Suspension Puzzle - ADVISE PLEASE

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4/25/2018 12:37 PM

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

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4/25/2018 12:41 PM

Sounds like the forks are too soft and you're bottoming or they are riding on the stiff part of the stroke.

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?

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4/25/2018 12:45 PM

Have you checked travel on the forks to see how much they are actually moving? Ziptie around one leg and see how much travel you are using. If I remember correctly 2-3 psi is a spring rate. I'm 45 and run 152 in my 350 on the mx track. I tried lots of setting down to 137 and it was super soft but I felt everything. I'm 175 lbs so I'm wondering if you are just too soft, not too stiff.

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4/25/2018 12:48 PM

rikhek wrote:

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

Contact Doc @ 812 Suspension in Texas. He is a suspension magician. 512-695-9645

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4/25/2018 1:08 PM

keep PSI the same and put your compression at 8-10 clicks out and see if it gets better. Youre likely pushing the forks further down in their stroke into the harsher part of the valving. you need to stiffen them to reduce what youre experiencing. move one thing at a time. compression first then bump up the PSI

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4/25/2018 1:15 PM

That air pressure is way low for having a heavier rear spring. I went one stage lighter on my rear spring (on a 250sx) and I usually run 142psi in the front.

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4/25/2018 1:51 PM
Edited Date/Time: 4/25/2018 1:54 PM

THANKS to all for taking time to help me out. GREATLY APPRECIATED. Just about everybody I've spoken to and who's responded to my thread points out the new, stiffer rear spring has changed the balance and is putting too much pre-load on the front. As such, the front is in the "middle" of the stroke as soon as I get on the bike. I'm embarrassed I didn't realize what was going on as I've been doing this for over 45 years.

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...

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4/25/2018 3:55 PM

Sounds like you're already on your way but

https://motocrossactionmag.com/mxa-race-test-2017-ktm-450sxf/

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4/25/2018 5:10 PM

rear shock spring will break in and could change your sag quite a bit.

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