Difference in travel, springs between Duke and RC 390?

[CDN Duke]

I need to lower the front end of our Duke 390 more than what I can achieve by raising the fork tube in triple clamp (~17mm) - in order to balance out geometry change from lowering rear ~45mm with T-Rex Racing's lowering link kit. http://www.ktmduke390forum.com/forum/appearance-modifications/33449-t-rex-racing-45mm-lowering-kit-review.html

I am contemplating buying the KTM 90212955044 lowering kit for just the springs but if I can achieve the lower ride height through internal modifications without those springs, I'd like to try that if possible. The parts listing I have for the Duke doesn't show fork internals or part numbers for the spring etc. Does anyone know how the two fork setups differ? The Duke has 150mm travel to RC390s 125mm - presumably the springs are different? The Duke 390 lowering kit lowers by 25mm - are they just putting in a spring similar to the RC390 spring by chance? albeit not a progressively wound one as the photos would suggest?



I borrowed the pic of the RC390 fork internals below from gbabyracing's thread on adding preload:

"This shows the fork cap removed from the damper rod. The first black ring between the cartridge and the spring is the 5mm spacer.

 

[CDN Duke]

Eg, I think I may be in the market for some (preferably lightly) used RC390 springs?

 

[CDN Duke]

Matt from Formula390.com has stated in another thread that the stock RC390 springs are 345.5mm, and the stock spacer is 5mm, for a total of 350.5mm length. I need to understand the Duke spring length now and how the overall travel is managed I guess.

 

[Chris RC390]

Eg, I think I may be in the market for some (preferably lightly) used RC390 springs?

I have some stock RC390 stock springs I can give you. The springs have about 200 miles on them.
Just pay for shipping.

 

[CDN Duke]

I have some stock RC390 stock springs I can give you. The springs have about 200 miles on them.
Just pay for shipping.

Thanks Chris, will send PM shortly.

 

[Diploman]

CDN, I think you may be making this more complicated than necessary. All you really need to do is to loosen the triple clamps on the fork legs and raise the latter by an amount commensurate with the distance you lowered the rear. Retighten. Presto, balance restored.....

 

[CDN Duke]

Already did that - you can only get 17mm adjustment if you stay on the smooth portion of the fork legs (at the lower clamp). I lowered rear by ~45mm, so trying to get closer to that in total.

http://www.ktmduke390forum.com/foru...49-t-rex-racing-45mm-lowering-kit-review.html

(Link advertised 45mm, I did take one measurement at stock setup, but it was on side stand and the new side stand will presumably affect the 'after' measurement, but I'll redo that once I get the bike back from dealer.)

 

[Deleted member 452]

You might want to get higher clearance rear sets when you're done because you're giving up a lot of cornering clearance by lowering the bike. I rode my first street bike on my tip-toes at 14-1/2. I was 5'8" and 140lbs and the 1981 Suzuki GS550L was 32" and 476lbs with a full tank.

 

[Deleted member 452]

FYI- I was 16 years old and 5'10" in that last picture

 

[CDN Duke]

You might want to get higher clearance rear sets when you're done because you're giving up a lot of cornering clearance by lowering the bike. I rode my first street bike on my tip-toes at 14-1/2. I was 5'8" and 140lbs and the 1981 Suzuki GS550L was 32" and 476lbs with a full tank.

Agreed, I'll look to purchase some eBay sets over the winter. I may also try modifying the seat if possible, to narrow it up and see if it provides a benefit. But yes, the ground clearance is a concern. The lowering is a compromised solution, I recognize that.

Granted, it's more for me that it'll be a concern. My wife is still a novice rider and is less likely to approach the lean angles or ride as aggressively where it may be problematic. I'll be verifying the final setup accordingly.

 

[Diploman]

Why is it necessary to stay only on the smooth portion of the forks? The triple clamps won't care if they are clamping a smooth or textured surface.

 

[CDN Duke]

Effectively the material is thicker where it's smooth - there's been aluminum machined away where it switches to the textured portion... I think that's the intent also of the difference in the finish as well?

 

[CDN Duke]

So, I owe a big thanks to Olivier Gregoire at Gregoire Sport in Quebec, aka www.dirtnroad.com. (For the Canucks here, they have a pretty good inventory of KTM bits online and for the Americans, the exchange rate is in your favor now and our Canadian $ prices may be pretty attractive).

Anyhow, he opened up the KTM 390 lowering kit, sent me the instructions, and measured the springs for me. Looks like they're 320mm in length. The Duke fork has one of two spacers it appears, a 30mm or a 50mm. If it's the 30mm, it's to be removed on the Duke (only). If it's 50mm (and resides in the leg, it stays?!) I don't quite understand that part. I think there must be two generations of spring lengths used on the two bikes actually. The final spring assembly, when it goes back in the bike, is expected to be 325mm in length, which is the replacement 320mm spring and the 5mm plastic ring (spacer).

The kit also comes with a ~25mm bushing which goes on the rod assembly. This looks to be additional bottoming protection perhaps?

So, if I understand correctly, the overall spring stack is possibly reduced by 25mm, which is the advertised lowering. There's an added bottom out protection added. I'm not sure how the RC390 has 25mm less travel out of the box though, if the OE spring length is 345mm? Overall fork length is different perhaps - the legs do have different part numbers between the Duke and RC.

 

[CDN Duke]

So, I was reviewing my basic arithmetics and if Duke fork has a 30mm spacer to start, plus spring, plus 5mm plastic ring B, with resultant lowering setup to remove spacer and 320mm lowering spring plus 5mm plastic ring is to be 325, the lowering is really a result of the removal of the 30mm (preload) spacer? Advertised lowering is 25mm. Total travel is limited by bottom out bushing that is added to rod assembly?

