Note: don't presume that all of the new material appears at the bottom of the page. There may be things that come up that will require some editing on earlier material, and since the page is organized in sections I may post information on a particular phase of the project that require updates in several spots. I might also decide on the spur of the moment to do a major reorganization if I think it is appropriate.
This project might well be termed "Son of EX250 + 125GP = ?"
There were issues with getting some of the EX250 engine modifications done, and as is not uncommon with project bikes the window of opportunity (and interest) closed. After deciding that the EX250 was no longer something that I wanted to play with I began to think of something else I could use to power a 125GP-class bike.
Down through the years many of my bikes have been four-stroke singles, and while I'm going to keep some twin cylinder bikes around I don't have any big desire for anything with more than two cylinders. For the speeds I'm interested in going a single cylinder bike can provide plenty of power while reducing the part count (and the related money expenditure) when compared to a twin.
In AFM 250 four-stroke singles can run in FIII (the 125GP class), and I've been watching the modern thumpers (and thumpettes) get more and more powerful while also becoming lighter and more compact. The standard EX250 engine was almost the exact overall size and mass as an aircooled 560 Rotax single. Even after serious work the EX250 engine would still end up heavier than many Open-class MX thumper engines.
In the fall of 2005 Cycle News had a comparison test on the current 250 MX thumpers. The horsepower charts from the dyno runs showed "out of the box" power ranges competitive with a first-rate 250 Production EX250. Stock. No modification needed. Easy. So I decided that I'd start researching the engines and their availability.
Honda is the only major manufacturer that shows any interest in selling "crate" engines. I did have a local KTM dealer check with the KTM importers, as the KTM looked like the nicest of the engines for a road race special. It was the only one that had a 6 speed transmission, and it also had the highest horsepower of any of the bikes. But the reply that came back to the dealer was one of those "yes, we'll grudgingly sell you something, but the terms are going to be so horrible you should be able to figure out that we really don't want to be bothered" deals.
I talked to a dealer who sold both Yamaha and Honda, and he said that the Yamaha, with the dry sump engine oiling, would be his recommendation over the Honda for road race use that would see long periods of full throttle. The Honda has separate oil supplies for the engine and transmission, and while that is nice for letting each side get the type of oil it wants to have and keeping transmission/clutch trash out of the crank/cams, it doesn't hold much oil, nor is there an easy way to add in an oil cooler. Also, the Honda SOHC doesn't allow individual rephasing of the exhast and intake timing. Since I want to start off with a stock engine, but would like to have some ability to retune it, a DOHC system is more appealing.
While I wasn't wild about the Yamaha YZ250F 5-valve head, it did seem to have a power edge over the Suzuki and Kawasaki 4-valve engines, so I started watching eBay. In December 2005 I saw a low time (under 10 hours claimed) 2005 YZ250F engine being offered on a "buy it now" and I picked that up. It came complete with all ignition/wiring components, 37mm FCRMX Keihin carb, and shift and kickstart levers.
When I got the engine and put it on the scale I was very pleased to see that the complete engine came in at about 54.5 lbf. And I'll remove the kick starter parts for track use.
So the YZ250F engine gives me a powerplant that:
The thumper engine is not readily available for a couple of hundred dollars the way the EX250 engine is, but hopefully I won't have to have quite so many spare engines.
When using a dirtbike engine in a road race application you need to investigate the gearing in the engine. Many dirt engines have very "short" primary gear ratios, and you just can't hang big enough counter shaft sprockets/small enough wheel sprockets on them to get a good top speed. If you don't have a factory manual you can sometimes find internal and external gear ratio information on the manufacturer's web site, though you may need to go to the Japanese "home office" website instead of the US distributor's site.
