Alligator build discussion

[3-Wheeler]

Originally Posted by Michael Moore

.....Jim, running the swing arm as you've drawn in may have been avoided by the Gurneys because of poor squat/anti-squat chain geometry. I did a quick model with a steeply angled swing arm and matching sprocket and it seemed to have a lot of squat under acceleration. With an already low bike having significant pro-squat characteristics could be a problem. You'd also have to move the top damper mount forward to avoid a falling rate, and that would then intrude on the rider space....

Michael,

I thought about this as well, in that the change in swing-arm inclination *could* affect the amount of squat.

The same is true of shaft drive bikes, but this is another topic for discussion.

However, being that my intent with a FF design, is to get better mileage, then acceleration and the amount of rear squat is pretty much a moot point, within reason.

A side note, you probably still remember the ATAC sprocket that was sold for a few years, way back in the late 70's if memory serves. These sprockets were supposed to help with the topic you speak of, but then they disappeared. Maybe they did not *fix* a problem after all?? Who knows.

To get a rising-rate shock setup, I usually think of the shock top being inclinated at about 45deg or so from horizontal, not the almost upright position seen in the Yamaha above. From a purely technical standpoint, you are correct, but I'm not sure it would matter enough for a fuel sipper.

Jim.

[beatr911]

Michael, Just curious. For the club racers that rode the FF, were the lap times comparable or did they at least feel the times could be comparable or better than thier regular bikes? Excuse me for this basic question but I just don't have a feel for the track performance other than it's enjoyable and fun.

I can see that using a 17" wheel is preferable for tire choice. The small cruisers use a 130/90-15 (~24" OD) tire which is about equal in OD to a 130/70-17 so that evens out with tire choice. I was hoping that the small cruiser 15" tires would be a height advantage, but this is not the case.


Any smaller and it's all scooter tire choices. Or heavy LRR 12" or 13" car tires only good for another ~1" lower swingarm pivot (2" smaller OD). Maybe not worth the tradeoff.

For a low power bike, lowering the swingarm pivot into pro-squat may not be a significant problem. Harleys (not that they have exemplary handling and ride) often have pro-squat swingarm geometry with much more torque. The handling performance envelope target for this bike would be at least as good as the Harley. Probably better.

Rereading, the Burgman 400 does look like a good prospect. It seems to have one of the most efficient transmissions of the scooters, being bathed in oil and such. Form factor is close. Searching, I have not been able to find one hacked into ultra low form. Do you know of any?

[Michael Moore]

Jim, I did a quick "low sprocket/low pivot" model in some of Tony Foale's software. I took the default settings for a simple "damper near the axle" model rear suspension with 133mm of travel. The rear ride height is shown as 830mm and the CG height as 650mm. That shows that 1G acceleration without anti-squat being considered. That changed rear ride height to 764mm and CG height to 618.5mm. Factoring in the anti-squat on that default model had rear RH at 790 and CG at 630 so you can see it kept the bike from squatting so much.

I then dropped the pivot 100mm and the sprocket height 130mm to keep a somewhat similar relationship of the chain. For this version the RRH is 830 and the CG is 650mm (no change). With the 1G accel no anti-squat is 764mm and 619, with anti-squat 777 and 624.

Even though the rear now squats an additional 13mm the CG doesn't drop as much because the front is presumed to stay topped out under acceleration.

830-777= 53mm, and again, this is from a "full droop" reference position.

I have little doubt but that juggling the pivot/sprocket locations might be able to improve things for either model, though the caveat with the anti-squat is that "improved" doesn't always mean the same thing to all people/uses. GP bikes often have adjustable pivots and they use that because the squat geometry will vary with sprocket size changes. But I don't think they normally change more than +/1 1-2mm at most once they find a relatively sweet setting.

To get a similar chart on the wheel rate the low pivot damper needs to have the top mount moved about 50mm forward, which is some but not a huge amount.

You are thinking of Horst Leitner's ATK sprockets, and that does actually reduce the variance in squat geometry a significant amount on a long travel dirt bike. There's less variance to cause problems on short travel so the net change there from the ATK is not as great

A commuter bike with 8 BHP that never exceeds 45mph can probably work adequately with an amazingly wide range of settings. You could even go with a hard tail, eliminating chain geometry concerns, and use a cushy seat instead. My concern is for people who start hacking away when they have little to no idea that what they are proposing to do has any effect at all. A crash at 45mph can still put you in the hospital or morgue. The 8 BHP engine may not have enough torque to cause significant loss of ground clearance even with hugely pro-squat geometry, but it is always good to do some research first and try to get at least a mid-level "OK" design. If you've made things so the bike can't ground when leaned over at full bump, you could still have problems if the pro-squat bottoms out the suspension while leaned over and the suddenly infinite wheel rate causes a loss of traction.

beatr911, keep in mind that it was just a few laps on a different bike. The track is very short so a lap is about 1 minute. I looked through some old emails and found these comments where Arthur said "BTW, Emo thanked me for the spin on the bike, and said he could easily get 5 seconds a lap with minor changes. I believe him; he's a seriously quick rider. He wasn't specific about what these changes were. I will ask next time, especially if I can get another rider to test it." and "Given that proper racer Emo can do a 1 minute lap on my bike 5 seconds quicker than I can" and "I thought when other guys said they were impressed at my being "only" 5 seconds off Emo's time that they were just being polite. . . " but 5 seconds off the fast guy's time was respectable. So the fast guy hops on, reels off a respectable time, and says he could get another 5 seconds off.

