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328
Joined
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
Usually a stiffer rear spring or more preload puts more weight on the front.
Are you sure the sag is set right? Do you have any static sag now with new stiffer spring? Did you have No static sag with the stock rear?
The Shop
I'll bump up the fork air pressure to get the front back up to the top of the stroke as suggested. Being an engineer by degree makes me feel even more of an idiot. Simple, straight forward geometry/dynamic load calculation. I was being an idiot not realizing lowering the pressure was making it even worse.
Again, thanks guys.
Rick
P.S. LOL. It was amazing how well it turned and held ruts. Too bad that's all it did well...
https://motocrossactionmag.com/mxa-race-test-2017-ktm-450sxf/
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