Upside down fork question...

[tkent02]

Anyone have a lot of experience with upside down forks for off road use?

[hampshirehog]

A little on smallish bikes. Why are you asking?

[tkent02]

I bought a bike with messed up 48mm forks, and a parts bike with 43mm forks, in very good condition. I have the 43s on there now, just bolted right up. So far have only ridden it around the back yard a bit, nothing bumpy at all.
It seems fine so far.

Is there any advantage to 48mm forks over 43mm forks for riding on rocky terrain, very slow steep technical stuff? I do ride faster stuff in between the steep sections, just want a comfortable ride, it's all pretty rocky around here. I don't care about landing off big jumps, don't really care about high speed in the whoops or any of that motocross type stuff.
I just need it to be smooth and supple in the rocky sections, and comfortable everywhere.


I would guess the larger fork tubes would be more rigid and thus better for things like motocross, with the huge jumps and the high speeds in the whoops and such, but that's not me.

It would seem to me the smaller diameter forks would be just as good for my purposes, maybe even better.

[koolaid_kid]

There were tradeoffs associated with USD forks off road. Don't ask me what they were, it has been a long time. But I remember reading about them.
If you are happy with the 43s, I suggest staying with them.

[tkent02]

There were tradeoffs associated with USD forks off road. Don't ask me what they were, it has been a long time. But I remember reading about them.
If you are happy with the 43s, I suggest staying with them.

I think the down side is seals leaking more often, and the fork tubes getting dinged up. Right side up cartridge forks are very good if they are set up right.
I just put some on my DR, it's pretty sweet.
I don't have any right side up forks for this bike.

Both the 43mm and the 48mm forks are upside down WP forks, just a couple years apart, very similar except for the diameter.

I'd stay with the 43, but I just spent the money on bigger springs and new seals and bushings for the 48, and the tires on the wheel for the 43 are from the stone age.

Just wondered if anyone knew if the fatter ones would be better for my purposes.

[salty_monk]

Stiffer with "Big Pistons" in the 48mm if I remember correctly. Supposedly the BP fork is a step up over it's small piston brethren....

What is it? A KX?

[koolaid_kid]

For your riding you should not notice any difference. A professional motocrosser going full tilt might notice a bit of a difference.

[tkent02]

For your riding you should not notice any difference. A professional motocrosser going full tilt might notice a bit of a difference.

That's kindof what I was thinking. I bet even this professional wouldn't notice until he looked at his lap times.

Well these forks aren't much good either. It seems OK on the big rocks and bumps, but the little stuff just tears me a new one, very rough and harsh ride...

Tried different compression and rebound damping, it's the smoothest with the comp turned all the way down, but it's not very smooth.

What I don't get is my 1979 YZ250 was as smooth riding as any motorcycle I have ever ridden, none since have been as good. Not even the beemers. I know the technology has improved in the last 35 years, so why do they all ride like crap?

Getting tired of taking forks apart.....

[Sci85]

The larger 48mm forks have bigger internal pistons as stated which allow more oil flow which in turn allows more tuning capability. They are also stiffer. As the size of jumps continued to get larger and riders were pushing the heights and distances farther, the older right side up forks could not handle the forces without bending and many times breaking so UPD forks with their larger diameters and stiffer tubes, not to mention internals, allowed riders to jump hundreds of feat without killing themselves. Suzuki tried to go back to RSU forks on the RM one year, I don't recall exactly now, maybe the 1995 models, but quickly reverted back. I agree that RSU forks tend to work more fluidly and why they use UPD forks on street bikes I can't say other than for looks maybe.

Note too that "stiffness" is definitely relative and defined in multiple ways. Keep in mind there are always trade offs when tuning suspensions. While we strive for a smooth stroke that can handle the littlest of irregularities while also handling the largest of jumps without bottoming, in reality nothings perfect so in most cases, some relative "smoothness" at slow speeds is sacrificed for the necessary "stiffness" required for large whoops, jumps, etc at high speeds. And typically, the faster the rider the more stiff the fork will feel at low speeds. A tuning specialist like Race Tech, Enzo, Factory Connection, etc will be able to get them a whole lot better for your weight and riding style. You would be amazed what they can do.

Good luck.

[tkent02]

The larger 48mm forks have bigger internal pistons as stated which allow more oil flow which in turn allows more tuning capability. They are also stiffer. As the size of jumps continued to get larger and riders were pushing the heights and distances farther, the older right side up forks could not handle the forces without bending and many times breaking so UPD forks with their larger diameters and stiffer tubes, not to mention internals, allowed riders to jump hundreds of feat without killing themselves. Suzuki tried to go back to RSU forks on the RM one year, I don't recall exactly now, maybe the 1995 models, but quickly reverted back. I agree that RSU forks tend to work more fluidly and why they use UPD forks on street bikes I can't say other than for looks maybe.

Note too that "stiffness" is definitely relative and defined in multiple ways. Keep in mind there are always trade offs when tuning suspensions. While we strive for a smooth stroke that can handle the littlest of irregularities while also handling the largest of jumps without bottoming, in reality nothings perfect so in most cases, some relative "smoothness" at slow speeds is sacrificed for the necessary "stiffness" required for large whoops, jumps, etc at high speeds. And typically, the faster the rider the more stiff the fork will feel at low speeds. A tuning specialist like Race Tech, Enzo, Factory Connection, etc will be able to get them a whole lot better for your weight and riding style. You would be amazed what they can do.

Good luck.

I believe those one year right side up RM fork is very popular for the stuff I use it for, rocks and technical stuff. Don't have a set though.

I know I need to work with a suspension guy, but I'd like to at least get it working somewhat properly first.

Not much point in having a KTM if it doesn't work anywhere near as well as my old Suzuki.

[Sci85]

Well, if it's an older KTM, you never will.

[tkent02]

2004, is that too older? Well, I like how light it is, and the engine is great. Brakes are excellent. Starts and runs beautifully, everything works perfectly, just rides like crap. I've ridden others just like it that were wonderful. This one will be, it's just a matter of how much time and money.

The good news is, I can get the forks apart really quickly now.

I have a theory as to why the original forks are messed up. They were mismatched, different springs inside. One has 28 coils, the other has 31 with a thicker diameter wire. I'm thinking someone crashed it, bent one fork tube and replaced only the one. Can't figure any other reason for the mismatch. The other must be a tiny bit damaged too. One side looked more worn inside too, more marks from tools, etc.

So I'm going back in again to check.

Any way to check for a slight bend other than rolling it on a flat table?

Any other things I should be looking for?

By the way, how does your '08 YZ 250 ride on rocks?

[tkent02]

I have a theory as to why the original forks are messed up. They were mismatched, different springs inside. One has 28 coils, the other has 31 with a thicker diameter wire. I'm thinking someone crashed it, bent one fork tube and replaced only the one. Can't figure any other reason for the mismatch. The other must be a tiny bit damaged too. One side looked more worn inside too, more marks from tools, etc.

Well that's not it. Both legs, same part number, same manufacture date. They are both straight as far as I can see. Someone put in two different springs for some reason. WTF? Who does that?

[koolaid_kid]

Nascar rider?
Go fast, turn left, go fast, turn left,...

[tkent02]

Nascar rider?
Go fast, turn left, go fast, turn left,...

Well now that's an interesting guess.