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Old 10-21-2011, 11:44 AM   #151
Wheedle
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Originally Posted by Houseoffubar View Post
Really depends on the bike though. I helped build a bike with a belly tank, and high exhaust, and airbox. It works wonderfully (wins every race)

I'd like to see the bike, I'm always interested in seeing creative engineering solutions that work. Honda went a little too far I think, they had lots of issues with weight transfer working the wrong way and fuel sloshing around creating some weird issues.
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Old 10-21-2011, 01:47 PM   #152
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Surely you jest!!






The guy on the Duc is still trying to ride the bike. If he could just get his foot down to upshift....
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Old 10-21-2011, 02:00 PM   #153
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"Champions never quit!"

Funny, I had that red & white team in mind when I said that.
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Old 10-21-2011, 02:51 PM   #154
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Originally Posted by troidus View Post
The guy on the Duc is still trying to ride the bike. If he could just get his foot down to upshift....
That guy on the Duc!!!!! No 46, thats Valentino Rossi aka The Doctor. Multiple multi class world champion...... Good as he is just cant get that Duc working for him, maybe his engineer needs to read this thread? lol
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Old 10-21-2011, 03:04 PM   #155
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>>My thoughts about relocating the front spring/damper lower were more about centering the mass than just
>>lowering the CG. From a rider's stand point,(where I have most of my experience), if you can lower and
>>center the mass it will make the rider's job easyer to go faster with more confidence. I race dirt bikes over long
>>distances and conserving my energy is a high priority.

For a roadrace bike a low C of G leads to slow turn-in performance. Mass centralization is nearly always good but sometimes you need to put something where it wants to be instead of where I want it to be. The front shock location provides a linkageless solution with the forces acting as directly as possible on the relevant parts to maximize feel and minimize play/hysteresis.

>>You can build the most advanced, kick ass bike ever to roll a wheel on planet earth, if the rider is uncomfortable
>>or unconfident on it, it will finish last every time.

Absolutely.


Chris
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Old 10-22-2011, 03:52 AM   #156
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Quote:
Originally Posted by Wheedle View Post
I'd like to see the bike, I'm always interested in seeing creative engineering solutions that work. Honda went a little too far I think, they had lots of issues with weight transfer working the wrong way and fuel sloshing around creating some weird issues.
Hey Man, here ye be! It's a Honda RS125 with a CRF450 engine in it. The fuel in the chin fairing, and the fuel tank is now the airbox (converted to down draft intake) The bike weighs 165lbs, and makes 71HP at the rear wheel. Lots of fun!!!

Sorry, I don't think I have a picture with the fairing off
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Old 10-22-2011, 06:16 AM   #157
DingDangKid
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Quote:
Originally Posted by Houseoffubar View Post
Hey Man, here ye be! It's a Honda RS125 with a CRF450 engine in it. The fuel in the chin fairing, and the fuel tank is now the airbox (converted to down draft intake) The bike weighs 165lbs, and makes 71HP at the rear wheel. Lots of fun!!!

Sorry, I don't think I have a picture with the fairing off

any more info? There's a debate going on over in the perfect line about hp vs top speed and they would like to see this.
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Old 10-22-2011, 11:50 AM   #158
Brad Felmey
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I appreciate your sharing of engineering concepts. I'm enjoying the read.
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Old 11-05-2011, 04:59 PM   #159
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i'm back....

Sorry for dropping off there, got a bit busy there and didn't have time for updates. I did get a chance to do some cool machining for the front suspension (how appropriate) that is on the blog.

The last post was starting on the backgrounds of why I didn't just bolt a pair of forks on the bike and called it a day. Those reasons, and also liking a design challenge, propelled me forward.

Since I had decided not to use forks the question was what to use? Examples of people from Tony Foale to John Britten showed there were many ways to skin the cat. Which method would I use? To help make a choice I looked at some of the issues current GP bikes were having. Those were late 1990s and early 2000s, the last years of the 500cc two stroke 4 cylinder bikes and beginning of the 990cc 4 strokes. The 500s were absolute beasts, 200+Hp and a power curve guaranteed to spit you off at some point. The 990s had a better torque curve shape but even more power. What the current designers were doing was stiffening up the frames to deal with the big slick tires and violent power delivery. This increase in stiffness had unwanted and unforeseen side-effects while the bike was leaned over and going over irregularities in the pavement.

When a bike is leaned way over the suspension is no longer in line with the bumps in the pavement so is less effective in absorbing pavement irregularities or even subtle braking and acceleration forces. As tires improved (due to competition) lean angles increased as a consequence of higher corner speeds. GP bikes were now starting to enter territory where corner speeds were high and the resulting lean angles were making the suspension ineffective.