Arthur's Kawasaki has a stock engine and doesn't run super-sticky tires as Arthur isn't interested in putting the money in it, he'd rather spend it on more track time. Other than the FF riding position the only real chassis change is the swing arm extension, so it probably is safe to say it will have performance much like a stock EX500.

The important thing IMO is that Arthur's bike shows that an FF riding position doesn't necessarily preclude riding in a respectably sporting fashion. And having a respected "fast guy" go out and come in saying he'd like to ride it some more adds some street cred to offset those who'd dismiss it out of hand.

One of the FFers was doing a Yamaha TMax with a Hossack FFE on it:

http://www.bikeweb.com/image/tid/101

And with that, I'm back to the garage to see if I can get things mocked up with wheels and the new cellulose composite chassis parts.

cheers,
Michael

[3-Wheeler]

Originally Posted by Michael Moore

....You are thinking of Horst Leitner's ATK sprockets, and that does actually reduce the variance in squat geometry a significant amount on a long travel dirt bike. There's less variance to cause problems on short travel so the net change there from the ATK is not as great
....

Michael,

Thanks for the detailed response.

As you referenced that the ATAC was used on dirt bikes, I do recall seeing them on road bikes and reading about the merits of such an install. But as mentioned previously, as good as they may have worked, they did not last long in the market, and thus my comment that they may have fixed a problem that did not exist from a practical standpoint, still stands.

Originally Posted by Michael Moore

....A commuter bike with 8 BHP that never exceeds 45mph can probably work adequately with an amazingly wide range of settings. You could even go with a hard tail, eliminating chain geometry concerns, and use a cushy seat instead. My concern is for people who start hacking away when they have little to no idea that what they are proposing to do has any effect at all. A crash at 45mph can still put you in the hospital or morgue.....

Well said.

From a practical standpoint, even a speed of 65 mph does not cause much squat in the rear because it typically takes about 8 hp to run at this steady state speed on a typical motorcycle. This also means that there is not much "load" on the rear swing, and thus the largest forces on the swing-arm arm from road imperfections, and/or cornering forces, as cornering would put more load on the suspension compared to 8 hp steady load (and resultant compression).

This still means at the acceleration I will be taking off at, the rear end squat is simply not much of a factor. In fact since one on a FF would be sitting lower in the frame, the overall CG is lower than a stock bike, and thus cause even less squat at the same acceleration.

I think if a person had no idea what they are doing in this regard, the project probably would not come to fruition in the end. At least I hope so.

Let's say a person botches the swing arm geometry, and it has a terrible amount of squat. The only time the motorcycle will be able to accelerate at full throttle, is with the bike upright. In other words, accelerating in a straight line.

Now let's say the squat allows the suspension to reach full compression. What will happen? Right, the under carriage will scrap hard enough to counter act the acceleration, and will quickly inform the rider that he/she has an issue to contend with. LOL!

Now if the same rider was foolish enough to try the same trick in a corner while being bent over at 45°, well then the rider would get a nice view of the road from the side of their helmet, as the under carriage would drag hard enough to make itself known very quickly.

I think this is what you are alluding to above.

Jim.

[3-Wheeler]

Originally Posted by beatr911

.....I can see that using a 17" wheel is preferable for tire choice. The small cruisers use a 130/90-15 (~24" OD) tire which is about equal in OD to a 130/70-17 so that evens out with tire choice. I was hoping that the small cruiser 15" tires would be a height advantage, but this is not the case.....

Back in the mid 1980's, there were 3 tire choices for sport bikes, the 16, 17, and 18 inch rim size.

My bike has a 16 front and 18 rear, and there was much discussion about this in the press.

My choice for the front would be maybe a 17 or 18 instead of the 16. The 16 is faster from a fast transition standpoint, but I prefer more feedback to steering in the front than the 16 provides. Just my preference. I've had numerous bikes with 18 sized wheels, and they felt "normal" to me.

Before 1985 or so, all the typical Japanese bikes came with 18's front and back, and then in the late 1980's switched to mostly 17's.

I really think it comes down to a personal choice.

Jim.

[beatr911]

One bike I am considering has a 16"/15" the others are 18"/15" wheels. I too prefer good feedback especially in the front. The rear not so much, but stability and durability is important. The 17s just have so many choices on width, aspect ratio and compounds.

As for squat, you folks definitely have the experience edge to figure out how much squat/anti-squat geometry might be acceptable/required for say 20hp and 4" of travel.
I'll have to experiment with Tony Foales software to gain a better understanding of what might work well.

