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KTM lowering 50mm, how much spacer on the shock

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I am going to help my friend lower his 2007 ktm 250 XCFW. He wants to go about 2", I figured with would go with 50mm (1.85") so that the spring change will be easier by using some Yamaha springs and we will make the limiting spacers. (its ok, I'm a machinist and slept at a holiday inn last night and I love working on suspension). How much will be needed on the shock to match the forks? Does the leverage ratio of 3:1 apply in this instance for a linkageless bike? So.. to get the matching 50mm drop at the seat to balance the chassis we will need to make a spacer 16.7 mm roughly ( 50/ 3 = 16.67mm) to go between sealhead and piston base clamp. thankfully his spring rate is right for his weight so no new spring will be needed for the shock. Not sure if any damping changes will be needed on the high speed stack to reduce bottoming for forks and shock, but I figured we could try it first and see how it goes. If you have valving recommendations I will gladly take them. He is a 160lb trail rider, C level/ novice, and riding northwestern single and double track and so far has been really pleased with the damping rates and ride. His only complaint is the seat height, as he is inseam challenged and is getting tired of picking up his bike in the super technical sections.. makes for a lot longer day and more fatigue/ less enjoyment.

essentially mimicking this KTM lowering kit

thanks for all.

 

Edited by dnsducks

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mesasure shock free lenght.

Put weight on bike until it lowered 50mm (it is 1,97" in my calculations)

Measure shock length again.

The difference is the spacer thickness.

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The 'leverage ratio' is close to 3:1 for linkage-less KTM's. 

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I will probably take the spring off go with the dropping the bike 50mm and measuring just to be sure I get it right exactly right the first time. Yes your math is right 1.968".. not sure where I was getting my math from..duh.

Any changes in sag measurements for a lowered bike? 30-33% of whatever the total travel is so 99-109mm. Does that sound right?

Thanks for the inputs guys.

Edited by dnsducks

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Assuming it starts with 335mm travel and use 110mm sag you remove 50mm you end up at 95mm sag

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Just measured by the method you suggested Cascao. I pulled the spring, then bolted the shock back up. measured shock free length, then measured to a mark straight up on the fender from the center of the axle. (similar to sag measurement). Then I put a floor jack under the back tire and start lifting the tire to get 50mm drop at the fender from the center of the axle, then measured shock length again. Ended up with a difference of 12mm at the shock. does that sound about right? getting a leverage ratio of 4.16 not sure if I am computing the leverage ratio right.. 50/12=4.16, but the bike is an 07. I have some delrin I will be making the spacer from..

fork springs: I got some Factory connection LSB-040 on order. same as stock rate .40kg but 50 mm shorter.

can anyone advise on what to expect as far as the damping is concerned. Don't have plans to make any valving changes while we are in there, as overall the rider is pleased with the stock damping. But if something certainly needs addressed becuase of the lowering we can make those changes while we are in there. any suggestions? thanks

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So here are the forks ready to go back together. Ended up getting 53mm of drop with the FC lsb-040 that measured 464mm long and pulling the 10mm preload spacer to get the same amount of preload. Stock spring  was 507mm. Then built a drop spacer our of delrin that was 53mm. Forgot to take a picture of that..

IMG_20170809_204641334.jpg

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Be careful with your rear shock delrin spacer design. I'm not familiar with your shock, but my shaft has a bleed hole in it near the piston assembly. If yours does as well, you don't want a tightly fitting spacer to block it. Race Tech sells spacers for my Showa that are looser fitting and designed to allow flow through the bleed hole. Just something to consider.

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motrock -yeah when I had it apart and looked at it.. those bleed parts where a concern. Did not want to obstruct those ports. even tho I was planning on of cutting some asymmetrical reliefs around the bore of the spacer to allow oil flow next to the shaft... i decided I felt better just making a whole new limiting spacer/shims stack base to go on the shock. I forgot to take a pick of the counter bore on the bottom side of it but it is roughly .5" larger in diameter (matches the dimensions on the stock unit) so there is plenty of room for oil to get to the ports on the rebound stroke. cross drilled .125" holes are to bleed off any excess. slightly smaller than the actual bleed ports in the shaft. probably not needed. but thought it couldn't hurt. made the spacer from 303 stainless. it came out pretty nice.

stock spacer front, new spacer plus 13mm back

 

 

IMG_20170811_164827124.jpg

IMG_20170811_165256608.jpg

IMG_20170811_165322518.jpg

IMG_20170811_173817388.jpg

IMG_20170811_193155409.jpg

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Cut about 1-1/8" out of the kickstand. Ended up with final length of 13-7/16" from bolt hole to end of kick stand where metal meets plastic foot. 

IMG_20170814_215923687.jpg

Edited by dnsducks

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I thought the pds has a lower ratio than 4:1 ?

winning by 4:1 lol

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3 hours ago, dnsducks said:

motrock -yeah when I had it apart and looked at it.. those bleed parts where a concern. Did not want to obstruct those ports. even tho I was planning on of cutting some asymmetrical reliefs around the bore of the spacer to allow oil flow next to the shaft... i decided I felt better just making a whole new limiting spacer/shims stack base to go on the shock. I forgot to take a pick of the counter bore on the bottom side of it but it is roughly .5" larger in diameter (matches the dimensions on the stock unit) so there is plenty of room for oil to get to the ports on the rebound stroke. cross drilled .125" holes are to bleed off any excess. slightly smaller than the actual bleed ports in the shaft. probably not needed. but thought it couldn't hurt. made the spacer from 303 stainless. it came out pretty nice.

stock spacer front, new spacer plus 13mm back

 

 

IMG_20170811_164827124.jpg

IMG_20170811_165256608.jpg

IMG_20170811_165322518.jpg

IMG_20170811_173817388.jpg

IMG_20170811_193155409.jpg

Nice work!

