Topic: KTM USD swap on a CB350f / CB400/4... direct bolt-on!

[Erwin83]

Just a little tip for people contemplating doing an Upsidedown fork swap in their CB:

A USD fork of a KTM 125/390 Duke or RC (Duke has handle bar mounts, RC has clip-ons) will fit without any hard work, and relatively low cost (I got a complete fork with yokes, handlebar clamps and front wheel axle for 300 euro).
- bottom bearing standard CB350f / CB400/4
- Top bearing ID 30 instead of 26mm
- Best bit: stem length is spot on.
- Even the steering angle stoppers fit.
- CB350f fork is 69cm long (wheel axle to top-cap), when parked on the main stand.
  - KTM fork is 72 cm long, when parked on the jiffy. KTM uses a smaller wheel of course.

Fitting as follows: The Allballs CB stembearing kit comes with 2 spacers, combined they are 7mm thick. Exactly what is needed to clear the KTM bottom yoke and the CB frame.
Top bearing: fit as standard KTM.

Keep in mind:
- I have not yet checked tank clearance.
- This fork set-up has a single brake disc only.
- offset of the fork is approx. 40mm. CB350f is approx. 60mm, so trail is affected. Don't know yet what this will do with handling, but I guess this is similar to most USD fork swaps.

Big thanks to allballs for providing their conversion tool (https://www.allballsracing.com/forkconversion/), and the KTM Duke rider who helped me measure the KTM fork before putting the money down.

[andy750]

Great info and I will be interested to see how this affects handling. Looking forward to seeing the finished bike.
good luck!
Andy

[SOHC4 Cafe Racer Fan]

Erwin,

Also keep in mind that the KTM likely has a smaller offset between steering head and fork tubes than the 350F/400.  If so, this can make the bike tend to want to fall into corners. 

[754]

He mentioned that in his post..
 I am wondering what bearings in the top, how it all fits.
 And the travel is probably a lot longer than stock forks. Are you cutting down the amount of travel, or increasing back travel ?

[Erwin83]

The 2013-2016 KTM Duke/RC is quite a sporty Street/City bike. I expect suspension travel to be less or more or less equal.

This fork is absolutely NOT a KTM cross or Supermoto fork.
Those forks will NOT fit like this one.

[Erwin83]

Erwin,

Also keep in mind that the KTM likely has a smaller offset between steering head and fork tubes than the 350F/400.  If so, this can make the bike tend to want to fall into corners.

Yes, the offset is 40mm, instead of 60 mm stock CB.
How this will affect handling, i don't know yet.
Let's be honest, people doing these swaps care about looks more than anything else. That includes myself, I confess.

[Erwin83]

I am wondering what bearings in the top, how it all fits.

I am using all balls bearings.
The top bearing can be ordered for this swap:
99-3541-5
It is basically a 48,5mm outer shell and the 30mm ID roller bearing, same as used at the bottom yoke.

It's rather unbelieveable how easy this fits as the steering pin is exactly the right length...

[754]

Erwin,

Also keep in mind that the KTM likely has a smaller offset between steering head and fork tubes than the 350F/400.  If so, this can make the bike tend to want to fall into corners.

Yes, the offset is 40mm, instead of 60 mm stock CB.
How this will affect handling, i don't know yet.
Let's be honest, people doing these swaps care about looks more than anything else. That includes myself, I confess.

I have a 43 mm ZX fork I want to put on a 750 this year, floating big rotors, 4 piston calipers. Like you I will mount it and try it and see. My forks are junk, I haven't had a bike that handles well in over a decade.. I just want brakes a damping..

[Erwin83]

Erwin,

Also keep in mind that the KTM likely has a smaller offset between steering head and fork tubes than the 350F/400.  If so, this can make the bike tend to want to fall into corners.

Yes, the offset is 40mm, instead of 60 mm stock CB.
How this will affect handling, i don't know yet.
Let's be honest, people doing these swaps care about looks more than anything else. That includes myself, I confess.

I have a 43 mm ZX fork I want to put on a 750 this year, floating big rotors, 4 piston calipers. Like you I will mount it and try it and see. My forks are junk, I haven't had a bike that handles well in over a decade.. I just want brakes a damping..

Googling around, I found that steering stem length is the biggest challenge with these conversions, as changing that requires pressing in a new pin, which can be expensive. Cognito motor sells conversion pins for instance. $$$ ouch.
Stem length is also a piece of data unavailable of most motorcycles, meaning many of us are left to gambling, and just sourcing the fork (gixxer, zixxer, r6/r1, cagiva mito, i’ve found many examples) and see what happens.


That’s why I am so chuffed to find out that this KTM unit fits directly

[754]

As soon as I get my lathe going I can make  my own stem, made a few before. On this ZX I press the original out, then a Honda one out, machine a sleeve to put between the two,  and weld sleeve to the stem , and press it in.

[Erwin83]

That is "traditionally" the way to go indeed.
I just don't have a lathe (yet)  and all "pressing" I can do in house is with my vise...

A boy can dream of an even better equipped garage though.

[Erwin83]

Some more images.
Literally fits like a glove.

[754]

Looks beefy , want to see it with wheels on it..

[Erwin83]

That's going to be the next puzzle.
I bought an aprilia 650 pegaso front wheel. Which has a 300mm brake disc, like the ktm.
Axle is slightly off, and brake disc won't fit the caliper.

So, getting it on both wheels will take some time.

[754]

Usually not hard to fix..except.. with radial caliper mounts it gets harder. If you slip the caliper on , does caliper need to go in or out  ?

[SOHC4 Cafe Racer Fan]

This little 4 is going to look great with the 17" wheels.

[Erwin83]

This little 4 is going to look great with the 17" wheels.

 
I'm going for 18" wired wheels. What you see in the pic is actually a 19".
The game plan now is a front hub of a suz DR.
Front rim 2.15-18, rear rim 2.50-18 on original hub.

[SOHC4 Cafe Racer Fan]

I must be losing my mind and eyesight! 

[Erwin83]

I must be losing my mind and eyesight! 

No worries, it's the enormous 300mm brake disc that throws you off, I'm sure.
I don't think I'll be keeping that b.t.w. The DR comes with a 280mm disc, and the KTM caliber has spacers that allow for a slightly smaller disc.
Goal is, again, finding a fitment that doesn't require any real machining work, since I don't have a lathe or CNC, or whatever.

We'll have to see how the DR hub fits though.