The problems of increasing engine power and lean angle have continued on to today, perfectly shown by Casey Stoner in the following image.



This image is stunning. 60+ degrees of lean is incomprehensible. I highly recommend looking at some of the 1000 frame per second video of Stoner going through a corner. His form of slightly spinning the rear wheel sideways while dragging his elbow is poetry in motion.

Because all GP and 99.9999% of production race teams use telescopic forks that have a round cross section the only option to introduce lateral flexibility to deal with bumps is to have the entire fork and headstock assembly move. Round cross sections have equal stiffness in all directions since all the material is the same distance from the center of the profile. If you have a plastic pen try to bend it. It provides the same resistance regardless of which direction you try to bend it in. Not try to bend a ruler along its flat direction then along its narrow direction, it provides extremely different stiffness depending on the direction because its profile is not symmetric about its center axis.

This is the core what I think is a big flaw with telescopic forks. If you make the forks big enough to provide stable braking using sticky tires and big brakes then the forks are too stiff when leaned over and the bike skitters over bumps. If you make the forks small enough to be supple under bumps then they deflect back excessively under braking resulting in an unstable bike under braking. Both situations are not good. Non-round sliding elements are troublesome in that any torsional load (which there is a lot of in a motorcycle front end) tends to make them bind, obviously not a good characteristic for a suspension element. We can see how the use of forks required teams to have complex frames that were stiff in one direction and less stiff in others. This made a frame designer's tough job even tougher. Now they had to hold the shaking engine, swingarm pivot, shock mount, rider seat, and handlebars all very rigidly yet allow the lateral flex needed by the front wheel/fork assembly. Not easy to do and not easy to subtly change one without affecting the other.

To me, this was a great reason to eliminate the sliding action of forks with several pivoting motions of linked control arms. It would allow me to use rigid structures to create the suspension movement and 'flexible' structures to allow controlled sideways flex.

Now I knew to use a control arm system the choice was which style to use? I wanted to easily be able to change lateral stiffness while keeping suspension movement accurate. Once you go to control arm style suspensions there are several layouts that can be used. Although they look very different from a physics perspective they are all identical:



Looking at these I felt the best solution for my priorities was a Hossack style. It would allow me to create strong triangular structures to control the suspension movement and easily predictable and replaceable structures to provide the lateral compliance. Difazio and hub center styles can have problems with steering lock and also have lower control arms that are loaded in multiple directions.

These are the stiff parts:



these are the 'flexible' parts:



After picking and choosing what I felt to be the best design points we had a front end that fit all my requirements:

-low stiction/friction
-stiff where needed
-flexible where needed
-easily changeable 'flexible' elements

My first design was a bit elaborate. it had roller bearings on all the upright pivots but had a complex welded structure as the upright. I may have went a bit overboard with these parts but it was my first time and I didn't want anything to fail under normal use.



This bike lasted 2 years and taught me a lot. A hell of a lot. For the first redesign I wanted to be able to make front ends more easily. I made the compromise to eliminate the roller bearings in favor of heavy duty plain rod end bearings which would allow a much simpler design.



I ended making a lot of these with various cross sections, wall thicknesses, offsets, etc, in the search for what produced the best results. We collected a lot of data and was even able to see chatter of varying frequencies and amplitudes in the suspension travel data as the design evolved. From this frequency information and the easily characterized rectangular upright legs it was a simple engineering exercise to change the stiffness to minimize or eliminate the chatter.

Front wheel chatter (purple) line:


The lessons learned through the past few designs have resulted in the latest design. It has all the benefits of the all roller bearing setup while adding the easily definable and changeable legs of the rod end bearing setup.




That pretty much covers the how and why of the FFE front suspension. From here I'll talk about general frame design and swingarm stuff.

That's all for now.


Chris
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Old 11-05-2011, 07:14 PM   #160
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Well worth the wait, looking forward to the next instalment.....
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Old 11-05-2011, 07:32 PM   #161
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Fascinating and extremely well told. Thanks -- I'm learning a lot here.
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Old 11-06-2011, 02:40 AM   #162
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great stuff
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Old 11-06-2011, 09:25 AM   #163
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Brilliant, thank you.

You're explaining complicated concepts really well, even I can understand!

-Simon
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Old 11-06-2011, 04:56 PM   #164
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Keeping me interested.
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Old 11-08-2011, 12:58 PM   #165
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Thanks. It is fun telling the story and having to type it out makes me stop and think about it a bit in a different way then when I was actually going through it. Thankfully I haven't had any 'what was I thinking' moments yet!

As an aside is there any way to download the entire thread? It would be nice to save the formatting and images now that I am spending a bit of time to generate and post it.


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