[redyaris]

For the WR250R lowered seat aero project, my plan is to keep the steering/handle bars fairly standard. This will probably limit the amount of seat lowering I can do. I want to keep the fabrication requirements within reasonable limits on a first attempt. It usualy takes about 3 attempts before one has debuged the whole thing.
The stock fuel tank is only 7.5 L total [2 US gallons] this presents an interesting challenge, how fare can I go on 5.5L [1.5 gal]? can I get the fuel consumption down low enough to get over 400 km [250 miles] on 5.5L? This is 1.375L/100km [171 mpg US]? I do have a 15 L tank for this bike so if all else fails I can put it on and get the range I want.

[Michael Moore]

Originally Posted by beatr911

The 17s just have so many choices on width, aspect ratio and compounds.

I'll have to experiment with Tony Foales software to gain a better understanding of what might work well.

When building a motorcycle project I think the first thing a person should do is ask "what are the best tires for this bike/application?" and once you've chosen those, they dictate what rim size/width you'll run. The tires are the one major item that you can't modify in the garage and that are a big factor in the ride qualities.

I had 16" wheels on my EX250, and tire choice was very limited and people would spend money to have their stock wheels converted to 17" (this was before Kawa switched to 17" on the EXs). 17" is where the tire companys have focused their efforts for the last couple of decades, but that focus is on 250cc+ bikes, not little bikes.

There's a fair choice in 18" because of the vintage bikes. The 250x18 Bridgestone BT39SS that came on the Dream 50 are very popular with the people racing CB160 Hondas, and they are very light and stick very well though they may be a bit too lightweight/soft for general street riding. Heidenau has several sport tires in small 18" sizes, and I think they also have some 17" for small bikes as they are used in "moped" race events in Europe. Heidenau also has some tires for different size scooters that are made with reasonably sticky rubber (as opposed to "rim protector" rubber) .

Heidenau Tires



Tony's software is very handy. The drawback is that it is up to the user to decide which set of numbers are the ones s/he want's to use; the software doesn't offer any "this is better" comments. It sure beats trying to do a bunch of manual computations but it can easily get to a "too much data, can't tell what is best" situation where I have to just pick one and go with it.

cheers,
Michael

[3-Wheeler]

Originally Posted by Michael Moore

....If you've made things so the bike can't ground when leaned over at full bump, you could still have problems if the pro-squat bottoms out the suspension while leaned over and the suddenly infinite wheel rate causes a loss of traction.....

Michael,

I agree with what you said above.

And to add to that.

Let's say we have a motorcycle that is setup with a good inclination of the swing arm while under power (anti-squat).

Now we are entering a curve, while still under power and we have adequate ground clearance to make the turn.

Suddenly a deer runs across the road in front of us, and yikes, we have to back off completely (let's not even talk about grabbing the front brake hard).

What happens?

The power is lost so the suspension drops and now the undercarriage hits the ground, making it very hard, if not impossible to make through the turn.

This is exactly what happens to a bike with shaft drive and a big, powerful motor too. It has natural anti-squat tendencies.

I would prefer to design the rear swing-arm so that it squats under hard acceleration and therefore does not "drop" when you remove the power source while in a bend in the road.

Normally when a seasoned rider is in a turn, there is a slight amount of throttle held on to keep the same momentum through the turn, and not scrub it off with the front tire. This action keeps the chassis stable and also keeps adequate ground clearance throughout the turn. One typically would only add an excessive amount of power in a turn in this scenario, 1) if they are wreckless, or 2) want the undercarriage to hit, or 3) want to see how close they can come to crashing.

I tend to use the "Kenny Roberts" technique of rounding a curve, which if anything implies that I ere on the side of too much throttle through a turn as this would kick out the rear slightly and still allow the front to steer the motorcycle. This action is a polar opposite of how Kel Carruthers (spelling) of the Yamaha racing team used to ride through turns. His front tire was gone after a race, while the rear looked pristine. The main drawback to this of course, is that if he pushes too hard and looses the front, the odds are that it's gone for good, and he's down!

Obviously this also implies that one is not going too fast through the turn to begin with!!

Sorry I did not think of this sooner as it came to me in bed last night.

Jim.

[Michael Moore]

Jim, another consideration, which Tony mentions in his book, is that having the wheel (either one) move up and back when hitting a bump makes for a better ride. With a dropped s/arm pivot you'll see the rear wheel moving up and forward.

FFers have mentioned that it is important to have a some "spring" in the seat back so that bumps don't jar the rider so bad. You can't "post" on the pegs over bumps as with a conventional riding position. Gurney may have gone to the extra bother on the Yamagator chain run to improve the way bumps are transmitted into the bike.

Owners of shaft-drive bikes (especially BMWs) do learn to not chop the throttle in mid-corner. I've never ridden one of the more modern shaft setups that float (like a BMW paralever) but they should have less jacking in the rear suspension from drive forces.

No doubt the importance of squat effects depends on how fast/hard/powerful the rider and bike are. For high gas mileage and gentle riding they are probably not a huge concern.

Sorry I did not think of this sooner as it came to me in bed last night.

Oh good, someone else who suffers from "project insomnia". I have a bad habit of waking up about 2-3AM and then getting started thinking (for the next 2-3 hours) about whatever project I'm working on.

cheers,
Michael