Fitting RC8 R forks to the 390….. Works Great !!

[jokerio]

I really didn't want to shell-out $1500 (+) for a pair of Cup forks from KTM So, I remember reading that the outer/upper tubes were the same diameter on both the bikes (and the same length ). Looking on ebay I found a nice pair from a 2012 RC8 R for only $400. So I thought I would take the chance and see if I could make them work on the 390.

I had to make some axle spacer/adapters, so I could use the stock 390 axle. Then for the brake, I used a 10mm off-set 320mm diameter brake rotor from a monster/916/748 mounted Backwards so it would line up in the center of the caliper.
Also had to slightly, machine off a millimeter or so from the front (now back) mounting surface face of the rotor carrier to get it all to line up correctly.

The other benefit was also being able to use a real Brembo caliper on the one side (awesome braking now !).
Of course being that the forks came off a heavier machine, I re-sprung them with lighter springs. It all works great and now with full damping adjustability, I have a wide range to dial them in !

(Oh yeah, the front fender is also from a RC8)

 

[psych0hans]

I really didn't want to shell-out $1500 (+) for a pair of Cup forks from KTM So, I remember reading that the outer/upper tubes were the same diameter on both the bikes (and the same length ). Looking on ebay I found a nice pair from a 2012 RC8 R for only $400. So I thought I would take the chance and see if I could make them work on the 390.

I had to make some axle spacer/adapters, so I could use the stock 390 axle. Then for the brake, I used a 10mm off-set 320mm diameter brake rotor from a monster/916/748 mounted Backwards so it would line up in the center of the caliper.
Also had to slightly, machine off a millimeter or so from the front (now back) mounting surface face of the rotor carrier to get it all to line up correctly.

The other benefit was also being able to use a real Brembo caliper on the one side (awesome braking now !).
Of course being that the forks came off a heavier machine, I re-sprung them with lighter springs. It all works great and now with full damping adjustability, I have a wide range to dial them in !

(Oh yeah, the front fender is also from a RC8)

Thank you so much for this, you've made my life a whole lot easier... Can you please share more details on the axle spacers/adapters? Pics maybe?

 

[jokerio]

Thank you so much for this, you've made my life a whole lot easier... Can you please share more details on the axle spacers/adapters? Pics maybe?

Let me see if I can find my notes and I'll get back to you, (with photos also….)

 

[psych0hans]

Let me see if I can find my notes and I'll get back to you, (with photos also….)

You're a godsend!!! Thank you

 

[jokerio]

You're a godsend!!! Thank you

You are welcome, just don't show-up on my turf !

 

[Hunterbear]

Nice setup!

I would be interested in your notes as well. It would be a cheaper alternative.

Not that it would be a deal breaker or anything, but can you still mount the abs ring?

 

[jokerio]

Nice setup!

I would be interested in your notes as well. It would be a cheaper alternative.

Not that it would be a deal breaker or anything, but can you still mount the abs ring?

The ring mounts on the wheel, so yes.
But you would have to find a way to mount the pick-up/sensor. I didn't care or bother because I removed all the ABS crap. That was a job in its self. After removing bodywork and tank, you can still barely get to the ABS control unit module. The unit and (4) brake lines way several pounds. I did however loose the speedo function, but oh well.

 

[psych0hans]

Let me see if I can find my notes and I'll get back to you, (with photos also….)

Hi buddy, any luck with those notes? Cheers!!!

 

[Old3]

Are the RC8 forks externally identical in length as well as fitting into the stock triple clamps? Same extended length and inner exposed leg length?

Thank you.

 

[psych0hans]

I have a set lying with me, what measurements do you need?

 

[Old3]

Axle to top cap, extended length if lower inner tube, or axle to fire seal maybe? I'm trying to find out if they are the same in length as the 390 tubes I.e. If installed flush, will the axle hole align in the same place as the 390 forks.

Thank you!

 

[psych0hans]

The overall length I believe is the same, from the fork top to the middle of the axle is approx 736mm. I'll find out about the other measurement today.

 

[Old3]

Thank you my friend!

 

[psych0hans]

Thank you my friend!

My pleasure, I'll also be fabbing up the spacers to install the stock axle and wheel, so I'll update you on that as well. Are you planning on changing out the oil and springs? These are way too stiff for the little RC and over damped too... I was thinking of a mix of 2.5 and 5wt oils. And .85kg/mm springs.

 

[Old3]

I figured if I go this route I'd be doing springs and valving to suit. Still up in the air for now but this could be a very good option.

 

[Old3]

Do you know the stock spring rate on the RC8?

 

[psych0hans]

Do you know the stock spring rate on the RC8?

It varies between generations, I have the 2012 RC8R forks which have a .95kg/mm Spring. You will also need a 320mm disc.

 

[psych0hans]

Just for reference a link to a comparison of various fork oils. Just because two different oils both say 5w, doesn't mean they are the same. KTM recommends Motorex SAE 5wt. I'm planning on using Motul Expert 5wt.
https://transmoto.com.au/comparative-oil-weights-table/

Brand/	Designation	cSt @ 40C	cSt @ 100C	VI	Type

Bel-Ray High Performance 5 W

Fork Oil

17.1

4.10

146

Mineral

Motul Fork Oil Expert - light

Fork Oil

17.9

3.90

112

Semi/Synthetic

Maxima Fork Oil 5wt

15.9

3.50

150

Petroleum

Motorex Racing Fork Oil 5.0

21.8

5.50

209

Mineral

Shell Advance 5

Fork Oil

21

5.00

153

Semi Synthetic

 

[MotoKam]

That is an extremely cool set up.

Thanks for doing the leg work (pun intended).

Any thoughts about using the whole front end with the wheel?

Also, has anyone tried to use the rear shock from an RC8?

Cheers

 

[psych0hans]

Currently ordered Maxima 5w fork oil(which is lighter than the 5w Motorex oil recommended by KTM) and 04-05 GSXR fork springs which are the same dimensions as the RC8 ones and have a stock spring rate of .85kg/mm. Looking forward to this build.