Posts
35
Joined
10/19/2021
Location
Peoria, IL
US
Edited Date/Time
8/25/2022 3:41pm
I always hear of riders/racers being partial to their sag being set, yet I never hear of anyone having a preference of where their rear tire is centered. Is this an actual setting or do I need to drink another one?
Different sprocket combos can have the same final drive ratio but put the wheel in a different place. I’m using 13/49 on my newer bike mostly for the way the axle is at the back of the slot.
It’s a trade off, like everything else!
Works the same on a BMX bike, slam the rear wheel and manuals/wheelies are a breeze! Push it out and not so much.
But basics are; forward for sharpness, back for stability.
The Shop
Previously, I was running 13/50 (one up on the rear from stock) and it was almost all the way forward. Really didn't like that. Felt like I was riding a unicycle.
I have yet to hear/see anyone say they like their wheel more forward on any bike. Everyone always likes them back a bit more, or just dead center/stock.
Axle position can slightly change your sag too because the leverage on the swing-arm is changed by that small amount.
Have you ever put on a new chain and sprockets mid-season and felt like your bike turns a little better?
Keep in mind the factory level racers are going out on the track with a new chain, so their axle position is more consistent through a season than average dudes like us.
It changes traction (think about your starts).
It changes weight-bias and cornering.
Half-links are weaker than normal chain links and I avoid running them, but if you want to learn what all this feels like on your own bike, go buy a half link and use it to try your bike with the axle all the way forward vs all the way back on a new chain. Check your sag in each axle position. Consider playing with fork height too. It's easy to get lost, so buy a notebook and make a setup table of the combos you want to try.
Anaheim 1 = rear axle max forward position.
I run mine pretty far back, about 1/2” from max with the forks about +2 in the clamps. Very stable and will still dive to an inside line if I want it to.
Torque = Radius * Force * Sin(angle)
Radius is set by swingarm pivot center to rear axle center. Thus, moving the axle inboard or outboard changes the radius and impacts the torque being applied.
I’ve always wondered how significant a change moving the rear axle has on the required spring rate for the rear shock, due to this lever effect. I know it impacts it, I just don’t have the knowledge on how to calculate it.
I have a 2023 KX450.. I just recently lowered the gering t
o 13/51 and cut a new d i d chain down to 116 links moving the rear wheel back in the swing arm a little past midways but definitely closer to the back which is what I wanted to do..I couple that with the pro circuit link and knuckle combined with a two rate stiffer (56kg) Race Tech shock spring..I also put Ride Engineering's 22 mm offset triple clamps to bring the front end a Little closer with fork height at 5mm up..I'm hoping to get the best of both worlds, with a little more stability and more accurate predictable cornering..
I
Haven't ridden the bike yet..still waiting on the shock spring to come in from motosport before I put the pro circuit leak and spring on.. should be here in the next 2 days..but I'll give a ride report as soon as I get a few hours on it
I remember the Joe Gibbs Suzuki team being so particular about their axle placement that they changed gearing with internal changes if needed to keep the axle consistent in it's position.
Pit Row
You can do this with changing front and rear sprockets to get the same gear ratio and different chain length 14/52=3.714 ratio 13/48=3.692 and the different size sprockets can have an effect on the rear suspension
Adjusting the axle position from fully forward to full rearward, is a change of about 4% in swingarm pivot to rear axle length.
Steps in rear shock spring rates are typically 6-7%.
I remember going up 2 teeth on the sprocket back in the day because I kept stalling my 250f. Bike started swapping all the time off jumps. This is how I run mine no matter what I’m riding now.![]()
All the "Factory Teams" do that. They can change the size of the gear on the back of the clutch basked by one or two teeth, so the bike ALWAYS handles the same. Unadilla tomorrow !!!!
That was a big topic a few weeks ago on Pulp, I think during one of Nicoletti's segments.
Interesting stuff for sure, being a weekend warrior I never gave it much thought. Like most, my bike gets a little longer every time my chain stretches 🤣
You have to take the 4% change into account twice. Once for how it affects force at the wheel, and again for how it affects travel at the wheel. So 1.04 x 1.04 = 1.082. Call it 8% change in 'wheel rate'.
The 4% was based on measuring my bike, 575mm minimum, 600mm maximum, swingarm pivot to rear wheel axle.
If changing from minimum to maximum length, any upward force on the rear wheel will increase the force on the rear spring by 4%.
To add, the farther the rear wheel is back the more weight it puts on the front wheel.
I like the rear further back, but just play the game with my gear ratio and chain length. To add another complexity to this, if you change your gear ratio it can also changes the bike anti-squat character.
Interesting topic and very much what i've been looking for.
Recently i noticed a spongy feel on the rear of my '22 KX250 and figured it might be from tightening my chain, so i checked race sag, adjusted it back to 103mm and then checked static sag which was then off by 5mm compared to the last time i set sag. 5mm is a lot, especially since it dropped from 31mm to 26mm and therefore fell below the recommended 30-40mm static sag - meaning the spring rate is too soft.
I went from a 5,65kg/mm to a 5,8kg/mm shock spring (both springs were actually tested on a calibrated shock spring dyno). After that my static sag was back to 32mm, but compared to when i ran the 5,8kg/mm spring a couple months earlier, my static sag dropped from 39mm to now 28mm!
I figured it might either be the rear axle position or the nitrogen already went through the bladder into the oil. After pouring the old oil (1 year old & 10 hours) into a pan, there were loads of bubbles, so my assumption was correct. I put new oil in my shock, bled it and put new nitrogen in, but the sag was still only at 29mm with the 5,8kg/mm spring. So off i went to a guy who used to do suspension for guys who race MXGP, his dyno indeed told us that something was off.
It turned out that the whole rebound shim stack was damaged by the washer which is just a stamped piece of steel instead of a machined part. The whole washer only contacted the first shim in 3 tiny spots. So we put new shims in, faced that washer on a lathe, put the shock back together and ran it on his dyno to check for improvement.
Last time i ran the shock it felt much better, but still spongy despite having a great overall performance without bottoming out. Something isn't right and i'm yet not sure if it's indeed just the axle position.
I never heard or experienced such a huge difference by just a few millimeters of adjustment on the rear axle, but i'm starting to think that this generation of Kawis reacts heavily to it's rear axle position. Next i'll try different locations in the swingarm and always adjust and measure sag accordingly.
Did anyone else ever come across such a big difference?
Uhh, wouldn’t you want to go down in spring rate if your static sag is under 30?
Under 30mm usually means too soft. He’s had to add too much preload to get rider sag.
Ahh thanks. I was imagining it not dropping as far, but it makes sense that the reason why is due to the preload and not the spring.
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