Monday, January 13, 2014

Suspension Lowering: KTM 350 EXC-F

I had George at Suspension 101 near San Diego, California help lower the suspension on my 2013 KTM 350 EXC-F.

I'm very happy with the work George did, I highly recommend his business. And I learned some interesting things in the process.

The difference in the "new ride height" vs the "amount of travel removed from the suspension" is interesting to note.

The thing I learned was that there is a measurement difference between shortening your suspension travel and lowering the bike's ride height. This means if you shorten the suspension 30 mm, the ride height will NOT be reduced a full 30 mm!

So, If you want to reduce the ride height by 30mm, you will have to shorten the suspension my more than 30mm! (- at last in the rear of the bike, more on that later.)

Some explanation
When you set the rider sag on the bike, you want to set it at a specific fraction of the total suspension travel. When you reduce the suspension travel you thus reduce the target rider sag. For example, if you want your rider sag to be 1/3 of the suspension travel and your original travel is 300mm, your original rider sag target is 100mm. If you reduce the suspension travel to 270mm, your new rider sag target will be 1/3 of 270 = 90mm.The new rider sag is 10mm less than the original rider sag. So, when you shorten the suspension 30mm, the ride height in this example only goes down by 20mm after you correct your rider sag.

Now, with most dirt bike forks, setting the front rider sag is tricky because stiction makes this very hard to measure. Also, the front fork rider sag is usually set to a smaller number than the rear, and thus the sag effects the lowering equation by less. Additionally the forks are at an angle, so the change in the length of the fork effects the height of the bike a bit less than you might think (Look up your trigonometry!) But you are concerned with the change in ride height, not just the length of the suspension.

What I did to my KTM
I wanted the bike lowered 30mm.
-The stock rear suspension travel is 335mm and the stock rider sag 1/3 * 335 =  111mm.
- If we shorten the rear suspension travel by 40mm to 295mm the new rider sag will be 1/3 * 295 = approximately 98mm.
- So the change in rider sag will be 111mm - 98mm = 13mm
- So shortening the rear suspension 40mm results in the ride height being 40mm - 13mm = 27mm lower. Close enough for me!

On the front fork, given the stiction issues and the smaller sag differences, we shortened the fork travel by 30mm AND we changed to a spring that was 30mm shorter but the same spring rate. Thus the fork was 30mm shorter, but with the same preload the sag should be approximately unchanged because the spring rate is the same. Upon measurement of the rider sag after reassembly, the front ride height was approximately 30mm lower.

Be aware that if you shorten your suspension 40mm, after properly setting your sag the bike's ride height will not be reduced by the full 40mm. Check your target rider sag and see how that effects the height before starting the project.

Here's a pic of my bike after the lowering. Also note my shaved seat.

I also had to shorten the side stand of the KTM to match the lower bike. Conveniently the KTM side stand is a hollow tube and has a removable foot. I pulled the foot off and cut the end of the stand off. You can see the removed piece has a flared end for strength, so my cut stand will be a bit weaker than original, but seems fine so far. Also, the inside of the tube has a bit larger inside diameter in the end to accomodate the foot. So I had to grind down the male part of the foot to get it fit in the new end of the stand.

Shortened stand with the foot re-installed.


  1. how tall are you? i am 5' 6" and thinking of this bike and a similar mod.