I-Convert transmission: How it works

Moto Guzzi V7 Sport, 750 S, 750 S3, 850 T, 850 T3, 850 T3 California, V1000 I-Convert, V1000 G5, 1000 SP, Le Mans, Le Mans II, Le Mans CX 100, Le Mans III, Le Mans 1000, 1000 SP III, 1000 S, California II, California III, California 1100, California Jackal, California EV, California EV Touring, California Aluminium, California Titanium, California Special, California Special Sport, California Stone-Metal, California Stone-Touring, California Classic, California Touring, and California Vintage models

Created: 2010 April 02

Updated: 2010 April 02

I extracted this information from Patrick Hayes and Jim VanDenBerghe off of the old Yahoo! MGconvert news group (which has now moved to Groups.io).

In Patrick's own words:

First, the Convert has a traditional, two-speed manual transmission. Two gears, low and high. There is no neutral position per se. The only way to obtain a neutral function is to pull in the clutch handle and hold it ( for maintenance purposes you wrap a bungee around it).

Second, this two speed manual transmission is driven by a traditional multi-plate motorcycle clutch. In this case, I think Guzzi stole the clutch bits from either Stornello or Nuovo Falcone.

Third, the clutch and transmission are driven by a a fluid torque converter mounted on the engine flywheel (hence the name CONVERT). At lower rps, the fluid slippage of the torque converter means that you don't have to pull in the clutch at idle. The bike will simply sit and idle until you give it the gas. Think of a humongous moped.

Fourth, because there is a fluid torque converter coupling between the engine and the clutch/transmission, there is very little engine braking effect. You have to use the brakes a lot more than on other motorbikes.

Fifth, the torque converter and the ATF fluid system only operate the drive function. Unlike your automatic transmission car, the fluid system on this motorbike has nothing to do with gears or transmission.

Sixth, the bike will go 0-70 MPH in low gear and 0-100 MPH in high gear. Obviously, acceleration is a bit sluggish in high gear but many people leave it in high and don't shift for a month.

Seventh, the transmission should not be shifted while the bike is in motion. Select low or high before you start out. If you need to change, stop and change. The purpose of the clutch is to soften the blows or impacts on the gear dog faces when shifting.

Eighth, the basic idea of a torque converter is like this. These are very simplified concepts. You take a chamber and fill it with fluid. Within the chamber are two propeller like devices attached to an input shaft and an output shaft. At low rotation speeds one propeller moves and the other does not. The fluid just swirls and slips around the moving propeller blades. As the RPMs are increased, the motion of the first propeller begins to add a lot of fluid drag and force to the second propeller, eventually moving that second propeller. At mid RPM ranges, there is still slippage and the input propeller is spinning somewhat faster than the output propeller. At very high RPMs, the slippage becomes almost negligible and the two propellers approach a matching speed synchronization.

Ninth, because of all this slippage in the torque converter, the Convert motorcycle traditionally gets poorer fuel economy than similar standard mechanical motorcycles.

Tenth, because the Convert is mostly driven in high gear, the engine seldom reaches any serious RPM level and more commonly is loping along at lower RPMs. An unfortunate side effect may be a poor battery recharge and a gradual decline in battery, requiring an overnight charger boost every week or two.

Patrick's reply to the question, How come the fluid needs to be under pressure to work? Is it just that it would get too hot if you didn't re-circulate it and cool it?

You've reached the limits of my knowledge on this one. Perhaps someone understands the SACHS torque converter more completely. As far as I can tell, the fluid doesn't do anything other than transfer drive force from the engine half to the transmission half. The slip and drag of the fluid generates a lot of frictional heat and it has to be circulated through a radiator. Also, any air contamination within the converter would lead to cavitation of the liquid and loss of drive force. Pressurized circulation helps ensure purging of air. I don't think pressurizing the fluid in the converter makes it work any more efficiently. Liquids don't compress much under pressure. I may be wrong and there may be some pressure advantages inside the converter. But, the pressure isn't moving anything (other than the fluid) unlike a car transmission where the pressure is utilized to move clutches, gears, valves, etc.

One other strange component of this torque converter is the central bearing. It has oval rollers rather than round balls. As speed and pressure are applied, these rollers stand up and wedge themselves between the converter and the transmission shaft. When the engine is off, they lay down and release the contact grip between the transmission and the engine. Thus allowing you to roll the bike around without having to combat drag of the drive system.

Some further clarification from Jim VanDenBerghe:

The ATF pump doesn't cause the bike to move. It doesn't pump the bike forward. The reason for the pump is to simply circulate the fluid that was warmed up (due to the slippage loss) through the ATF cooler where it is cooled down so it's temp doesn't exceed the maximum design temperature of the ATF fluid.

Those oval rollers are called sprags. They are used in a lot of auto applications too, and I bet in other power transmission systems. They allow torque to be transmitted in only one direction, i.e. clockwise or counterclockwise, but not in both directions. In the other opposite non-driving direction the two shafts slip without torque being transferred (free-wheels).