Posts
1363
Joined
11/5/2015
Location
Grapevine , TX
US
Edited Date/Time
1/4/2022 10:02pm
I am finally getting around to the final assembly of this project after a lot of research, collecting parts and a fair amount of machine work and mods. The last generation of Honda two strokes are my favorite bikes of all time so I decided to pull out all the stops and build the best CR I could with the latest tech and trick parts available (within reason)
I really wanted to think outside of the box and make mods to the bike that make it a better machine, not just bolt on a bunch of bling that does nothing to make the bike better. I did have a few pet peeves with the CR250 that I could fix and I wanted to address them as well.
Originally I just had a few simple requirements on my wish list:
1. Lots of horsepower
2. As lightweight as possible
3. No air forks
4. Reliable
Then my OCD took over and I ended up with quite a bit more of an exotic build than I had originally planned but I think it's turning out great so far. I will post more updates, details and pics over the next few weeks.
Frame prepped and ready for final assembly.
All of the brackets have been removed from left frame spar and all of the electrical components will be relocated towards the center of the bike and in the airbox.
I removed the coil brackets from the down tube of the frame and moved it near the rear shock tower. Anyone that has done a top-end on a 2002-2007 CR will know why. The location of the coil is a total pain in the ass when pulling the cylinder and this mod makes it much easier. The bonus is that the weight is moved to the center of the bike and that is never a bad thing.
I added a return spring to the rear master cylinder eliminating the need for the tab on the frame or the OEM spring. Added a billet brake clevis and Ti hardware to finish it up.
Because the ignition box will be relocated to the center of the bike I was able to utilize the old mount on the head tube to fabricate a mount for a Honda HPSD steering damper.
I really wanted to think outside of the box and make mods to the bike that make it a better machine, not just bolt on a bunch of bling that does nothing to make the bike better. I did have a few pet peeves with the CR250 that I could fix and I wanted to address them as well.
Originally I just had a few simple requirements on my wish list:
1. Lots of horsepower
2. As lightweight as possible
3. No air forks
4. Reliable
Then my OCD took over and I ended up with quite a bit more of an exotic build than I had originally planned but I think it's turning out great so far. I will post more updates, details and pics over the next few weeks.
Frame prepped and ready for final assembly.
All of the brackets have been removed from left frame spar and all of the electrical components will be relocated towards the center of the bike and in the airbox.
I removed the coil brackets from the down tube of the frame and moved it near the rear shock tower. Anyone that has done a top-end on a 2002-2007 CR will know why. The location of the coil is a total pain in the ass when pulling the cylinder and this mod makes it much easier. The bonus is that the weight is moved to the center of the bike and that is never a bad thing.
I added a return spring to the rear master cylinder eliminating the need for the tab on the frame or the OEM spring. Added a billet brake clevis and Ti hardware to finish it up.
Because the ignition box will be relocated to the center of the bike I was able to utilize the old mount on the head tube to fabricate a mount for a Honda HPSD steering damper.
The Shop
I also agree on the last generation frame of the Honda two strokes (had an '03 myself). To date, in my opinion, still the sexiest aluminum frame.
Any info on the shock swap that you're willing to share, what did you do to get it on there, change the length, linkage etc ?
We are still working on the details and tying up a few loose ends but the transmission will be a lightweight 3 speed version that saves about two pounds of rotating weight inside the engine. It would never work offroad or for trail riding but for MX I think it will be awesome. We will see, I haven't tested it yet.
Good luck none the less, I look forward to seeing the end result!
After seeing that bike and also noting how KTM has redesigned their entire engine (raising the transmission shafts and clutch over an inch) to reduce the drag caused by the gears running through the oil I couldn't help but wonder what effect that would have on the good ol' fashioned 2 smoker? We shaved over TWO POUNDS of weight in the transmission alone and eliminated 4 cogs spinning through the oil.
Here is the pic of Max's Cosworth engine:
The shock was much more involved than I originally thought it would be. I will post more on that later (pics as well) but I will sum it by saying that I am glad it is finally finished but it took a ton of tweaks and mods to make it happen. I thought it was "close" to a bolt on. I couldn't have been more wrong, and truth be told I would not do it again. It was more work than I think it was worth in my opinion.
If you look closely at the pic below taken of MC's bike in 2005 you can see where Team Honda mounted the box, right above the kickstarter. This bike has it mounted in the same place. I just fabricated a bracket out of carbon fiber plate retained by the head-stay hardware and mounted the OEM aluminum bracket to it.
