If you had a fixed spring rate in your front forks, but could replace your rear spring rate to whatever you needed, you would figure out your front sag, and adjust your rear spring rate to match the front sag, right? the reason i'm asking is because i have a fork kit for my mini that is already installed on the bike along with an aftermarket rear shock, and i am wondering if my rear spring rate is too stiff. 1) what symptoms would the bike create when riding it that suggest to you the rear spring rate is too stiff? 2) what symptoms would the bike create when riding it that suggest to you the rear spring rate is too soft? 3) what symptoms when setting the bike up for a rider would suggest to you the rear spring rate is incorrect? Thank you in advance. The bike in question is an Aprilia RS50 with a Honda CR125R motor, marzocchi 35mm front forks with .80 race tech springs and gold valve emulators, with adjustable preload. the rear is a DNM MT-BAG rear shock with a 1000lb spring, if it matters. I weigh probably 180 lbs with gear on. about 167 lbs in underwear
You should be able to identify if the spring rate is too high or low by your sag numbers on the rear by looking not only at the rider sag but also bike sag. Too much preload to get the right rider sag will result in too little bike sag.
Roughly 5% of the travel for bike sag is the number that seems to crop up for a starting point. So it will depend on suspension travel front and rear.
Take a grease pen and draw a line on the shaft of the shock. Run a session. If you're bottoming out the shock, put on a stiffer spring. Sag is a destination, not an origin. Likewise, look at the seal head, if there's witness marks from the bump stop, you're out of stroke and working the tire too hard. Granted, this is just a snapshot of what has happened once, but short of a data acquisition system and potentiometers or a very tuned in ass dyno, it's a good start.
Not usually. There's usually a lot more travel in the forks than there is the shock in roadrace applications.
As said sags a baseline and each rider and styles a bit different along with speed etc. just whatever changes you do linit the # of changes at once and be methodical about it. Write down everything.
Wow, thank you! What would you look for if the rear spring rate was too stiff? I am actually fortunate in that the bike already has a full data acquisition system on it. I just have to add the potentiometers. Perhaps that will be something I do next year. Thank you. For sure one change at a time is plenty. I will approach it the same way i approach jetting. Having a logbook helps so so so much it's not even funny when it comes to my bikes.
I missed that it was an RS50. I did two shocks for those in the past for Brandon Paasch. Well, one was for Paasch and one was for a fast adult about 180lbs and I believe we ended up with a 1300 lb spring with very light installed preload. IIRC, there was no linkage on the shock, so no mechanical advantage.
Very smart. No idea how many tines people complaining bike went backwards but they changed geometry, springs, shim stacks, old tires vs new, cold vs hot
I used to cringe even when throwing calculated hail Mary's at the bike bike because you were out of time.
yeah i am currently at 1000lbs. no linkage, no mechanical advantage. i built two virtually identical bikes with the same rear shock. when braking hard, what little engine braking the 125 engine has in it is enough to chatter the rear tire on both bikes. tires are in the range as specified by the manufacturer, so i am wondering if its an issue with spring rate, or whatever. i never noticed it at jennings, only on the small karting track.
crazymofo: don't know. I have not done the grease pen technique on them. I re did the front forks adding damping rods to both legs, put stiffer springs, 30 wt oil, and race tech gold valve emulators in them, along with external preload adjustment knobs... did my best to set my sag at ~30% by myself using my iphone and photos as references with the bike standing up. So there is definitely some room to improve there. the forks are 35mm marzocchi traditional forks if that matters. Since putting the emulators in there they feel a lot smoother , but genuinely, what do i know. they might just be smoother cause my wallet is lighter.
