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Old 11-07-2012, 02:09 PM   #31
naginalf
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Quote:
Originally Posted by robtg View Post
http://www.tonyfoale.com/ -- freeware -- steering geometry calculator.
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).
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Old 11-07-2012, 03:56 PM   #32
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Quote:
Originally Posted by Rucksta View Post
Sprung to unsprung weight ratios on the heavier bike work in your favour.
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

Quote:
Originally Posted by Rucksta View Post
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.
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)
Quote:
Originally Posted by Rucksta View Post
300mm travel on my G/S has the sump guard dragging before bottom out.
If you break something before bottoming out you're "bottom" is to far away.
Quote:
Originally Posted by Rucksta View Post
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.
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 :) )
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Old 11-07-2012, 05:55 PM   #33
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Quote:
Originally Posted by Dmaster View Post
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

I was aware of the heavy rear end I hadn't quantified it like that I think my unsprung weights are a bit lower and the rearward weight bias is a little more but it's enlightning to see it expressed like this and most likely the reason the rear end is the weak spot in suspension performance to the point that any further improvement to the front end suspension is a diminished return.


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)

Agree. It's nice to have big numbers available although control via spring an damping rate is more important. My experience is improved damping characteristics were more important to the performance than big travel numbers

If you break something before bottoming out you're "bottom" is to far away.

Exactly my experience 270 mm is to much and I'm happy with 220 + reserve

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 :) )

I haven't made an insane drop onto a front wheel landing yet as I can usually get the back wheel down with a bit of throttle. Downhill front wheel landings off erosion mounds are OK at trail speed picking the air time for comfortable take off and precision selection of landing area.
Enduro speed would be a different story I expect and that's sort of integral with the bike set up; it is not enduro it's a mid weight ADV bike and even if it's ancestors won 4 Dakars it is never will be an enduro bike.

I did take out a big roo at speed last year and expect I would have used some of the reserve beyond 220mm but was too shaken by the experience and cracked frame to measure the fork travel.
Really glad I had something in reserve.
Don't know what the roo thought about it as I didn't bother to stop and ask with him being road kill and all

I picked an overall short Showa USD to give a raise of around 50mm over stock on the front end to match the lift in the rear. Initially I thought 270 would do and would have prefered more.
Initial setup saw 260 in use and as I tuned the mid stroke characteristics, preload and airgap if found I was using less and less travel settling on the 220mm for normal "heavy use"
I have to call it a compromise as any setup is a copmpromise between conflicting needs but the front end does not feel compromised at all.
Nice talking to you.
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Old 11-08-2012, 06:59 AM   #34
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I'm glad this discussion has taken off,
last few posts have been great!

Quote:
Originally Posted by naginalf View Post
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.



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.
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.
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Old 11-08-2012, 07:35 AM   #35
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Quote:
Originally Posted by ontic View Post
. 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!

.
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.
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Old 11-08-2012, 03:25 PM   #36
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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.
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Old 11-08-2012, 06:26 PM   #37
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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...cycle_geometry
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Old 11-08-2012, 06:32 PM   #38
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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.
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Old 11-08-2012, 07:38 PM   #39
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Originally Posted by LoJack View Post
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.
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.




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Old 11-08-2012, 07:55 PM   #40
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Awesome, thanks!
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Old 11-09-2012, 03:12 PM   #41
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Originally Posted by Airhead Wrangler View Post
Tony Foale's book Motorcycle Handling and Chassis Design.
Wow, thats interesting thanx!
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Old 11-10-2012, 11:48 AM   #42
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[QUOTE=Rucksta
300mm travel on my G/S has the sump guard dragging before bottom out.[/QUOTE]



After using 250 mm travel this is where my wheel would be. So the sump will be dragging through the mud indeed.

Trying to get more (usable)wheel travel would mean the bike has to be higher. Not something I personally would like.
This limit does mean the height on the front is sort of limited as well if you would like to keep the bike handling ok!

Could people give some feedback about how there bikes are handling with the new triples, forks or longer swing arms compared to a stock GS or G/S.

