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Lately I've noticed a lot of guys replacing their two stroke swingarms with mostly 2009 YZ250F and 450 four stroke swingarms. I first thought to do this when I saw a YT about Wil Hahn's YZ250 he built at Star a few years ago but they don't go into a ton of detail on that particular mod. Recently I'm snailpace working on my 125 again and I don't want to really go through blasting the coating off my original swingarm so I start looking for alternates to buy without spending a ton of cash. I love the look of the fourstroke SW but I'm at the point of pure curiousity as to why anyone would do this mod unless they needed to replace a bent or damaged SW.
Is there some kind of handling performance related gain or is it purely a trick look?
Seems like much work for little return….maybe better parts availability ?
I think people want the linkage bolt to be in bosses under the swingarm rather than having the bolt run through the swingarm to make it more modern-looking.
When Yamaha changed the four stroke swingarm in '09, they claimed less weight, better control and comfort. Each year the flagship bikes get tiny improvements that add up to something worthwhile over a generation or two. I'm not convinced that I can go any faster on a 2025 model than I can on a 2023 model of any specific bike. I don't care what my old two strokes look like and I don't typically buy engine hangers, triple clamps or other such items for any of my bikes. Stock is usually pretty good for the average guy.
Yamaha hasn't done any major changes to the YZ250 chassis in 20 years and I'm not complaining. I know how to make them work well for my level of riding / racing. I have run the four stroke swingarms on my last couple of YZ250s. I'm not even sure if I'd notice the difference in flex, etc if you secretly swapped them between motos on a race day. Still, if I can save a pound or two off the bike here and there without going crazy on cost, why not? The swing arm swap and Titanium spring is a better weight savings for your money than buying thousands of dollars in Titanium fasteners or carbon fiber parts that are prone to crack and fail due to vibration.
Here's a link to MXA's test of the 2009 YZ250F (first year of the swing arm change) and I clipped some quotes from that article:
https://motocrossactionmag.com/mxa-race-test-the-new-2009-yz250f-four-stroke-suspension-settings-jetting-specs-likes-dislikes-plus-much-more/
(2) Swingarm. The new YZ250F and YZ450F share the same hydroformed swingarm, which changes the rigidity balance and flex. It is 12 ounces lighter than last year’s swingarm.
Q: IS WHERE YOU LOSE WEIGHT MORE IMPORTANT THAN HOW MUCH WEIGHT YOU LOSE?
A: Yamaha engineers deserve applause for focusing on reducing unsprung weight. Backyard engineering calculates that unsprung weight is roughly seven times more important than sprung weight. Since unsprung weight (weight below the suspension) has to be stopped, started and damped every time the suspension moves, the weight of the tires, wheels, swingarm, linkage, brake rotors, calipers and chain is critically important. The 2009 YZ250F is lighter in terms of unsprung weight as opposed to simply tipping the scales at a lower number. Kudos to Yamaha.
Q: WHY IS THE UPDATED SWINGARM SO IMPORTANT TO THE CHASSIS OF THE YZ250F?
A: Yamaha’s focus of the hydroformed swingarm wasn’t just centered around unsprung weight; they also tried to alter its “rigidity balance.” Flex is shorthand for “rigidity balance.” Yamaha engineers felt that they could improve the cornering characteristics and traction capabilities of the YZ250F by increasing vertical and twist flex (while reducing horizontal flex). As a result, the 2009 YZ250F is improved in the areas of traction and cornering. The swingarm change is most noticeable in rutted corners and berms, as steering is very precise.
(3) Swingarm. In an effort to reduce unsprung weight, the new hydroformed swingarm not only achieves its goal, but it does so in a technologically advanced way.
(5) Unsprung weight. Yamaha’s engineers understand the benefits of cutting down on unsprung weight. Dropping unsprung weight is more important to the performance of the machine that overall weight.
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This thread got me interested.
What do I need to replace / change if I am swapping to a YZF swingarm?
Do the swingarm fit just right in between frame and at the backside of engine?
Does the axleblocks I have now fit so I can use my rear axle I have?
You need the complete swing arm and linkage assembly. It bolts directly to the aluminum frame YZs from 2006 on up to current models.
The 2009+ four stroke axle is larger than a YZ125/YZ250 axle and it's shorter.
The hub and wheel bearings are different as well.
You need the four stroke axle, blocks and hub.
Or you need custom axle blocks to run your stock axle and hub.
I machined a thicker set of blocks so I can run the stock two stroke axle and wheel.
The axle blocks make up for the difference in axle length. It's about five or six millimeters.
If you happen to be running a set of tusk wheels or at least tusk hubs they have a set of spacers might work if I am remembering correctly. I had tusk wheels on my yz(s) and the set came with a whole bunch of different hardware combinations so you could use the wheels for either a 2 or 4 stroke.
Talon hubs work this way for sure.
I just did some custom work on a 1990 YZ250 swing arm to adapt a modern talon hub with a modern YZ250 rear brake and axle.
The Talon hub fits a YZ450F and a YZ250 by simply changing the seal collars.
The smaller two stroke collars have a sleeve that goes inside the bearing to bring it down to size.
You don't need the bigger axle or rear hub setup. Use the wheel spacers off the wr fx 250/450. I forget what years now. Whenever it went to the newer style swingarm. Think that was in '14.
It still uses the older hub n 22mm axle setup. Might need axle blocks also at most. I did this on my 06 yz250.
Post a reply to: YZ125/250 Swingarm Upgrade to 2009 YZ250F/450F Swingarm - Why? Looks? Performance?