Unholy Union geometry

[Dmaster]

Originally Posted by LukasM

It's pretty simple really, if you want to keep the stock handling but with better quality suspension then just copy the stock total offset (triple clamps + fork lugs), fork length, and stroke.

The thing is, i do add a bit extra height and especially stroke. (I'm after suspension travel)
So what to do to make it steer like stock (or at least feel like as much as possible)
Do I need more trial because of the height increase, and how to get that.
Or stock trial and another steering head angle?

[LukasM]

Unless you are going to have it professionally done I would forget about changing the actual steering head angle on the frame.

To compensate for the longer forks you would want more offset to keep the same turning ability. Of course it's a balancing act to see when it's too much etc.

Or just increase the rear travel and height the same so the bike stays balanced?

Keep in mind that only a part of the additional travel will be actual height increase due to additional sag and the fact that the forks are at an angle.

[Prutser]

Originally Posted by LukasM

Hi Bas,

Probably the open cartridge 48s from a 690 or 950, have both here. Not sure if the wider or narrower spacing between the upper and lower clamping area is better?

My friend is on a tight budget so we'll have to look for a cheap set of KTM OEM clamps that will work well. I'm thinking 950/990 SM or Duke will have the highest offset.

Erik is going to make the clamps for me and Dmaster. But he is also going to do some in the WP size for the X with 210mm
The OEM 190mm WP triple size does effect the turning circle a lot !

[Clay Spinner]

Anything useful in this....

[LukasM]

Originally Posted by Prutser

Erik is going to make the clamps for me and Dmaster. But he is also going to do some in the WP size for the X with 210mm
The OEM 190mm WP triple size does effect the turning circle a lot !

Interesting! He should advertise them on the forum, I bet there will be some interest. Erik really needs to get a helper to put all his stuff online..

I'm pretty sure that the big KTM street bikes (and even the LC4 SM) actually have 210mm spacing, so that should help.

[Prutser]

Originally Posted by LukasM

Interesting! He should advertise them on the forum, I bet there will be some interest. Erik really needs to get a helper to put all his stuff online..

I'm pretty sure that the big KTM street bikes (and even the LC4 SM) actually have 210mm spacing, so that should help.

Lets test them first before advertising

[Dmaster]

Originally Posted by LukasM

Unless you are going to have it professionally done I would forget about changing the actual steering head angle on the frame.

A higher or lower back would give the same result (I won't expect i need a huge difference in this one)

Originally Posted by LukasM

To compensate for the longer forks you would want more offset to keep the same turning ability. Of course it's a balancing act to see when it's too much etc.

So you would like to have less trial? Why? (technical explanation otherwise its not true )

Originally Posted by LukasM

Or just increase the rear travel and height the same so the bike stays balanced?

Well the added height will effect the steering because the center of gravity is higher. At least thats what I still think.

Originally Posted by LukasM

Keep in mind that only a part of the additional travel will be actual height increase due to additional sag and the fact that the forks are at an angle.

I know about that one ;)

How are all measurements taken anyway? (like, steering head angle, trial, wheelbase)
Static, dynamic or high in the sky? If you compare those things all can be dramatically different.

[ontic]

Originally Posted by LukasM

KTM triples are available in a variety of offsets, although nothing below 11mm or above 35mm I believe. Fork bottoms on the WP43 and WP48 have the axle leading by 35mm if I remember correctly.

What ktm triples came with anything close to a 35mm offset?

Also, by "axle leading by 35mm" do you mean how far the axle centre is forward of the fork tube centre? Ie axle offset?
35mm sounds about right for the USD forks, and that is the figure R-dubb was working with in this thread, so I am assuming that is what you meant.
But if anyone has these forks please measure them.

Considering my rough measurement was brought up at the beginning of this thread I thought it was about time I revised it.

I just had another look at the G/S forks verses my WP50's
Now that I've got a spare set of WP50's on the bench I could give them a proper measure.

I would like it if someone else with WP50's and G/S forks would have a got at these measurements, as it is a little tricky.

I also measured the G/S triple clamp offset because I had never actually done that before and was just assuming the reported figure was right.. it was

G/S triple clamp offset = 38mm

G/S Axle Offset = my calculations were 24.65mm

WP50 Axle Offset = my calculations were 40.26mm


I'm going to call that 25mm and 40mm until someone corrects or confirms and call that 15mm difference of axle offset between the G/S and WP50's,
and if the 35mm axle offset on the WP USD forks is right, then a 10mm difference on those forks.

Most intend to extend the front at least a bit to get some more travel and/or ground clearance.
Some people are extending the swing arms or (like my plan to move the lower shock mount forward) just raising the rear a little, to accomplish the same things and balance changes on the front end.

We worked out in another thread that a 50mm swing arm extension (with no major changes in sag) nets around 12-15mm more height, doubled to 24-30mm for a 100mm swing arm extension..