I was able to come up with all the gear ratio information for both the YZ250F and a 1997 RS125 Honda two stroke road racer. I also found a chart showing final gearing on the RS125 for many tracks which was a big help. I determined that a final ratio around 2.05:1 on the Yamaha would give similar overall rear wheel speed to the RS125 (and since I'll be using 125 slicks I don't have to factor in different tire diameters). I like to run fairly large counter shaft sprockets as it makes life easier on the chain. Also, I hoped to run a 428 chain instead of the 520. The 428 chain is lighter than 520, and the sprockets are smaller in diameter (for the same tooth count), all of which helps reduce the rotating mass. But the countershaft sprocket also needs to be big enough to allow the chain to clear the swing arm pivot.
16/33, 17/35, 18/37, 19/39 and 20/41 all give around the same ratio.
I checked with Chris Products, the distributors for PBI Sprockets, and found that they were able to supply countershaft sprockets for the YZ250F in the 428 size up to 20 teeth. I ordered 2 each of 18, 19 and 20 teeth sprockets. I'll probably order 38, 39 and 40 tooth rear sprockets to start, and with the three different countershaft sprockets to mix and match I should be able to get something useable at most tracks. Here are the overall primary/transmission ratios for the YZ250F, and you can compare those to some vintage road racer transmission ratios on my Motobi project page.
YZ250F % drop 1st gear 27.0821 2nd gear 22.1155 .1834 3rd gear 18.3243 .1714 4th gear 15.5062 .1538 5th gear 13.1682 .1508 Overall: 2.0566
You can see that the Yamaha is fairly close overall compared to some of those vintage singles and twins, but evenly spread across the ratios. Having the percentage drop decrease as you go up in the gears can be a help. But I'm hoping that the engine will have a reasonable spread of power as well as significantly more power than those classic 250s.
OK, now I've got this nifty engine and some sprockets. As you'll recall for the EX250 project I'd obtained some Honda RS125 specification magnesium Dymag wheels and those are still on hand. I had been intending on using a Hossack-style FFE (funny front end) on the EX250 and I'll carry that concept, and the bag of race-quality rod end and spherical bearings, over to this project as well. More details of the FFE will follow in a bit but you may want to review the information on Norman Hossack's website as well as this article on alternative front steering/suspensions at Tony Foale's website.
I was originally planning on doing a somewhat conventional bike (other than the FFE) that might have ended up looking something like Norman Hossack's 125 Suzuki racer.
That plan changed recently. Another concept I've been interested in for 20+ years, and that I have wanted to build a bike so I could try it out, is the Feet Forwards (FF) motorcycle. I could go into a long explanation about it, but it is a lot easier to just refer you to Bike Web which has masses of articles and photos that address this subject.
On the mc-chassis-design list we'd been talking on and off about the FF concept for use on the track for the past few weeks, and after a comment along the lines of "well why don't you FF folks build a track bike and find out if it works or not" was made I decided that I may as well combine two projects (the thumper racer and an FF bike) into one and reduce the amount of work I have to do.
Obviously, researching everything you can about a project is a major design tool. On another page of my web site there's a list of useful technical books that I've put together from my personal collection. The books by John Bradley and Tony Foale should be at the top of your "must buy" list if you don't have them already.
Special interest email lists and groups on the Internet are another big source of information. My own mc-chassis-design list, the Feet Forward list at Yahoo Groups, Bikeweb.com (link previously given) which has a lot of FF material contributed by members of the FF list are all getting a workout on this project. Expert advice (if you can find it) is always a good idea when designing and building a potentially lethal device. I suppose that I should put in the next bit of text:
Since you are reading this it seems likely you have access to a personal computer. Many vehicles (and other mechanisms) have been designed without anything much more sophisticated than chalk drawings on a floor. But there are software tools that are very useful, and they can, at minimum, save a lot of time.
Notes kept in your favorite word processor or a notebook are very handy. I suspect I may not be the only person who finds that a crucial bit of information that was on a small bit of paper "somewhere around here" can at times not be found when you really would like to find it. Full-blown project management software is probably not needed, but organizing things (or at least jotting down random thoughts in one easy to locate spot) can be a help. The more you document, the better your chances of not overlooking something. I use a simple text editor (Notepad) for most of my notes. Don't forget to make a backup copy! Or TWO!
