CDN Duke
Member
Thanks Chris, that's got to be part of the difference allowing for the RC390's longer 345mm spring vs the Duke's 320mm.
Difference in travel, springs between Duke and RC 390?
CDN Duke
Member
Had my forks apart Weds night - couldn't get the bottom of the fork legs off as didn't have enough clamping grip on leg to allow enough force to remove the fork bottom, even with enough heat applied for the loctite that the fork end started to smoke a bit. As a result, we couldn't remove the bottom 50mm preload spacer that is held in place by an o-ring around it's circumference. I ended up reassembling the legs using the original spring. I have a feeling it will be too softly sprung for the reduced 125mm travel. I won't know until I can ride it in the spring...
Duke 390 fork innards. White 30mm spacer, 320mm spring and damper strut assembly.
Duke damping rod (top) vs RC390 damping rod. I purchased the RC390 damping rod from parillaguy with whom I met up with when he came out my way to buy a race bike.
Inner bits appear to be exactly the same but the external parts differ a bit. Top locknut in Duke 390 is very shallow, you can see it propped up next to top cap along with the minimal bottom out bumper, small black rubber ring stuck to the top locknut piece. Compared to the bottom out bumpers on the RC390 damping rod, two conical pieces, larger one is harder plastic and smaller one is softer, more rubbery. Under the Duke damper, you can see a white preload spacer, about 2.5mm in thickness.
My plan was to pull the stock Duke 320mm spring and replace with the RC390 345mm spring (Chris RC390 sent me a pair), but I was going to have to pull the 50mm preload spacer out of the fork as part of the reconfiguration, but as we discovered, it's got an o-ring around it to keep it from moving in the leg and that which prevented us from tapping it out (photo is from one of the 4 spares I now have). So, you're supposed to pull the fork bottom off the stanchion but that proved difficult without a proper tool to hold the chrome stanchion. We heated the fork bottom and removed the set screw, but could only get it to turn about a 1/16 of a turn after quite a few attempts. (You can see the magic marker reference line to on chrome to right below the set screw). Note that these bottoms are reverse threaded, need to turn clockwise to unscrew. We had put an allen key in the fork tube to help keep it from rotating in our rigged up clamping assembly but after breaking the key off once and additional attempts, we realized we ended up deforming the hole and I had to try and knock down the ridge that resulted. I was able to polish it up and correct the damage but that was a learning experience.
So, the primary intent of this exercise was to install a travel limiting spacer on the damping rod. The KTM kit pictured above has a black 25mm bushing. I searched around for something to use and decided on a length of 1" diameter solid Delrin rod to use. However, when we realized the difference between the two damping rods and the fact that the RC390 unit had these two black bottom out protection pieces, we decided to try those for now and see how they worked. The two together came to ~27.4mm. I pulled one of the 2mm spacers I had from the RC390 spares, and it looked like the total would equal the 30mm spacer I was going to remove from below the spring. So, we added the two bumpers onto the damping rod to act as extension limiter and added the black 2mm spacer with the white 2.5mm spacer on top of the spring.
We reassembled the legs, refilled with 5 wt oil. I had two bottles, one Amsoil 5 wt and one Honda 5 wt, kept them separate for respective legs, one green, the other red (pink).
My feeling is that this setup will be too soft as the stock spring is intended to cycle through up to 150mm of travel but I have limited the fork to ~125mm now (reduced by 27.4mm from stock). I won't be able to ride and determine for sure until I get the bike out of storage in the spring unfortunately though. I weigh 150lb nekkid, wife is lighter at 125 lb. It might be fine, but I expect it will be soft... Should it be too soft, my plan will be to get a proper fork leg clamping tool (DCMoney has graciously offered to loan me his), pull the fork leg bottoms, remove the 50mm spacer, put the 30mm white spacer back with the RC390 345mm spring. The RC390 spring is designed for 125mm travel so presumably better for my intended setup. We shall see.
Duke 390 fork innards. White 30mm spacer, 320mm spring and damper strut assembly.
Duke damping rod (top) vs RC390 damping rod. I purchased the RC390 damping rod from parillaguy with whom I met up with when he came out my way to buy a race bike.
Inner bits appear to be exactly the same but the external parts differ a bit. Top locknut in Duke 390 is very shallow, you can see it propped up next to top cap along with the minimal bottom out bumper, small black rubber ring stuck to the top locknut piece. Compared to the bottom out bumpers on the RC390 damping rod, two conical pieces, larger one is harder plastic and smaller one is softer, more rubbery. Under the Duke damper, you can see a white preload spacer, about 2.5mm in thickness.