Most of us, myself included will try lengthening the forks quite a bit more than that, so even with getting more travel at the rear end, chances are for those that aim for a taller bike, the end result will be an increase in trail...

thus reducing trail at the triples or the axle with a nice big offset to get a taller bike with more travel might be a pretty good thing...

complicated stuff.

I would really like to understand it all better, but I'll be happy if I can just end up with a bike that rides like I want it to

[Clay Spinner]

Originally Posted by ontic

complicated stuff.

I would really like to understand it all better, but I'll be happy if I can just end up with a bike that rides like I want it to

Exactly... I guess that is why there aren't more people building these things in their spare time.

[Rucksta]

Has anyone with a well set up transplanted front end measured how much travel they actually use?

220mm out of 270mm available is all I get from a fork that will take a front wheel landing or trickle through a rock garden wiithout skipping.

I'd be interested to find out how much others use.

[Phreaky Phil]

Originally Posted by Rucksta

Has anyone with a well set up transplanted front end measured how much travel they actually use?

220mm out of 270mm available is all I get from a fork that will take a front wheel landing or trickle through a rock garden wiithout skipping.

I'd be interested to find out how much others use.

That would be interesting to see. I never measured my travel used but when I had the USD front end in my GS (travel shortened to 220mm) i dont remember ever bottoming it. Probably due to the far better damping than the original forks had and possibly better bottoming resistance than original forks.

[Dmaster]

I know have 265 or 270mm stroke in my DRZ fork, and I can bottom it out if I want.
Take a high jump with a ruff landing or something and its done.
Same at the back.
In my opinion if you can't bottom it out, 1 your driving to slow, 2 your springs are to stiff/ to small air gap.
You can even bottom out a proper set up MX bike if you really want to. And if you mount such stiff springs you won't all other times it will be to stiff and you will never use the stroke you actually have.
And please notice the last piece of stroke is a hydraulic stop, if you end up at the end of that one you're doing insane stuff ;)

[naginalf]

Originally Posted by ontic

complicated stuff. I would really like to understand it all better, but I'll be happy if I can just end up with a bike that rides like I want it to

I found this very helpful to understanding it, it's really not that complicated, just hard to visuallize. Personally, I wouldn't raise the front without equal in the back, the rake on a GS is already 28°, a KTM is 25-27°.



And, if my trigonometry is correct, the following formula should get your trail (in mm):
X = trail (the important part)
F = offset, combined from triples and axle. More offset makes less trail.
N = rake angle of the forks at the steering head. This will go up from stock if you raise the front and not the back, thus increasing trail.
r = radius of the wheel/tire at the road surface. This should be a range from the exact measurement, up to 15mm or so less for a compressed tire. A 90/90/21 tire has a 347.7mm radius, probably compresses down to 330mm.

X = (r SinN) - F
.........CosN

If you don't remember your algebra, open the calculator from windows accessories, click view, click scientific, then do the following. Take the sine (push 28, sin) of your rake angle, multiply by the tire radius (330 to 343mm depending on your tire, inflation, and weight), press equal then subtract your offset. Now press equal again and divide by the cosine of the rake angle (push divide, 28, cos), and equal again for your trail.

For example, the trail on my GS with WP4860s and HPMGuy triples will be, let's see 35mm axle offset, 41mm triple offset (this was for the WP50 triples, not sure if it changed for the 4860s), 28° angle, that makes it.... 89 - 96mm of trail.

As for what trail should be, I have no idea , I was just having fun with math. However, using the KTM example, the stock 640adv has 124mm of trail, some companies make custom triples that increase offset and thus decrease trail, perhaps as much as 115mm. That makes me worry about having such low trail and high offset with the HPMguy triples. Perhaps raking it out with longer forks isn't such a bad idea. Although by my calculations, the stock GS has around 93mm of trail. I think that the more rake you have, the less trail you need, but that's just my assumption from what I'm seeing.

[Rucksta]

Originally Posted by Dmaster

I know have 265 or 270mm stroke in my DRZ fork, and I can bottom it out if I want.
Take a high jump with a ruff landing or something and its done.
Same at the back.
In my opinion if you can't bottom it out, 1 your driving to slow, 2 your springs are to stiff/ to small air gap.
You can even bottom out a proper set up MX bike if you really want to. And if you mount such stiff springs you won't all other times it will be to stiff and you will never use the stroke you actually have.
And please notice the last piece of stroke is a hydraulic stop, if you end up at the end of that one you're doing insane stuff ;)

Setup on a mid weight is a different ball game to an MX or Enduro buke.
Even if thats where you source the grafted front end.
Sprung to unsprung weight ratios on the heavier bike work in your favour.
Controlling weight transfer is challenge
Ability to cope with high speed running and varying load conditions e.g. luggage 0 to 45lts of fuel is another.

I'm suggesting big numbers on travel is not the holy grail of supension set up and "use it cause you have it" is not the best approach to a balanced setup.

300mm travel on my G/S has the sump guard dragging before bottom out.

[robtg]

http://www.tonyfoale.com/ -- freeware -- steering geometry calculator.