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I did the exact same thing but since i already had a potent '98 around i wanted to go full out and also opted for a modern CRF chassis...thus building a AF which now is a real 4-stroke beater!
I still have a '98 CR 125 around which is still a great weapon up to date although people tend to talk $hit about those stiff aluminium chassis. With good suspension it is still a great bike.
I'd say i became a guru regarding updating old 125 CR machinery. What i like about your project is your concern about the light weight as this is the most overlooked thing when tuning bikes. On my AF build i went out to the max so i could reach the lower weight limit of 88 kilos / 194 lbs. In the end i managed to save 8 kilos / 17,5 lbs and i can only tell you that such a light bike with a potent engine is not only light but also much faster and easier to ride - MUCH !!
IF you indeed reach the lower weight limit of 88 kilos your bike will be
-about 12-15 kilos / 30 lbs lighter than a 250F
-about 15-18 kilos/ 35 lbs lighter than a 450F
THAT'S HUGE !! When i'm riding my friends bikes i always have the feel of riding a Paris-Dakar bike with a huge gas tank ...they all feel extremely heavy and slow handling!
Lighter weight helps 4 times:
-faster acceleration
-later braking
-easier to flick around and swap lines
-much less tiring...you last longer thus can go faster again!
But it is not so easy to reach that goal. In order to get there you really have to dig deep and have your balance ready when shopping for new parts! And every tiny little bit helps! Expect a stock 125cc to weigh around 94-95 kilos so you need to save about 7 kilos...that's a big task! I did it and on my Flickr-account you can find many,many pictures of parts on scale so you get an idea:
https://www.flickr.com/photos/ottospeed/albums/72157632570549639
https://www.flickr.com/photos/ottospeed/albums/72157649383024720
Most of the weight is hidden in the wheels. But if i read that you are looking into different rims be careful and have them weighed before! A lightweight 125 doesn't need sturdy wheels! Usually stock Honda wheels are the lightest around. I'd rather look into stock CRF wheels made to fit in there!
When i read you plan to plant the whole front-end off a KTM in there remind you you will add weight as KTM wheels are much heavier! I'd suggest you get some Motostuff rotors insead which are really light while offering all the power you might need. Me too i had a KTM in '12 and when coming off a KTM on a Honda you are looking for better brakes (and a hydraulic clutch). Needless to say i got both but definitely stay away off the KTM stuff !!
Tires:
Dunlops are heavy.plain and simple. Pirelli tires are already 3-500g lighter (per tire!) and just yesterday i got to know that the new Michelin Starcross 5 soft rear tire is a whopping 2 lbs lighter than the Dunlop MX32. (100/90-19 = 4157g). Pirellis are around 4700g while Dunlops are at 5000g.
Tubes:
again a lightweight 125 doesn't need reinforced inner tubes. I experimented with innertubes from MTB Fatbikes and have to say that after 1 year of use i'm really susprised they hold up. The lightest i could find are those by german manufacturer Schwalbe. 380g is about 2 lbs lighter than your heavy-duty MX innertube. And we are talking non-suspended mass here...!! Lighter wheels also accelerate faster (they need less energy to spin faster) and also your suspension will work much better having lighter weight.
Regarding the engine:
I know most people love the 6s tranny of the 90-97 models. BUT i actually prefer the 5s. It's basically the exact same transmission except they got rid of the 6th gear. When was the last time you were in 6th pinned on a MX-track??? I never reached 6th anyway as i have such a potent engine that i added 1 teeth to my rear chainring thus making for longer gears. I hardly ever reach 5th anyway so the 6th gear is really useless. Gearing ratios are identical as well (only minor differences in 2nd if i remember right). I also have bikes with 6s trannys and couldn't tell a difference at all. Those doing GNCC or Enduro riding might need the 6th for decent top-end but you don't need it on a MX-track.
Big-bore:
well - if you think that's the way to go then do so. I on the other hand would rather suggest you keep it at 125. I can only tell you that if you could get a Mugen kit for that engine you'd have a engine that outpowers your big-bore any day! As others mentioned already learn how to ride a 125 and let it sing.You don't need the added cc when you have a high-revving,potent engine. My 1990 Mugen is still as fast as any '16 bike (including the new KTM !!). Even my '98 outpowers the '16 YZ 125 by far....
Suspension:
hm-back in the day (87-90) WP forks were seen a lot on CRs but these days? Also if you want to reach the lower weight limit you might have a look in actual air-forks! I mount a Showa SSF air fork off a '15 CRF and it's 1,5 kilos/ 3,3 lbs lighter than the Kayaba spring forks it replaced. Without such forks i really doubt you'll reach your goals....
In the back i mount a titanium spring (-600g / 1,3 lbs)
Bolts:
Almost all titanium and some aluminium as well. I left alone the 3 main axles (front,rear and swingarm) as those would cost 1000$ and savings would be just about 300g / 0,7 lbs) which to me wasn't worth it. I ended up getting all standard sized Ti-bolts from Ebay (China) and only the special sized bolts from specialized companies which also ask steep prices. But anyway - By spending about 700$ in Ti i managed to save 836g/ 1,8 lbs
Seat:
definitely try to get a closed cell foam! My Thinktechnology seatfoam saved 550g / 1,2 lbs over the stock CRF seatfoam. And it doesn't stop here: also seatcovers differ quite much....as mentioned already have your balance ready when shopping for new parts! My 100% seatcover was 60g lighter than the stock CRF cover.