A spreadsheet program is handy for organizing repetive calculations, and you can also use it to hold notes relative to those calculations.
Something else I am not is an artist. I can do some drafting, but my sketches are typically pretty sketchy. I've found that if I can get an electronic form of a side view photograph of a motorcycle similar to the one I'm thinking about using in a project I can import that into a very basic graphics program and do some useful "sketching" with the image saving me having to draw a lot of extra detail. "Paint", which comes standard with the Microsoft Windows (TM) operating system fits the bill for me. I can import an image, use the eraser tool to delete stuff I don't need (and give a nice clean background for the sketching), capture the image of a wheel and move it to a different location, paste in some useful bit from a different image, capture screen snapshots from other programs, draw lines and circles etc etc. I do recommend that you save periodically, especially if you get a good "base" image to work with. That way if you get it too messy you can just pull up a fresh copy of that "base" and start your sketch over. It sure beats sharpening pencils and brushing eraser dust off of a piece of paper. You'll see some examples of this soon.
Drafting software is very useful, as it lets you draw parts to an accurate scale, and then read off accurate dimensions from the resulting drawings. Two dimensional (2D) CAD (Computer Aided Drafting) programs are really all you need, but you may find that you feel more at home with 3D CAD that lets you look at solid models from different angles. You can find free or very inexpensive CAD programs on the Internet.
Here's what I have for CAD:
The motorcycle specific software I have was written by Tony Foale. http://www.tonyfoale.com I am very pleased to be able to claim Tony as a friend, and the creation of some of the small calculators he offers as freeware on his site are due, I think, to my pointing out to him how helpful they'd be to the less informed enthusiast (like me), and how if I had something like that he wouldn't have to answer quite as many questions from me. :-)
Tony has also written his "whole motorcycle setup" software which is an amazing tool if you need it, and very reasonably priced too. If you are trying to do a clean sheet design, figuring out how to make a new suspension linkage, wondering what effect a change of sprockets will have on rear suspension squat characteristics, or anything like that YOU NEED THIS. Tony is also finishing up a user manual for a smaller program that works like the rear suspension portion of the big package for designing various types of FFEs. I'll be using both of these programs, and some of his calculators as things move along.
Another helpful offering from Tony can be found in the back of his book. This is Tony's pal Fred, who is a 2D ergonomic mannequin that happens to have the center of gravity (CoG) location for each main body part indicated on that part. I put a scan of Fred (in dismembered state) in as a background image in Rhino and traced around him, then assembled him and figured out the location of his overall CoG (this was done in a spread sheet) for several different riding positions. You can find jpg images of those, as well as .dxf (Autocad Data Exchange Format) and .3dm (Rhino) files in the http://www.eurospares.com/graphics/fred_ergo/ folder on my website. We'll see Fred in action shortly.
This isn't software, but it is a design tool. If you look in this folder you'll see a quick lean-angle/clearance gauge that I built. I'm going to use 50 degrees of lean to either side with the suspension bottomed out. Rather than spend hours (at this time) trying to get an accurate model of the engine I took some boards, a couple of hinges and some cardboard to make this gauge. The included angle between the boards is 80 degrees and is set by the cardboard. To use the gauge hold it up to the underside of the engine. Put the upper vertical edge of the cardboard on the wheel center line you've marked on the engine, and then raise it up until it contacts some low-hanging bit of the engine. Read off the scale to see how far it is from the vertex of the 80 degree angle up to a handy spot on the engine (on the YZ250F I just went to the center of the swing arm pivot hole, as that is what I'll be using as a reference). You can look through the cardboard and around the edges of the boards to see just what on the engine is the first thing to drag. You do need to account for the tire having a rounded profile, but you'll see that done shortly. But this quicky lets you see how high the engine needs to be for a given lean angle with a fully bottomed out suspension.
