will help the stability and reduce frontal area some.

I'm guessing the best you'll see on this motor with easy street riding is about 35-40 mpg. The motors are just designed to run best near or at WOT and consume alot of fuel in the process. They are great at what they are designed to do.

Please report back to prove me wrong though. Really. I'm very curious to see what an off-road 2 stroke can do.
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Last edited by beatr911; 03-25-2013 at 06:16 PM..
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Old 03-25-2013, 08:01 PM   #9 (permalink)
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On a similar note (sort of), I saw a RD400 near the Huntington Beach pier Staurday night. Boy, I would love to have one. Ahh the sound and smell of a 2 stroke.
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Old 03-26-2013, 12:56 AM   #10 (permalink)
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Aeromodding a dirt-bike is nearly impossible

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