Dirt Rider Magazine

midlifemx

Years ago, before there were four stoke motocross bikes, I had heard that the 125 class made better lap times than the 250 (two stroke) class.

It might have just been the amateure class.

Does anyone have any facts or comments on this?

NZrider

at my provincial championships a few weeks ago the 1st and 2nd fastest lap times were set by 250fs the 3rd fastest was a 450. at a local 6 hour cross country a couple months ago the fastest laptimes and overall winners were a pair riding a yz125 and crf250.

its not too uncommon for less powerful bikes to lap faster than their more powerful rivals.

Mark47mx

If you watch some Nationals and Supercross from the 90's on you tube you will find from time to time the 125's did faster lap times. I saw this more in supercross than Nationals the hills really slow down a 125

That really doesn't mean the 125 was faster though.
On the right track with the right obstacle placement a 125 would carry momentum through the corner to clear the obstacle
or maintain a faster line through a series of obstacles.
in this same corner the 250 because of the additional power is able to take an inside line and still clear the obstacle the inside was often slower but on the 250 was the line of choice for the leader.
Why a slower line would be a line of choice?
125
the lead rider will maintain the faster line in this case the outside because he doesn't have the power to get over an obstacle from the inside nor does his opponent so a pass from the inside will be short lived.
250's
The inside line is slower but is the preferred racing line for the rider in the lead on a 250 , the inside line protects from being passed which if the 250 rider gives up the inside his opponent is able to pass and hold the lead because the power allows him to clear the obstacle.

The 125 was easy to maneuver both due to weight and lack of power. Most offroad/motocross races are not top speed races
a 125 pilot who can carry his momentum can often run on pace with the bigger bikes.

125 racing was a lot of fun to watch because it really lead to a lot of block passing due to the lead riding needing to maintain the outside so often.

midlifemx

Your responses remind me of watching the races at Englishtown, NJ. The 125s would come screaming by. The 250s almost as much. But the racers in the open class would barely touch the throttle by comparison and it seemed like they were going much slower.

Kiwi450x

Don't let Coyotekiller hear you talk like that or we'll get another 10 post lecture on the viability of mid-80's open class MXers.. LOL

coyotekiller

i have been working on improving my responses Kiwi. lol.. actually i'm glad that there are few who like open class bikes because both of the open class bikes i have bought i got cheap because the owners were scared of them. lol.. i will have to start another thread on that subject sometime.

i would like to hear how the open class fits in with the others like Mark posted with line strategy and such.

midlifemx

I was reading a Dirt Bike Magazine from 1979 and it had the
horsepower ratings in their tests.   The
125s were around 21 and the 250s were around 31. I used to race the 125 and 250 amateur class
in the early 1980s. After a serious
accident, I stopped riding for 25 years.
I got back into it about 5 years ago, just as a fun hobby with my kids
so I bought a 2007 CR125.   From what I
read, this bike has around 32 horsepower.
It’s been a long time since I rode a 250 two stroke (1981 KX250) so I really can’t remember how much power my old
250s had, but ... could that be right… a 125
now has the hp of a 250 back then?

sweetbrue

Someone will know for sure, but I think its very possible. The technology that has been developed in the past 30 years in all types of motors including vehicles is impressive. Just look at the power a V-8 had back then and compare it to those of today, or even the high output V-6 motors in some of the sports cars.

Mark47mx

The 1973 Elsinore 125 put out around 20 horse power at 8000 RPM and the 250 right at 29
There have been a lot of advances since those early days. Here is just a short list.
First a modern 125 doesn't even start making power until 8000, peak HP is predicated on air flow reaching its terminal velocity of 650 ft/sec a small bore has to rev very high in order to actually reach its potential peak (http://www.gglotus.org/ggtech/hp-theory/hptheory.htm) This coincides with better /more accurate porting specs for a 2 stroke.
Liquid cooling which keeps the engine at a more precise operating temperature
Better knowledge of crank volume
The advent of the power valve in the early 80's
For 250's the use of the 66.4x72 bore and stroke, this is interesting Honda started using this around 82, Kawi jumped on the band wagon around 93 Suzuki 96 and Yamaha in 1999. This seems to produce the best HP/Torque. Before that the were the most popular sizes were 67x70 and 68x68.

Mark47mx

Coyote, In my opinion the more you could flow your lines and carry momentum the better off you were. 250's are a better stop and go bike IMHO, here's my nutshell on why.
The open class bike had one huge advantage but that was also a disadvantage. POWER. You could go any where and jump any thing with a touch of the throttle. But power is also a disadvantage you have to find traction to use it, as I mentioned if you could maintain momentum it was be a big advantage however the EVIL twin was always waiting for you to get happy so he could put you on your backside at any time. Respect and throttle control.
The additional weight with little engine breaking to assist in slowing made it difficult to hit lines you have to plan ahead. It is harder to set up suspension with the extra power and the chassis doesn't seem to settle into corners as well as lighter/less powerful bikes do.
Vibration and gorilla grip, I've have my hands cramped for almost a week after a long day of riding when it was really technical with a lot of stop and go...
I loved open bikes when I was young and often have fantasies of getting a Service Honda AF then I wake up and pay a few bills...My 250 suits me fine in fact I love playing on my sons big wheel 85 so much you might catch me on a 125 some time soon.

