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328
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2/26/2012
Location
Tulsa, OK
US
A mechanical issue required me to replace the OEM shock spring on my 17 KTM 450 SX-F. The stock spring was a 4.6 kg/mm and I replaced it with a 4.9 kg/mm. The 4.9 is the next step up on the stiffness
The 4.6 worked fine but was at the limit with pre-load for my weight and riding style/skill/speed. I weigh 200 lbs and the 4.6 is said to be good up to a 189# rider.
I'm a 57 year old who'd been riding MX for 45 years. I weigh 198 naked and ride at an intermediate vet pace.
After installing the new 4.9 spring and setting sag at 105. I've ridden the bike twice since and have the rear shock dialed in.
Problem is the forks are now beating the hell out of me. They were PERFECT prior to installing the new rear spring. I've been running 135 lbs in the forks since buying about 1.5 years ago with 22 clicks out on the comp. LOVED the forks.
However, now the forks are just terrible. So harsh the outside palms of my hands hurt to touch after riding all day yesterday on a somewhat rough track. I progressively backed the compression out all the way (i.e., 36 clicks) and no better. I reduced air to 130 pounds and not really any difference. I have Motion Pro bleeders and vent/release outside chamber air pressure every time I go on track. I have not had ANY increase in air pressure since the problem developed.
Anybody have any thoughts on what might be the issue. Keep in mind the ONLY variable to the bike is the different, stiffer rear spring. This is NOT a subtle feel change, it's harsh as hell. Funny as it turned better and held ruts better but everywhere else it hurt....
TIA
Rick
The 4.6 worked fine but was at the limit with pre-load for my weight and riding style/skill/speed. I weigh 200 lbs and the 4.6 is said to be good up to a 189# rider.
I'm a 57 year old who'd been riding MX for 45 years. I weigh 198 naked and ride at an intermediate vet pace.
After installing the new 4.9 spring and setting sag at 105. I've ridden the bike twice since and have the rear shock dialed in.
Problem is the forks are now beating the hell out of me. They were PERFECT prior to installing the new rear spring. I've been running 135 lbs in the forks since buying about 1.5 years ago with 22 clicks out on the comp. LOVED the forks.
However, now the forks are just terrible. So harsh the outside palms of my hands hurt to touch after riding all day yesterday on a somewhat rough track. I progressively backed the compression out all the way (i.e., 36 clicks) and no better. I reduced air to 130 pounds and not really any difference. I have Motion Pro bleeders and vent/release outside chamber air pressure every time I go on track. I have not had ANY increase in air pressure since the problem developed.
Anybody have any thoughts on what might be the issue. Keep in mind the ONLY variable to the bike is the different, stiffer rear spring. This is NOT a subtle feel change, it's harsh as hell. Funny as it turned better and held ruts better but everywhere else it hurt....
TIA
Rick
You need to recheck your shock static sag and then the race sag and if they are not within spec's, which they will not be, it has to be corrected first.
Go to race tech on line and use the spring rate calculator to figure out the correct rear spring needed.
Paw Paw
The problem is your air pressure in fork.
Im 195lbs, 17 450 SXF. 48nm 36mm static (~105 sag). I ran my forks today at 153psi and that was almost to soft.
So set it to 150psi to start with, and adjust from there up in 2psi steps
Stiffer is softer. Dont understand why people dont start at what the manual says, it is not wrong. Running rear spring for 200lbs and fork pressure for 150lbs will give you this result.
The Race Tech calculator recommends the 4.9 I now have as do the guys at Factory Connection. It's a FC spring AND I picked/ordered it based upon their recommendation. I even spoke to them this morning and they are puzzled...
Please note the bike worked very well with the lighter, OEM 4.6 spring.
The Shop
that said, It sounds to me that you have got the shock setup better now. This is holding the bike up better, and putting more bias on the front end- this is why its turning better, but it might be riding too low in the stroke- by taking out PSI or compression damping you are making it worse.
In order to compensate, aees is right- try adding air pressure and/or compression damping.
As such, I'm into the "mid stroke" of the forks as soon as I get on the bike? As the forks gets progressively stiffer/harsher I'm already into the "stiff" portion of the stroke while sitting static?
If this is what is going on this would also explain why the bike was corning better and holding ruts better than it ever has but once I'm through the turn/rut it's like riding with a rigid suspension?
I greatly appreciate you taking the time to help me out.
Rick
I apologize for writing my prior post before reading your response. I was typing a response as you were typing/providing your solution.
MUCH APPRECIATED.
Rick
P.S. I'm a little embarrassed. I'm an engineer by degree and should have realized the change in the rear spring changed the pre-load on the front and that I need to use air to get the front raised back up to counteract the additioanal pre-load imposed on the front by the stiffer rear spring.
Paw Paw
Paw Paw
Been through and have at home 42,44,46,48,50,52 rear springs that i have tested with and 48 is just the right one. 46 is to soft, but might work if you are a really short person that dont move much on the bike. 50 might work if you are really really fast and tall guy. But then we are gona end up with 158-165psi to get it balanced.
No need for him to go there.
A couple others hit on it here. To me it sounds like now with the slightly stiffer shock spring you are riding front heavy, so you must add pressure to the air fork. Id be willing to bet when you dropped pressure your forks are in the middle of their stroke which is more harsh. In other words by reducing fork pressure you are riding lower in the stroke, which in turn makes the bike feel more harsh. Move clickers back to stock positions then add air pressure to the forks (slightly more than prior to stiffer shock spring) then work from there. Once you are close then start adjusting with clickers.
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