# Unholy Union geometry

Discussion in 'Airheads' started by Ras Thurlo, Nov 3, 2012.

1. ### PrutserLong timer

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Lets test them first before advertising
2. ### DmasterBeen here awhile

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A higher or lower back would give the same result (I won't expect i need a huge difference in this one)
So you would like to have less trial? Why? (technical explanation otherwise its not true )
Well the added height will effect the steering because the center of gravity is higher. At least thats what I still think.
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.:huh
3. ### ontic…

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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
4. ### Clay SpinnerBeen here awhile

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

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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.
6. ### Phreaky PhilLong timer

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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.
7. ### DmasterBeen here awhile

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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
8. ### naginalfHandy Schtroumpf

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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.
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mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0in; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} </style> <![endif]-->[FONT=&quot]N[/FONT] = 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.
9. ### RuckstaSS Blowhard

Joined:
Oct 26, 2008
Oddometer:
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Location:
Gold Coast
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.

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11. ### naginalfHandy Schtroumpf

Joined:
Jan 29, 2010
Oddometer:
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Location:
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Thanks, that confirms that my math is correct, it matches exactly what I get, except this calculator figures in the tire compression for you. Nice, thanks for posting. That's much easier than my method (although not nearly as fun imho).
12. ### DmasterBeen here awhile

Joined:
Apr 3, 2012
Oddometer:
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Location:
Netherlands.
The unsprung weight of our bikes is quite a lot at the back. Example (this doesn't make real sense but will give you some ruff idea (although you probably already know ) :

My G/S: wheel + tire: 12kg
final drive (inc. oil brake shoes...: 8,5 kg
and lets forget about the shaft, tunnel,..
Weight bike at the rear wheel: 93kg sprung weight vs 20,5kg unsprung weight = 1:4,5

My XR: wheel, tire, brake disk, chain disk: 9kg
brake caliper (don't know exactly: 0,5 kg
and lets forget about the rest,... the main wight of the swing arm is at the front anyway.
Weight bike at the rear wheel: 60kg spring weight vs 9,5 kg unsprung weight = 1:6,3

Big numbers are helping for offroad usage, but only if you can control the movement by damping, and setting it up with in mind the smallest amount of travel (front vs rear)
If you break something before bottoming out you're "bottom" is to far away.
If you make an insane drop (i mean a jump or drop you already regret before you land it) and use only 220mm of travel why did you wanted 270mm in the first place? (just asking :) )
13. ### RuckstaSS Blowhard

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Nice talking to you.
14. ### ontic…

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I'm glad this discussion has taken off,
last few posts have been great!

Cheers,
yeah I get the basics on how changes in front and rear effect trail, and what those changes should theoretically mean, and no, the algebra stuff isn't what rocks my boat what makes these things most confusing to me are instances of well reported counter intuitive results. You know, where a decrease in offset at the triples actually makes a bike turn in quicker and easier, when it should be 'increasing stability'. There are obviously (to me) interactions and effects going on that a kinda simplistic understanding of 'trail' doesn't really illuminate too far into. I've heard some pretty interesting things (first hand) about mucking around with offsets, trail, suspension travel, and whatnot on airheads, not all good!

Regarding the R-Dubb/HPMGuy triples- they are all 38mm offset, not 41mm, regardless of what fork they are bored for.
If you read one of the threads I linked to, here, you can see some of the process that R-Dubb went through. Originally he wanted to increase the offset more than 38mm. But this was all fitting into a particular plan.

The biggest lesson for me running these triples and forks so far is how different fork lengths made a difference. The difference in length wasn't all that great. At first I had them, full length but slid up through triples as far as I could get them, then I shortened them 3 inch or so but had them flush with the triples. maybe an inch, inch and a half difference- big difference in feel.
Whatever the case, these triples and forks do feel great.
I don't have much confidence in being able to answer the big complicated questions about bike geometry, but I do feel perfectly confident that these forks and triples, with a little tweaking front or back, are going to make for a perfect (to me) riding bike.
15. ### RuckstaSS Blowhard

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This is intersting.

I've observed this happening and have yet to hear an explaination I can
1. Understand
2. Believe.

I will comment that increased trail increases the tendancy of the wheel to return to straight ahead.
It doesn't affect tip in as much as it affects how much bar pressure is required to hold the bike in a turn.
16. ### ontic…

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these guys try to give a little explanation- (bearing in mind they also sell aftermarket triple clamps)
http://www.mb1suspension.com/id103.html
seems to boil down to reducing offset is like moving the engine forward (relatively) and puts more wheight on the front wheel, trail is increased, but the bike feels more planted and confident in the turn... more stability, but also better turning... something like that- and all from a change of only 2mm!

More weight on the front wheel making for a better planted turner of a bike is something I have experienced, so I can kinda believe that explanation a bit. But like most of this stuff, even if it is right I doubt it would be a hard and fast rule for all bikes and geometries.
17. ### LoJackLong timer

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B.G., yo!
The write up Ontic posted explains a lot. I know bicycles are a lot different than motorcycles, but that's what most of my professional experience is in, having been building custom bikes for the last 12 years. I've felt that how a bike handles and "feels" has a lot to do with front end geometry and not as much how high (or low) the weight is.

It's funny, but I've gotten shit in the bicycle circles for talking about my F650Dakar which has the Touratech WP suspension kit and 18"/21" wheels. The bike handles so much better than the standard F650GS that I had before it, but it's four inches higher off the ground. It has sharper, more nimble steering at slow speeds, and doesn't feel like it's going to fall over on me like the other bike did, though it has happened more than once. I just wish I had taken measurements to compare the two, but I suspect the Dakar has less fork offset and has a steeper steering angle. But that's just an assumption, so don't quote me on that.

Of the different bicycle geometries I have ridden, bikes with more fork offset tend to wash easier in dirt. I tend to think of it as a lever around the steering axis, a longer lever having more, um, leverage to push the wheel aside. Otherwise the rotational inertia of your wheel keeps you going straight. Steeper steering angles are better for woods bikes, being more agile at slower speeds than slacker steering angles, which may be more suited to open terrain and higher speeds.

There is also what's called wheel flop. I don't want to get into it too much, because all this stuff makes my head hurt after a while, but it's basically how the frame drops as your front end turns, and the tendency for it to continue to drop without rider correction and wheel inertia.

Wikipedia is another site with a good write up on all this stuff:
http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry
18. ### LoJackLong timer

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I read a review a couple years ago on a book someone wrote (maybe in the '70's) about steering geometry. The author took an r75/5 and did some pretty heavy modifications to it for experimentation. I think he made an adjustable offset fork, and also welded a new head tube to the frame to have a 0degree rake.

Anyone know what this book is? I've been wanting to read it for a while and can't remember what it's called or how to find it.
19. ### Airhead WranglerLong timer

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Tony Foale's book Motorcycle Handling and Chassis Design. Good book. Here are the pics, but with Russian text. There are copies for sale on amazon, but there's also a PDF of it floating around the web.

20. ### LoJackLong timer

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Awesome, thanks!