coyotekiller

That is a lot of great info but I would like to point out that comparing dyno sheets from different days is sketchy so comparing across 3 decades is even more sketchy. plus in the early 90s there was a change in the way they measured dyno hp by more than 10%. i will have to dig out my old papers as i used to have a conversion factor to use when comparing a dyno test from the 80s with one from today.

i know my wine so for example the 85 yz490 and the 86 kx500 were the peak years for hp on both bikes after which they detuned them and a dyno sheet i have comparing the 86 kx and yz has them at 47 hp. if you were to look at a dyno sheet from a 2004 kx500 it would read 55+ hp even though it is producing slightly less hp.

coyotekiller

thanks Mark, that is exactly what i was looking for and you also described the most endearing trait to me of open class bikes . the double sided sword of hp.

Mark47mx

Sorry midlifemx we've kinda jacked your thread for a very interesting discussion.

Coyote, you are correct there really isn't a direct comparison from dyno to dyno day to day however a 12-15 hp spread on the 125's and 15-20 for the 250's from 73 to present far outweighs any comparison tolerances or a 10 percent change in measuring.

Just a quick example of how porting can really mess up a bike the 1998 RM 250 produced 38 hp on the MXA dyno. Seems they wanted to call it a 200 with the way it ran. Suzuki made to ports way to small a skilled craftsman could pull 44-45 depending on the dyno without increasing compression.

The first 2 stroke I ever rode was a 1973 CR125 that was in 1973 I can assure you it was no where close to as fast as the last 125 I rode an 02 YZ 125. But for the day it was a blast to ride on the cutting edge.

midlifemx

Not a problem... glad you guys furthered the discussion. I've learned a lot.

At the end of the day, I would like to know if my current 2007 CR125 is as fast as my old 1981 KX250... probably because I'm wondering if I am too big (190) and old (47) to be riding a 125.

coyotekiller

i am 45 and i am 6'1" tall and weigh 300 or so lbs but i would still like to have a good 125 in my stable. there is nothing more fun than using everything a bike has to give and i am a long ways from taxing a 125. considering i have hypopituitarianism, emphysema ,COPD, and have to carry oxygen nowadays i should probably be shopping for a xr100 instead of a 125. lol

Mark47mx

midlifemx, you left the "ideal" weight for a 125 behind about 25 lbs ago. Hey I'm 52 yrs 6 1 and 198 lbs as of the completion of my workout a few minutes ago. I weigh in before and after workout to monitor fluid loss.

Is your bike faster than the old 250 id say yes and no.
Just off the top of my head
the 07-
modern chassis, suspension, ergonomics and you have a power valve which while small bores don't make a lot down low it makes a big difference in ride ability. Modern brakes. Much more aftermarket performance options.

the 81,
no power vavlve your either on the pipe or your not.
air cooled engine, these bike have an optimum temperature air cool just can't always do it plus they tend to be heavy.
It should have more torque than the 125... big plus even if HP is equal.
It will weight a lot more than your 125
limited suspension

If you are looking at racing it's tough to win a gunfight with a bb gun but for just pure fun nothing puts a grin on your face like a 125. Get the suspension set for your weight this will require heavier springs and maybe a re-valve, and go collect some bugs on your teeth..

midlifemx

Guys,

I really appreciate the feedback.

Mark, I just watched a video of me going off a 5' tall jump and I am bottoming out on the landing. I just went to Motosport and it looks like new springs front and back will run around $250. That's about half what I thought it would be. Looks like I have a winter project.

Coyotekiller, I have a 1982 XR100 that I ride in the backyard when I can't get to the track... it's the most fun you can have on a dirt bike on 1/3 of an acre.

Thanks to both of you!

coyotekiller

i would seriously like to get a xr100 but trying to explain it to my Dad when i bring it home would be too much!!!!!!! LOL!!!!!!!!!!! he is the type of person that wears shovels out and he would not be able to grasp the xr100 concept. LOL!!!!!!!!!!!!!!!!!

glad i could help.

coyotekiller

i don't mean my Dad is stupid, quite the contrary. he is just a strong believer in the liberal use of elbow grease. lol

midlifemx

I can respect that.

It's all a balance in life... mow the lawn vs. go ride.

But sometimes it's about compromises... use the XR to mow the lawn :-)

coyotekiller

boy don't tear up grass in the wrong place around here. lol you guys that ride on public property have it easy. i rarely ride on my Dads place now that i have my own but even then i limit what i do to save the pasture. i guess you could say that i tread lightly. i only give my 490 her head on the poorer producing parts of our places.

i do have a small track and stuff to play on though.

midlifemx

I assume with a 490 you could trench for corn...LOL!!!

coyotekiller

i don't do corn farming. but the 490 will dig a trench.

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