A digital camera can be very handy in the design phase. If you can't find an existing photo of a bike or engine, you can take your own. Not having to wait for film to be developed is a big plus - a few years ago I finally pulled the battery out of my film SLR as I can't imagine ever using film again. When you take your photo take it from as long a distance with as big of a telephoto as you can as this will help to "flatten" out the image. Also, if you put a ruler or other straight object of a known length in the photo as in this photo of the right side of the engine you'll have an easier time of scaling the photo to size. You'll also want to keep a camera handy in the shop so you can take photos of the project as it progresses. I very much regret not having photos of many of the bikes I've owned or the pieces I built for them.
A note on measurements: I grew up with the U.S. version of Imperial measurements and the metric system was taught as something we weren't likely to have to deal with. I can deal with either one but most of my measuring instruments are marked in the inch system. However, Tony's software expects data to be input in the metric system. What I'll probably do is mostly work in inch and then convert where I have to, but I'll try to post data in charts in both units. Please bear with me!
Now we've got all of our pencils (or electronic equivelants) sharpened and neatly lined up on the side of our desk, and are faced with a blank piece of paper, a blank CAD screen, or very possibly a blank mind. :-)
I find it very helpful to do some general concept sketches. As I mentioned, pulling a modified image of an existing bike into a graphics program can reduce the amount of work for this. Hmmm, I'm not finding an awful lot of photos of race-track bound FFs with FFEs falling to hand, so I'll grab something that is close and see what I can do.
When I go looking for a photo of a really trick thumper race bike with a Hossack-style FFE I usually head over to look at Chris Cosentino's Rotacular. This is the latest version designed and built by Chris and his co-conspirators who make up Team Incomplete. I took an image of the Rotacular, moved the rear wheel waaay back (for a 58" wheelbase), copied in some chunks of swing arm where there was now space, and trimmed off some bits of frame that I wasn't going to be using. I then imported that as a background image into Rhino, scaled it, and plopped Fred down on it as you can see in this image. Now that is a fairly upright version of Fred (more like you might see on the Gurney Alligator). I decided to replace him with his brother Laid-back Fred.
Since I was sharing these images with the chassis and FF lists Chris was probably getting worried about people asking him about that crazy version of the bike he was building. I did some quick lines in Rhino tracing over the link, upright and frame on the Rotacular, and then substituted a side view of the YZ25F engine. Since the engine was facing the wrong way I mirrored it, so that is why the countershaft sprocket is on the right side instead of the left. That gave me this version. The round red circle is showing the space between Fred and the expected location of the back wall of an airbox. This drawing also has the wheels/tires drawn in at the size of the 125 slicks, rather than the 250 slicks used on the Rotacular. The tires I have dimensions for are the Dunlop KR149 front (3.31"/84mm maximum width and 22.76"/578mm maximum diameter) and KR133 rear (4.57"/116mm maximum width and 23.67"/601mm maximum diameter) which are meant for the mandated 2.5x17F/3.5x17R 125 wheel rims.
http://www.dunlopracing.com/pages/sizes99.htmUse of the lean angle gauge showed me that the engine could be lowered about 3 inches from where I'd originally placed it, so I did that in this image. This also shows the rear wheel/swing arm and a guesstimated chain line with the rear suspension bottomed out. Fred needs to clear the tire and chain at all times!
That should give you an idea of the "let's see if this is completely hopeless or not for having things fit together" process. Now let's look at things that more directly relate to this project.
You'll notice that Fred's feet are on either side of the rear of the front tire. I don't want my feet dangling out at the leading edge of the tire in case that edge suddenly moves back about 6 inches due to impacting on something. But if the wheelbase doesn't expand to accomodate the feet being completely behind the front wheel/tire assembly, the rider is going to have to bend his legs at the knees a lot more. That may be OK to do - beats me. Right now I'm leaning towards having the feet behind the front axle, which would seem to offer a fair bit of crush space should push come to crunch.
If the feet are alongside the tire, they need to allow space for the tire to move side to side as steering takes place. Granted, for most of the "at speed" situations the wheel only needs to move a few degrees, coupled with some alarming amount of roll. But many race regulations require 20 degrees of lock to either side. I drew a top view and rear/front view of a front tire to get some idea of what is going on. This was done in SmartSketch.