The good news is that compared to some of the magic that the top level tuners perform in regards to ignition mapping on the latest 4 strokes it is still relatively simple.
Fair enough removing gears, if light weight is the goal. Though, you need to consider that the 3 spd Bike It / Cosworths Yamahas ( no idea on the KXfs, but I assume they might have tried the mega revs / minimal gears concept) were engines that revved to 16,500 rpm, and perhaps more. One Pom mag that regularly tested KTMs Works Bikes ( they tested Ferris's Bike It 3speed YZf that he did so well on, too) each season had, one year, the 250fs as having 4 gears - once again, engines that rev to the moon. A 2502T - not so much - most have peak power at between 8 and 9k. Yes, you can tune it for much higher power at higher revs, but it won't be much of an MX engine.
You can always carry a variety of sprockets and chains. Hell, in (many, many) years gone by, in OZ SX , teams / riders would sometimes run no gear lever, to avoid missed shifts/ impact / center punch shifts, on our tiny little AX style SX tracks of the era.
That ignition from one of the former Czech / Slovak countries? If it's the one I'm thinking of, they also do new powervalve motors, which, I've heard, are difficult to get, ex Honda.
The gen3 CR 250 frame is still my favourite Honda AF chassis.
Pit Row
It's going to take some work to sort it all out but it seems like a fun challenge to me and I think it will pay off in the long run.
In regards to the transmission, I basically eliminated 1st and 5th and raised the overall final drive ratio ever so slightly. In my personal experience, I have found the CR250 has fairly wide powerband with a lot of over-rev once it is set up correctly.
My thinking on this 3 speed concept was really basic and simple:
The only time I have ever used 1st gear on 250cc two stroke was on my way to the starting gate, and more often than not I immediately shifted to second because I am so impatient. Very rarely do I ever use 5th gear when riding MX. I typically run stock or taller gearing to spread the power out and help with traction as well. When you combine that with a factory level modified engine that has a ton of over-rev you have a bike that is capable of jumping 100-120ft. obstacles WFO in 4th gear (?)
Considering I am at the +40 stage of my career I am good with that. Once I started paying attention to the gears I actually use on literally every track I ride I realized the only time I used 1st was in the pits and the only time I used 5th was when I took a trip to the dez once a year. Eliminating those two gears sounded crazy at first but made sense after I started paying attention to what was really happening on the track.
I know of several riders that have confirmed they ride an entire SX track in one gear, with the exception of shifting up for the whoops. Even better, AC222 defeated Alessi at the Glen Helen GP in 2010 while riding the KTM 350 after his shifter fell off in one of the the motos. He raced the entire moto in 3rd gear. At the end of the day I just do not feel a bike used exclusively for MX needs 5 speeds but I could be wrong.
My 500, I think I could get away with just 2 gears, 3rd and 4th, that's if I ever again spent much time on an MX track......
And, it's not hard / expensive to have a couple of sprocket sets and dedicated chains for different tracks in your tool box. I made mention of the 4ts mind boggling rev range , well because of their mind boggling rev range...... And feared you might be shooting for crazy revs. It was either Dirt Bike Rider or the ( now E only) MOTO mag ( British mags) that did those works bike tests, and they regarded the Bike IT conventional / carburettor Yamaha engine 3 speeders level with, or perhaps higher than the Works KTMs they tested. Those 4ts sure can rev - well, that's how they ( even the 450s) get their power, as well as twice or 1.75 times the Capacity advantage......
That series of modification articles I mentioned - I think the engine work might have been done by Terry Varner. It's good to see someone Not automatically trying the earlier mechanical PVd engine. The RC engine has plenty of potential, it's just far few people tried / too many found the ( nowadays pathetically low level of) 'E' tech a bit intimidating. Quite Laughably simple, nowadays, as I interpreted you as implying.
I recall either Dirt Bike or MXA did an article on McGraths 250 he used just prior to going to a 450, just prior to retiring. Mid/ high 50hp levels I think, whilst being quite civilised - Varner might have been the tuner?
I feel the RC valve engines are deceptively fast because they are more linear but in reality I think they will make 3-5 more HP than the mechanical PV engine in most cases.
Hopefully this bad boy ends up being everything you wanted it to be.
Can't wait to see and hear about the finished product.
Good luck.
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