Tank:
For the CRF chassis i was able to get a carbon tank which saved 560g / 1,2 lbs. I'm doubting you can find such for an old CR though.So again you'll have to dig deeper to get the weight down to where you want it!
Transmission parts:
you'll have to get the lightest chain and sprockets and here my Flickr pictures might be of a help as i posted several weights on all those parts. I found the EK MRDL6 chain to be the lightest (about 200g lighter than your typical DID ERT chain) and still be very durable.
Talon sprockets are lightweight! I have yet to find a lighter one.
Anyway - if you go through my pics you'll find a lot of items that help to bring the weight down. I also did a list with the biggest savings so you can take a closer look as well:
But in the end...all that counts is you got a lightweight weapon and rest assured it is indeed capable of leaving the heavy 250 & 450Fs behind! You will be surprised to see how much faster such a bike can be when you also bring the weight down. Only on deep soil and on really fast straights you are behind (a bit) but on average lap times such a 125cc bike can still be very competitive.
Me beating thumpers this summer . The Mugen engine is 24 years old (1990!) while the rider is 48
(just kidding)
Good Writing Nino.
i Think its common to forget the weight when people buy "cool stuff", like billet gastank cap, it can acually be heavier then stock plastic one, and in a bad place also..
but its not the Mugen itself that beats the thumpers, Think there is some skills also..
I have a question though... Why is it so much faster than a Honda cast cylinder? Can't someone just copy the port size/timing, head squish, dome shape/volume etc. to replicate the way a Mugen cylinder runs? I mean, they are so desirable (and expensive if you can find one). After all this time, there's no alternative??? What makes it so different?
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As mentioned - just lately i was able to testride brandnew 2016 Yamahas when there were test-days over here in Switzerland. My Mugen-equipped '98 was much faster than the '16 YZ 125 it was really a shame for a bike with a actual engine. My '98 has a '91 Mugen (24 years old!!!) and could draw circles around that YZ....what a shame really!
VERY well said, sir! Good stuff!
I'll never own a Mugen or an aluminum frame conversion, but threds like this are what makes Vital worthwhile. And your points are applicable for any dirt bike -- I'd love to take some of your suggestions for future builds (like my 04 RM250 or some of my steelie CRs).
You are making me want a Mugen cylinder. I can tell if I like this project the engine will become an open book for improvements and power placement. I owe you much thanks for doing so much of that legwork in regards to these older CR engines.
Also I will be focusing HEAVILY on weight on this bike. (pun)
I rode a 450 for about 10 years before completely quitting the sport and honestly wondering why I ever truly LOVED it vs other hobbies I've been into.
Well after training some young up and comers on 125's I was invited to race his 2006 YZ125 (stock everything) last year and the fun factor was off the charts but most importantly I was able to rekindle that pure passion and truly indescribable nature that MX is supposed to bring.
Without it this is much too dangerous and expensive.
But the light weight of the 125 really is what changed my mind. It was actually fun in the corners instead of working to hang onto the 450. Reminded me of being 15 again!
The 150SX is a fantastic bike and especially with the A kit, but I think at 203 pounds there is much more for me to experience as I get this CR into the 190's and 180's if possible.
Thanks again!
I might try riding for the forst time in over 6 months just tomorrow.
Regarding the ATAC engines: no, i haven't ridden one yet. I am just now finishing another 89 which i bought as a spare bike for the vintage races i'll do. I hope to have it ready within a week or so.
There is another thread on that build on Thumpertalk:
http://www.thumpertalk.com/topic/1188961-cr-125-1989-rebuild-for-vintag…
And pictures as well:
https://www.flickr.com/photos/ottospeed/albums/72157668784699162
I sure think the 89 is still plenty fast. However the ATAC is a far cry from the later HPP. So if you make a fast ATAC engine it might require more riding skills than modern 125cc bikes.
Just the other day i could ride a 2016 KTM 150 and i have to admit those are rocketships! I rode a stock one but hen there was also this. My friend has built a awesome AF using a 2015 Honda chassis with a tuned KTM 150 SX engine....you might have seen pictures of that bike in the internet. That bike is incredibly fast. It comes on strong down low already...kinda like a 250F. But then it lifts off. I rode it on a flat field though. No track. But for me THIS is the ultimate 4-stroke beater! The 2016 KTM is by far the most powerful smallbore and when tuned it has more ponies than a 250F, coupled with light weight....that thing is soooo much fun and incredibly fast.
Bump. I just did a search on 95 cr125 & I found this. The complete 95 125 season is on yt. Interesting that all the 95 cr125 sounded different. There’s quite a few that yr. Puzar,s bike sounded the best. The other Italian,s bikes sounded real good. Jimmy Button,s 95 sounded different. I think he had 94 hrc kit. Ever finish the bike ?
Get a Kawi... & save some money on hospital bills when it headshake's you over a berm...
Nino, do you run the Stic?
No i don't ! Back then i got early prototypes and while it made for a insane overrev it also made for a severe engine failure because of excess heat and detonation....at the time they (and I) couldn't figure out a carburetion setting for my bike.
Maybe now they can but i'm hesitant to risk another failure on my precious Mugen kitted engines. I asked for an updated version with carburetion suggestions but they wouldn't come back so for me that's the end of the STIC story.
That's too bad, it sounds like there was power to be harnessed.
I understand that you don't want to toast a Mugen. I like the way your bikes sound even more than the way they look.
Pit Row
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