My plan was to pull the stock Duke 320mm spring and replace with the RC390 345mm spring (Chris RC390 sent me a pair), but I was going to have to pull the 50mm preload spacer out of the fork as part of the reconfiguration, but as we discovered, it's got an o-ring around it to keep it from moving in the leg and that which prevented us from tapping it out (photo is from one of the 4 spares I now have). So, you're supposed to pull the fork bottom off the stanchion but that proved difficult without a proper tool to hold the chrome stanchion. We heated the fork bottom and removed the set screw, but could only get it to turn about a 1/16 of a turn after quite a few attempts. (You can see the magic marker reference line to on chrome to right below the set screw). Note that these bottoms are reverse threaded, need to turn clockwise to unscrew. We had put an allen key in the fork tube to help keep it from rotating in our rigged up clamping assembly but after breaking the key off once and additional attempts, we realized we ended up deforming the hole and I had to try and knock down the ridge that resulted. I was able to polish it up and correct the damage but that was a learning experience.
So, the primary intent of this exercise was to install a travel limiting spacer on the damping rod. The KTM kit pictured above has a black 25mm bushing. I searched around for something to use and decided on a length of 1" diameter solid Delrin rod to use. However, when we realized the difference between the two damping rods and the fact that the RC390 unit had these two black bottom out protection pieces, we decided to try those for now and see how they worked. The two together came to ~27.4mm. I pulled one of the 2mm spacers I had from the RC390 spares, and it looked like the total would equal the 30mm spacer I was going to remove from below the spring. So, we added the two bumpers onto the damping rod to act as extension limiter and added the black 2mm spacer with the white 2.5mm spacer on top of the spring.
We reassembled the legs, refilled with 5 wt oil. I had two bottles, one Amsoil 5 wt and one Honda 5 wt, kept them separate for respective legs, one green, the other red (pink).
My feeling is that this setup will be too soft as the stock spring is intended to cycle through up to 150mm of travel but I have limited the fork to ~125mm now (reduced by 27.4mm from stock). I won't be able to ride and determine for sure until I get the bike out of storage in the spring unfortunately though. I weigh 150lb nekkid, wife is lighter at 125 lb. It might be fine, but I expect it will be soft... Should it be too soft, my plan will be to get a proper fork leg clamping tool (DCMoney has graciously offered to loan me his), pull the fork leg bottoms, remove the 50mm spacer, put the 30mm white spacer back with the RC390 345mm spring. The RC390 spring is designed for 125mm travel so presumably better for my intended setup. We shall see.
D
Deleted member 452
Guest
Thanks for sharing this info with pictures!
CDN Duke
Member
Yeah, makes more sense when you can literally picture it...
For reference, here's a larger photo of the OEM KTM lowering kit bushing on the damping rod, what I was trying to replicate. (photo above in thread just thumbnail)
CDN Duke
Member
I now have close to 200kms on the bike since the modifications. Not enough time yet but weather hasn't been cooperative here in Ottawa. We now have rain for the next few days which should wash away the remaining snow and salt...
Fork now has ~135mm of exposed stanchion total. This will be less than RC390 due to fact that I've removed the bottom out bumpers and moved them onto the strut assembly.
I'm currently using just over 100mm of travel. I haven't measured sag properly yet, my main concern was that I would blow through the travel but that doesn't seem to be the case yet, but I couldn't say I've hit any harder dips or anything yet either so bit premature to make any final assessments.
I should have another ~25mm of travel. I'll keep monitoring. At this point, I'm still planning to swap the RC390 345mm spring as I think it may be more stiffly sprung than the Duke but I have no idea really...
Fork now has ~135mm of exposed stanchion total. This will be less than RC390 due to fact that I've removed the bottom out bumpers and moved them onto the strut assembly.
I'm currently using just over 100mm of travel. I haven't measured sag properly yet, my main concern was that I would blow through the travel but that doesn't seem to be the case yet, but I couldn't say I've hit any harder dips or anything yet either so bit premature to make any final assessments.
I should have another ~25mm of travel. I'll keep monitoring. At this point, I'm still planning to swap the RC390 345mm spring as I think it may be more stiffly sprung than the Duke but I have no idea really...
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CDN Duke
Member
I swapped to the RC390 springs... last I measured, 20mm static sag, 37mm rider sag when setup with 10mm preload spacer. Quite happy with the outcome. Now trying to dial in my new YSS MZ506 rear shock.
CDN Duke
Member
Reduced oil volume by 50ml each leg, from 450 to 400ml now. Have not been getting close to what I believe is full travel, still maxing out around 100mm. Re-measured static sag, closer to 25mm now. Will take more measurements at track day on Monday and see if I'm able to get more stroke. I think I've been locking out hydraulically with the full volume of shock oil.
Fangfactor
Member
I came to the same conclusion. Now running 400ml oil volume. and can get around 122mm travel now compared to 100mm before
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