I'm still trying to determine spring length of stock Duke spring. One of the members on the Duke forum has probably measured before he ordered some aftermarket springs - just don't have a response yet.

 

[CDN Duke]

So, got some updates from the Duke forum members LNICK and Trimapa - stock spring on Duke 390 is 320mm and is setup with both a white 30mm and a black 50mm spacer. Spring is progressive, same as RC390, just shorter at 320mm vs RC390 at 345mm.

Is it correct to understand that the RC390 forks do not have either of the white 30mm or black 50mm spacers?



KTM lowering kit appears to use a linear spring, compared to stock progressive, removes 30mm white spacer (but retains 50mm black) and then adds what I think is a bottom out protection 25mm black bushing to piston assembly.

So, bulk of lowering is actually removal of 30mm preload spacer combined with the new spring, results in a 25mm lower ride height.

With my ultimate goal of achieving a similar 25mm drop, or possibly more (but really, don't want to go too low), would using Chris RC390's 345mm (25mm longer than stock Duke 320mm) spring combined with removal of the 50mm spacer achieve similar result? I don't have the bottom out bushing however but perhaps the ostensibly stiffer RC390 spring will reduce the need for that?

Thoughts?

 

[ToraTora]

CDN, The stock spring employs a progressive spring because the actual rider weight would be undetermined. When lowering the bike like you are with the suspension they are probably assuming that it's going to be a lighter weight rider, and thus can go with a specific spring.

For others reading this thread.

This is a really horrible way to effect a bike to create a lower seat hight. I understand that it's hard to find a bike if you are vertically challenged, but destroying the suspension design of a bike is exceedingly unsafe, and probably the worse solution you could employ.

If you can't find a bike that fits your body there are other options. You can modify the seat, and if that's not enough find a bike that has a removable subframe. Then either create your own, or get a shop to make a subframe that lowers the seat (this can also be done on a frame that doesn't have a removable subframe, but requires more commitment). You may also have to modify the bars to be lower depending on the bike, and your reach. The new Duke 390 has a removable subframe, as to many other bikes.

Also, you don't actually have to have both feat on the ground. If you look at motocross riders they ride bikes with enormous amounts of travel which stick the bikes way up in the air. They straddle the bike with one leg hanging on the seat. Yeah taller folks don't have to do this, but then smaller folks have other advantages. Humans are really good at adapting to things, and bikes just happen to be another opportunity to adapt.

​

And there's this guy who is probably a lot shorter than most.

 

[CDN Duke]

ToraTora - KTM sells a lowering kit. If you lower equal amount F&R, which is what I'm trying to achieve, how is the geometry affected negatively? Granted there is less ground clearance and I recognize that and will ride the bike accordingly. It's the same with my car - it's lowered and I can't drive it the same way under certain conditions as a non-lowered vehicle. Similarly, people use ride height adjustable rear shocks and raise/lower the tubes on their forks all the time. At what point does it become really unsafe?

For my wife, who is still a novice rider, it's not really an option to slide her leg over on the seat just yet. Once she has more experience, that will be easier to do. I will look at modifying the stock seat to make it narrower and reduce height if I can as well.

 

[Fasteddy]

FWIW,

I concour based on experience that the way you are going about lowering your bike is going to likely cause unexpected results, as it did when I put FZR600 clip ons onto a 1984 GPz 750. It was for other reasons then lowering but required raising the fork tubes in the triples at least the same amount you have, it certainly was much snappier steering response wise. Since the start of this thread I have searched the web for information about how what I / you did with sliding the forks. From what the racers have said on this forum I have learned that it can have a noticable effect.

So the only thing I came upon, which was most interesting to read, was a book preview by Tony Foale, as I read through the parts that were available on line I suddenly realized why on at least a few occasions that old GPz setup the way it was would for what seemed like no reason would have the front wheel flop out from underneath it.

It is worth a read to see how the changes effect steering geometry, He even has some free software for down load and the book is available fot purchase on his site.

TONY FOALE DESIGNS - Home page

 

[CDN Duke]

Thanks Fasteddy, I'll take a look.

I think I've just realized what actually effects the lowering in the KTM kit - it's the 25mm bushing applied to the rod assembly. It limits the extension? I thought it was for bottom out protection. The new spring has a different rate, so combined with less extension, the 30mm preload spacer can be removed. I simply didn't understand the mechanics of this spring assembly - come from a mountain bike background and have never worked on upside down forks like this.

So, I just need one of these bushings, and then an appropriately sprung spring and I should be good to go?

 

[Chris RC390]

So, got some updates from the Duke forum members LNICK and Trimapa - stock spring on Duke 390 is 320mm and is setup with both a white 30mm and a black 50mm spacer. Spring is progressive, same as RC390, just shorter at 320mm vs RC390 at 345mm.

Is it correct to understand that the RC390 forks do not have either of the white 30mm or black 50mm spacers?

View attachment 3276

KTM lowering kit appears to use a linear spring, compared to stock progressive, removes 30mm white spacer (but retains 50mm black) and then adds what I think is a bottom out protection 25mm black bushing to piston assembly.

So, bulk of lowering is actually removal of 30mm preload spacer combined with the new spring, results in a 25mm lower ride height.

With my ultimate goal of achieving a similar 25mm drop, or possibly more (but really, don't want to go too low), would using Chris RC390's 345mm (25mm longer than stock Duke 320mm) spring combined with removal of the 50mm spacer achieve similar result? I don't have the bottom out bushing however but perhaps the ostensibly stiffer RC390 spring will reduce the need for that?

Thoughts?

My RC 390 did not have the white spacers, only the black spacers.