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Correct method to measure race sag?
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Posted by: sngltrk---------------------
What is the proper method to measure race sag?
1. Measure vertically straight up from the center of the axle to a fixed point on the fender.
or
2. Measure from the center of the axle to the center of the seat mounting bolt.
These two methods seem to be the most accepted. Problem is they can and do give different results.
Thanks for the help
Scott
Posted by: marcusgunby---------------------
I go vertical.
Posted by: WhiPit---------------------
Actually, (and I think Mr. Wilkey will back me up on this) you're supposed to swing an arc from your swing-arm pivot.
In other words, put the bike on the stand so the rear wheel is off the ground and use some type of straight edge to make a line from the swing-arm pivot, to the rear axle. Now, holding the "straight edge" in place at the SA pivot and keeping your other hand on the straight edge where it meets the rear axle, swing an arc upwards and find a handy reference point (on the rear fender or closest proximity).
You are correct that the "vertical" method and the "seat bolt" method will give different results. The most accurate way is as I described above - swinging the arc because your rear axle doesn't move towards the seat bolt or "straight up" when the shock compresses.
Good luck!
Posted by: marcusgunby---------------------
WhiPit i think the theory of what you say is good -however i believe for consistancy its easier to go the vertical route.In the end it doesnt matter as long as you standardise your method.Say my method gives 95mm and the other method gives 100mm-the number isnt that important its as long as you know how it handles at a given measurement so you can tune from there.Ive known riders to go between 80 and 115mm of sag so we have a vast range of feeling as to whats right.Just ensure you use the same method throughout.
Posted by: sngltrk---------------------
I believe the vertical method will yield much more accurate results. My reasoning is this; the swing arm (SA) does rotate about the SA pivot, but it never rotates as far as the fender. The tire might touch the fender, but the axle never comes close. On my bike a ’99 CR250 the manual claims 12.8” of travel. After some measurements taken from the bike, the SA pivot to the axle is about 23”. (center to center) When you put all the numbers and angles together the axle only moves front to back about 0.9” when travelled 12.8” up and down. Any measurement taken at any angle except vertical will have some front to back component along with the up and down component. I think we only want the up and down component for race sag, so the vertical measurement seem correct.
When suspension tuner quote race sag numbers, along what line are they using? If it is at some angle about the axle, what is angle?
The Honda manual says to use the axle to seat bolt method, or points close to them anyhow. This introduces a large front to back component to the measurement and changing angles, how can this be accurate.
Posted by: WhiPit---------------------
With all due respect, I still maintain that the method I described IS the most accurate. Of course, the axle only comes to within 20 some-odd inches of the rear fender, but it IS headed in that direction!
As you sit on the bike, the bike gets longer as the suspension collapses. If you draw a vertical line up from the axle when the bike is on it's stand, your vertical line will be in a different position once you sit on the bike.This throws off your measurement - not by much, to be sure. But going vertical isn't the most accurate.
To get an accurate measurement, you have to follow the arc of the rear axle. If you're just trying to get a basic "general" number, you can use Marcus' method (which in most cases is probably close enough).
Posted by: kevin5758---------------------
I don't even measure sag any more. I just turn the rear spring +/- until it feels balanced with the forks. At that point, if the forks get any modifications to change/improve them, the rear spring gets readjusted to balance with the forks.
Actual numbers: I like the forks firm, so the rear sag is approximately 90-95mm measured straight up to balance with the forks.
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