This image is looking from the top, with the center of rotation right on the axle (think zero rake and zero offset). The cross-hatched area is the tire. This quick sketch shows that in this case the outer sides of the tire basically need a minimum of +/-5 inches of clearance from the center line of the bike. If the feet are alongside the front wheel, then the inside of the feet/lower legs needs to be 10 inches apart at minimum.
I'll digress a bit here. The numbers I have for rake and trail for a 1997 RS125 Honda are 23.5 degrees and 3.3". However, that is "ground" trail, which is the conventional number. If you use Tony Foale's rake and trail calculator it will tell you the offset from the steering axis that you need for a given "ground" trail, but also what the "real" trail number is. Real trail represents the lever arm that is actually coming to bear on the front end, and is what you should be using to compare different trail numbers for different bikes as the ground trail gains and loses effectiveness as the rake angle varies. The real trail for the RS125 is 3.04", and the offset is 1.41".
Tony's rake and trail experiments showed that little to practically no rake is needed, but some amount of trail seems to be needed for having a rideable motorbike. Let's use the RS real trail number of basically 3" as a rough goal to start with for laying out things.
If you look at the Hossack bikes Norman kept fairly conventional rake/trail numbers built into the FFE. This will normally mean that the axle is offset forwards of the steering axis. In the case of the RS125, that offset is 1.41". What happens if we go to a very steep rake? 6 degrees of rake and a negative 1.79" of offset (think of forks with the axle 1.79" behind the center line of the steering axis, or "trailing axle" sliders) gives a real trail of 2.98". Effectively the same stabilizing effect, but potentially about 3.2" difference in axle location!
If we go to zero offset (the axle is right on the steering axis) then for the same tire radius 14.75 degrees of rake with zero offset gives a ground trail of 3 inches and a real trail of 2.9 inches.
So if rake is not a very big factor here (though increasing rake means that the front end of the vehicle rises and falls more as the steering is turned) and any of those three might give similarly stable front end geometry. Pretty cool, eh? Not to mention confusing. But Tony did find that reducing the rake to that roughly 13-15 degree range with zero offset seemed to give a noticeable improvement in handling and indifference to side-loads from bumps or longitundinal grooves in the pavement.
Now for this project lets look at what happens as we change through those three rake/offset scenarios. With a near vertical rake the center of rotation of the wheel moves forward, so for a given steering angle (20 degrees) the back of the tire is going to swing out even wider. That means the feet would have to go farther apart. With a conventional rake (like the RS) the tire moves less, but the experiments showed that steeper rake seemed to work better, and modern bikes have been going steeper and steeper over time so we may as well stay ahead of the curve.
One advantage to having the zero offset is that is the situation where the wheel has the least amount of leverage on the steering when exposed to a side wind. If you have a wheel that is offset 1-3 inches from the steering axis then you have a greater wheel surface area on one side of the axis than the other, and so a side wind will tend to push that part of the wheel off-line and impart a steering angle to the bike. If the wheel is equally disposed around the steering axis, then things should pretty much balance out. Tony did one of his other FFE conversions on a Gold Wing that brought it more into the "equally disposed" range, and it was reported that it did seem less bothered by side winds. That also had a 16" wheel which reduced the amount of area exposed to the wind.
For a first approximation, I think I'll go for a zero offset front end. 13 degrees gives 2.56" of real trail, 14 degrees is 2.76" and 15 degrees is 2.95". It doesn't hurt to know that the later Rotacular moved to a 15 degree/zero offset front end and it is reported to be quite stable. Zero offset may also make for an easier to construct upright (the "fork" part that holds the wheels). With a longer wheelbase and other changes the YZ250F-FF may be able to do with less trail. It looks like the Guzzi GP singles of the 1950s ran conventional rakes (probably about 28 degrees back then) but only around 2.5" of ground trail and they were noted for their stability. Let's pick 13 degrees with zero offset but we'll make sure to have some method of adjusting in some more (or less) trail if we need it. That could be just screwing the rod end in or out of the pointy end of the upper A-arm. Moving the pivot like that does slightly change the effective length of the A-arm, and swinging the upright around the lower bearing does move the wheel, changing the wheelbase as well as the trail. It would be possible to have a set of simultaneous adjustments that kept the wheel in the same spot and only varied either the rake or the trail (or kept those fixed while moving the wheel) but doing that adds a lot of complexity to both the design and the fabrication phases.
