Friday, January 15, 2010

What's the deal here... Summary, Overview, and Technical BS

DISCLAIMER:If you're reading this, a fair warning: This is a long-winded post about how we plan to get this all put together, if you're not interested in such details then please skip this entry.

Firstly, a few things about our engine. Snowmobile engines are two-stroke, and for our Yamaha V-Max 750, produces about 140hp (when new and running). They rotate in the "conventional" direction, same as most cars (except old hondas, I'll get back to this later). It drives another shaft through a set of gears which runs the primary sheave (or pulley) for the belt drive. Here's a picture so you get the idea:

The sheave here is on the left hand side of the picture. This runs a belt to another sheave, mounted on the jackshaft, which is just a regular shaft with a sheave on one end, a couple of bearings in the middle, and essentially what is the "final drive" on a snowmobile the details of which aren't important here.

In order to retain the CVT function of the snowmobile engine, we're planning to couple the jackshaft end of things to the car's transmission. This will mean making a new "jackshaft" which can couples to the splines on either the clutch or the input shaft of the transmission. Yes we haven't quite worked out the details here either.

If you're with me so far good. Now the engine drives the cvt, which drives the gearbox. Why retain the whole gearbox? We'll, for one its less machining, and less expensive stuff. Secondly, it allows us to retain the original differential and axles from the car. Since this is a budget build for an endurance racer, we're trying to minimise unproven and untested components. There are downsides too, which we'll have to deal with eventually.

The question has been asked if we're planning on shifting the transmission's gears as we drive. Likely not, we want the car to drive with the cvt as much as possible. Maybe we'll shift from one ratio to another, to give us a high-low range type of arrangement. Because the CVT effectively acts like a disengaged clutch when you're off the throttle, we probably won't need a clutch to shift gears. If we do retain the clutch it is for other reasons (which will be discussed eventually). This arrangement also gives us a reverse gear.

Now, for those of you who can visualise all of this with your head, congratulations. There is an design compromise to be made here with rotational directions here and how we choose to mount the engine. If we wanted to just take the front end of the Firefly and stick it in the back of the Geo (imagine a rear-end collision of sorts), then the snowmobile engine would have to be mounted offset from the center of the car, meaning it would have to be placed outside on the passenger side. obviously not a good thing. This has to do with how the car's transmission is placed. Look inside your hood (for a FWD), the engine is roughly on one side of the car, and the engine is on the other. In the case of our car, the engine is on the passenger's side, and the transmission on the driver's, everything being split roughly down the centerline of the car.

The way we will overcome this, is to place the firefly subframe backwards. Meaning that we're cutting the front of the Firefly and placing it back to back in the Geo. This allows us to keep the engine within the confines of the car body. Yes, everything should look "normal" from the outside. We have plans for the radiator and fuel tanks as well.

Now before somebody decides to put my idea to paper and see for himself/herself, this does mean we are going to be running the transmission in a reverse rotation as it would normally. Another way to put this is if your car engine were turning in the opposite direction as it would normally. If you've read this far I'm amazed. There are two points to be made about this.

Firstly, we could of used an old Honda, because they had everything backwards from the conventional positioning of things. This would have allowed us to run the transmission in it's normal orientation. However, we decided not to pursue this on the basis that even an old civic hatch (1991) is 4" wider than a Geo, this we feel would negate handling A LOT!

Secondly, for those of you who may know a thing or two about gears, is what exactly does it mean to run a transmission backwards. Well, helical gears produce thrust loads, axial to their shafts. This thrust load's direction is dependent on the rotational direction of the shaft. This affects bearings inside the transmission. Long story short, and you may feel free to ask me about it if you need clarification, but we need a transmission with thrust load capacity in both orientations. Picture below is from a 4 speed Swift transmission. Notice the tapered roller bearings at both ends.

I'm betting this is the same configuration found in our 5 speed. More research to be done on this soon...

Update: research suggests that the input shaft on the 5 speed has angular ball bearings, I hope this works.


  1. Why not just add one gear set between the cvt output shaft and the transmission input shaft. In other words, rather than have the cvt output shaft be the transmission input shaft (which requires similar rotations) just put a gear on the cvt ouput that messhes with a gear on the transmission input. Voila: counterrotation.

  2. That's a good suggestion, to be honest hadn't thought of doing exactly this. I can see two issues: First is speeds,which would be high enough to require good lubrication. Second is torque, its enough to require a good quality gear, which makes it harder and more expensive to make.

    While its always possible to scrounge something to do this and build a casing around it, we're going for simplicity. I think if I'm going to design and build a custom gearbox, the right way to do it would be to drop the metro gearbox completely and make something that uses only the stock diff and axles.

    While that sounds really cool, its simply much more effort and money than the guys are willing to put into the project. That being said, If our gearbox explodes, I may steal your idea.