The only rider I know who used aluminum lowering spacers (forks) ended up with binding while riding.  He had the forks serviced by a different tuner, who found aluminum shavings in the fork oil.  The spacers were re-made in delrin with no more problems.  Just another concern.

Mog pointed out that another method of reducing shock travel (lowering) is to make the same depth spacer externally to recess the seal head.  This eliminates the bleed hole issue completely.  I'm not familiar with your PDS shock, but since you're so handy at machining/fabricating this may be another good option.  Others may want to chime in.

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4 hours ago, dnsducks said:

motrock -yeah when I had it apart and looked at it.. those bleed parts where a concern. Did not want to obstruct those ports. even tho I was planning on of cutting some asymmetrical reliefs around the bore of the spacer to allow oil flow next to the shaft... i decided I felt better just making a whole new limiting spacer/shims stack base to go on the shock. I forgot to take a pick of the counter bore on the bottom side of it but it is roughly .5" larger in diameter (matches the dimensions on the stock unit) so there is plenty of room for oil to get to the ports on the rebound stroke. cross drilled .125" holes are to bleed off any excess. slightly smaller than the actual bleed ports in the shaft. probably not needed. but thought it couldn't hurt. made the spacer from 303 stainless. it came out pretty nice.

stock spacer front, new spacer plus 13mm back

 

 

IMG_20170811_164827124.jpg

IMG_20170811_165256608.jpg

IMG_20170811_165322518.jpg

IMG_20170811_173817388.jpg

IMG_20170811_193155409.jpg

Do you have a workbench on wheels?

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motrock- I have seen limiting spacer before done out of billet aluminum when doing seals on a friends bike revalved and lowered by MX-TECH. I didn't notice and unusual amount of wear on the shafts or excess chips on the oil,  didn't notice any seals or bushings in those spacers but there could have been. for this job it just made more sense to me to use something a little softer than the surface it was riding on. delrin is not necessarily easier to work with when you are trying to dial in cuts by the .005'. lots of flex., a little frustrating at times. 

mog- you are right. after installing the shock and measuring it on the bike WITH THE SPRING ON THIS TIME READY TO GO with preload.. I ended up with only 43mm of drop. Exacty 10 less than I was shooting for. there are a couple of possibilities for this. but ultimately I think it comes down to the added pressure with the preload on the spring forcing the shock further down by smashing the bump stop a bit more than when I measured before without the spring.. assuming the limiter I made is 13mm taller which I am fairly certain it is. I am calculating the leverage ratio to be 43/13= 3.3. my guess is not having the spring on the shock when doing the measurements contributed to the 3mm difference. does that sound likely?  that would mean I should have made a spacer that was 16.06mm instead of the 13mm.

Kinda bummed. shock is filled with N20 and I don't really want to tear it back down. I will probably wait for next oil change or if I revalve it to make the new spacer.

Having said that. what do I do with the 10mm difference in ride height? 53 in front 43 in back. Just run a little more race sag? Say 105 instead of 95mm? rider was riding with the forks up in the clamps pretty far up before the lower job. guessing he won't notice a real difference in handling even running 95mm sag with the new set up. Trail riding only. you thoughts?

redrider- the tool box is on casters.. so, technically yes.

IMG_20170815_124446984_HDR.jpg

Edited by dnsducks

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What about simply matching the front to the rear since the forks are apart? It might be easier to adjust the front at this point, and test. It may be lowered enough to get the ride he's looking for. If I've done the math correctly, you can put the 10mm spring spacers back in to compensate.

The rear leverage ratio changes throughout it's travel, so a straight ratio calculation will probably be a little off. I'm not familiar with PDS, so I'm not sure how much different the ratio is at 53mm travel. Maybe you can back up your math calculations by measuring the shaft again on the bike.

Good luck!

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good point. Everything is back together an installed on the bike tho. chalkin this project up as done. Just need to check sag. according to my math it should be somewhere around 96mm, but for the time being I will probably just have him run about 105mm of sag and leave the forks at 5mm above the triple clamp and see how it feels and go from there.. I think it will be good there.. if need be I can move the forks down to flush in the triple and run even more race sag if it's too twitchy. My guess is it will be fine tho. the ride height is good. He can touch with both feet and seems pleased. it's going to be a difference maker for him on the trail, i have no doubt. i thought 2" was going to be too low.. turns out it is just right for him. Both feet touching on the balls of the foot only, perfect.

Yeah I am a little upset with myself for not getting the math right on the leverage ratio. I thought the method of static measurements would get me a lot closer but so much for that idea. Oh well i can live with 10m variance and chalk it up as a learning experience. I am hopeful the extra 10mm of rear ride height with actually be a handling improvement for the riding he does. Hope that is not just wishful thinking tho...

Edited by dnsducks
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I think I would work with what you have now

winning by 4:1 lol

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