Now we've got a tentative rake angle and offset (which luckily matches the tracing we did off the Rotacular). What about foot peg height?
This drawing done in SmartSketch shows our 50 degree lean angle, a 10" rear of the front wheel "swing" and some feet (or foots) to either side. You'll recall that I mentioned that when using the clearance gauge you needed to allow for a rounded profile to the tire and you can see that here, with the tire not fitting all the way down into the vertex of the angle. Our minimum spacing of feet/legs to the wheel, and still having everything not drag at a 50 degree lean angle at full suspension clearance (which I'm realistically expecting that if I should reach that state I'm probably midway through falling down) means that the heels need to be about 9 inches or so, plus suspension travel of 4 inches, or 13 inches above ground with a fully extended suspension. A 17" wide seat is about at the same spot, but that is several inches below the swing arm pivot in the engine and there needs to be chain clearance under the seat.
We can use that as a good starting point. As with other measurements, I expect that as the process moves along I'll find that one thing needs to be shifted, and then everything else needs to move with that, which may require yet another iteration to take place. But starting from a "pretty close" spot is better than some spot that isn't even within sight of the solution.
With an FFE and the chance to tailor dive/squat at both ends of the bike being able to figure out with some accuracy the eventual location CoG of the assembled vehicle is important. There are ways to do this by taking the completed vehicle and elevating each end while weighing the other, but that isn't going to be too helpful in the design process! There are symmetrical objects like a tire or wheel that are easy to spot the obvious CoG. But what about an odd shape like an engine? What you can do for the other items is hang them from something. The CoG will be directly below the spot it is hung from. Take a photo and extend the line from the ceiling down through the part, then hang it from a different location (say the cylinder head instead of a motor mount) and repeat the process until you have intersecting lines in several planes. The spot where they all meet is the CoG. This photo shows the YZ250F engine when it was being suspended by the swing arm pivot. The cord is a plumb line that I dropped from the point I was hanging the engine from. The red line (done in Paint) was drawn in after taking another photo with the engine hung from the bottom motor mount. For the FFE and rear squat determinations we only need the side view CoG. It won't hurt to know the lateral CoG as you may end up with a bike that is more heavy on one side than the other, and you can shift batteries, tanks, and the like to the other side to balance things. But generally even fairly lopsided engines (like a unit single BSA) are something you'll adapt to within a few feet of setting out, and it isn't a very big concern.
Before we get too far along I'm going to need to start collecting data on various components and their locations. I may as start the that here, and I'll return to it as I either get a dimension or think of something else to add to it. I'll add a section for the weight on some standard sizes of tubing since I'll have to figure out the CoG of the frame parts, and that is probably going to be easier to do by pinpointing the CoG of individual lengths of tube.
The weights were obtained from the calculator at Principal Metals The Principal Metals calculator only shows the wight in lbf. For converting between units I use the handy utility at convert-me.com
If you use Tony's structural sections calculator it will give you the cross-sectional area of the tube or solid which you can then multiply by the appropriate material weight.
Here are some basic weights and dimensions. I'm not going to include anything for aluminum at this time other than a standard alloy sheet used to make tanks. I do most of my structural fabrication from steel.