I have an idea about my ST with 50 mm longer swing and full length WP 48mm with kite triples.

But I'm really interested in all the "pros and cons" that you all notice comparing the stock to the modified bikes.
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Old 11-10-2012, 07:40 PM   #43
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Quote:
Originally Posted by Prutser View Post

After using 250 mm travel this is where my wheel would be. So the sump will be dragging through the mud indeed.

But I'm really interested in all the "pros and cons" that you all notice comparing the stock to the modified bikes.
Can't look at either end in isolation, one affects the other so thanks for this.
I notice you have a long centre stand.
Do you have a long front end already?

Best all round geometry I've had is stock.
Next best was 1" rise in the front and 1.5" rise rear.
Bike became a slider with seamless transitions in and out of a slide.
It used more gas as it sat higher on the road and created more drag.
Top speed was down.
Bike became a bit harder to flick through esses on road and became more lively pushing it down into a turn on dirt while maintaining an upright body position.

Tyre selection is becoming more agressive as the thing becomes a dirt bike.

Changes to swing arm
- shorter better turning. twitchy under power. great wheels stands. reduced travel. didn't like it.
- longer more progressive transition into slide and able to feed heaps more power through the slide to maintain it
without the back wheel chasing the front. Less so returning to staight tracking about the same as stock.
- Longer tavel.

Longer also meant it was harder to break into a slide and the forward weight bias made the front stick better which was OK for 'wheel in line' dirt riding but made it a leap of faith to break the bike into a two wheel slide.

On road behaviour is starting to fall off now as the lean angles required to turn the bike are becoming more than the knobby tyres are prepared to give.
The bike is now a dirt road screamer with OK road manners and the potential to be an off road bike except for the front end It has to go.

With a front end replacent and a long swing arm I have a great off road touring bike with heaps of clearance good travel great weight distribution when carring a load, stable in a straight line and hard to get a leg over worse with luggage on the rack.

Fork offest is 20mm with Axle lead at 30mm travel is 270/240 and 'Im using 260/235.
Bike is stable especially in a straight line even in sand but it is hard to dig the front end out of a rut using throttle.
Weight transfer on the suspension needs to be controlled and some planning is required to throw the bike around.
Dive under brakes is controlled by lever modulation.

I want most of that but long distance offroad touring is not the the bikes primary or only use.
I want to ride dirt roads, fire trails, farm tracks, and enough single track to join them together.
I want to keep pace with the traffic on the motorway and have effective passing performance from legal speeds on two lane and single lane back roads.

Changes were required.

I now have a bike that sits on the standard centre stand with both wheels on the ground under compression (stable on the forecourt of a gas station but only just)
Ride attitude is level as stock with about a 2" rise front & rear.
Swingarm is std length with mods described elsewhere
Wheelbase is near standard (shorter on suspension extension longer on compression)
Travel used is 220/210
Triple clamp offset is 25mm
Axle lead is 30mm.
Weight bias is towards the rear over the stock bike.

I'd like some more axel lead.
I think it will get me closer to the sweet characterisitcs of the original bike.
I'd also like a little more rear wheel travel and / or a lot less unsprung weight in the rear.

For me any comparison of the pros & cons has to be filtered by the bikes intended use and what compromises the owner is prepared to make to achieve what he preceives as a pro.
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Old 11-10-2012, 08:38 PM   #44
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Originally Posted by Rucksta View Post
...
How do you take such good notes while you're riding? Do you put a dictaphone in your helmet? I come up with a lot of the same type of observations when I have my "alone time" in my helmet, but 98% of it is gone by the time I get to the next gas stop. Your descriptions of handling traits associated with the various geometry changes you've made are very good. Thanks.
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Old 11-10-2012, 11:57 PM   #45
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AW: Never thought about the how too much.
When I make a change I test.

Does it go better that it did before the change?
Does the change prerform as expected?
Are there any unexpected side effects?
What else could I have done to achive the desired result?
I get off the bike and make observations and even reride a section to validate the results.

I also have ethe beneifit of a 30 year database of observations.
If I forgot one before the fuel stop it would have got a second & third oportunity to present itself.
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