Round steel tubing - weight per inch/25.4mm:
Outside diameter Wall thickness lbf/inch kgf/25.4mm .500" .062" .0241 0.01093 .750" .062" .0379 0.01719 1.00" .049 .0414 0.01878 1.00" .062" .0517 0.02345 1.25" .049" .0523 0.02372 1.25" .062" .0655 0.02971 1.50" .049" .0632 0.02867 1.50" .062" .0793 0.03597 1.75" .049" .0632 0.02867 1.75" .062" .0931 0.04223 2.00" .049" .0850 0.0385 2.00" .062" .1069 0.04849 2.50" .062" .1345 0.06101
Fuel, oil and water are typically measured in gallons/quarts in the U.S.A, Here are some weights that I found:
This looks like a reasonable spot to start recording some component weights:
I spent some time today with Alibre. I got a rough engine model done - the motor mounts and swing arm pivot mount should be pretty accurate though. I found a drawing of a case half in the factory shop manual which I scaled and then used to generate an outline of the case. The front motor mount is the widest of the three points, so I extruded everything out to that width, and then extruded cuts back in to narrow up the other two mounts. The top end, clutch and flywheel covers are approximations. The countershaft sprocket should be pretty accurate for offset and location relative to the swing arm pivot. To tell the truth at my current level of skill I think I could do a "good enough" paper or 2D view faster. But speed will hopefully come with practice. I did find out that the assemblies are more friendly if I periodically save them and shut them down and reopen them. Those and a reboot seemed to get rid of some glitches after I had opened and reopened a lot of different Alibre windows. I think I was probably running low on memory as things were getting kind of slow.
Here are a series of screen captures as the different elements of the engine are created. You'll notice a long list on the left side of the screen, and a blue "dog bone" that moves along. Anything below the dog bone is being excluded from the current view, which is how you can go back to an earlier sketch and make changes.
I did some work turning Fred into a person of some substance, The dampers are just stuck in there to get an idea of clearances, and that is not necessarily the final location for them.
This image shows an assembly, which is what Alibre calls taking different parts and putting them together. This is a 58" wheelbase, 13 degree upright angle, and 13.5" high swing arm pivot. The pivot area of the swing arm is going to be very close to (or exactly) 2.5 feet long on either side of the engine. Other than that and the IDs of the pivot and axle holes the swing arm is a very rough "placeholder" approximation. Since I don't know what path the front wheel will travel the engine is just comfortably far away from it at this time.
It has been a week and a half since my last update. I've spent some time thinking about the project and fiddling with Tony's software looking at anti-squat effects. The very long swingarm pivoting off the back of the engine in this case seems like it may not be the hot setup for anti-squat effects (based on my no-doubt inadequate understanding of the issue).
Another problem is the chain is in the way of getting the rider any lower. One reason to build this thing is to see just what a very low overall vehicle/rider CoG does. If Fred is sitting bolt upright on his touring bike with a 32" seat height and then is dropped straight down to an 18" seat height his personal CoG drops quite a lot. If Fred is in a racing tuck and sitting on an RS125-spec 27" high seat his CoG ends up not being a great deal higher than when in an FF position on a seat that is only "somewhat low".
So what's the next step? After some thought, it seemed that moving the chain has to happen, as that is what is preventing the seat from getting lower.
Once there is no need to have a direct chain connection between the engine and rear wheel things look a lot different. The swing arm can be shorter and stiffer, the swing arm angle can be changed by moving the pivot point (which affects the anti-squat) and it is possible to consider having a sprocket running coaxially on the swing arm pivot. It could be possible to have one very long chain with idler sprockets to guide it, but I'm thinking that three separate chains might work better. That would be from the engine to a jackshaft somewhere under the rider, one from that to a jackshaft near the swing arm pivot (or to it if the pivot is coaxial with the jackshaft) and then from there to the rear sprocket. If two or more of the chains can be the same length it would make it easier to carry spare chains. If a coaxial rear sprocket/swing arm pivot is used all chains would then be free from the need for excessive slack to accomodate swing arm motion. Chain life might be improved from being able to tension them correctly.
multiple chain version with lower seat
Sorry, I haven't gotten this far in the process.
© 2006 Michael Moore, last update for this page 03 November 2006
http://www.eurospares.com/yz250fff.htm
