OK so when the 4wd switch is on & the rear tires are not spinning the front wheels are not tied to the rear at all?

CORRECT

Once the rear tires spin the front engages?

YES provided RPM are below 3500

If the switch is left on will the front disengage at some point?

Yes just as soon as the rear wheels no longer want to slip.

How is the front/rear wheel speed measured? IE, how does it know the rear is spinning?

The front / rear gearing ratios are slightly different so that the front wheels will want to turn slightly faster than the rear wheels.

What happens physically to engage the front?

When the AWD switch is on a current is drawn by the armeture in the front hillard cuasing a slight drag on the sprague carrier. Once the rear wheels slip, they are now turning faster than the front wheels (and the drive shaft is also). This alllows the sprague carier to rotate in the differential slightly forcing the rollers in the sprague to tighten on the front drive shafts engaging the front wheels. Once all four wheels are driving and the rear wheels have stopped slipping the rear wheels will want to go slower than the front cuasing the sprague carrier to rotate slightly in the opposite direction aloowing the rollers to drop back into the at rest position and the front drive disengages.

This is how it works, I may of got the front rear ratios mixed up but same principal, it is purely a mechanical thing orf the rollers in the sprague carrier being allowed to move outwards to grab the shaft when the rear drive over powers the front wheel speed.

 

Very helpful, thanks.

 

This is the best explanation for the Hillard clutch that I've came across:

The Hilliard clutch is an over running clutch. It differs from a normal over running clutch in that it can engage in either direction. This allows the Polaris to have 4 wheel drive in both forward and reverse.

Like any over running clutch, it will only engage when the input speed attempts to exceed the output speed. The way we use this feature is we connect the input of the over running clutch to the front wheel half shafts and connect the output of the over running clutch to the wheel. Then we can cause the over running clutch to engage by causing the front half shaft speed to attempt to run faster than the front wheels. This is exacly what happens when the rear wheels start slipping. So the over running clutch engages when the rear wheels start slipping.

In reality the rear wheels are always slightly slipping. However, we do not want the Hilliards to be always engaged. So the way we solve this problem is by running the front differential at a slightly lower gear ratio than the rear diff. This causes the inputs of the over running clutch to run slightly slower than the front tires. The ratio difference is about 20%. Now when the rear wheels slip at 20%, the Hilliards engage and power is transfered to the front tires.

Now a normal over running clutch only engages in one direction. However, we need a bi-directional over running clutch for our Polaris. The way we make this happen is to allow the over running rollers to be held in a "cage" and we apply drag to this cage. When we apply drag to this cage AND the input speed attempts to exceed the output speed, the rollers will jam themselves between the ramps in the over running clutch and the smooth surface of the inside of the wheel hubs. Understanding this drag is key to understanding how the Polaris system works.

The way we create this drag is to send current through an electro-magnet. This magnet attracts a plate that connects to the "cage". It is this electromagnetic attraction that causes the drag.

 

axle ratio question

In reality the rear wheels are always slightly slipping. However, we do not want the Hilliards to be always engaged. So the way we solve this problem is by running the front differential at a slightly lower gear ratio than the rear diff. This causes the inputs of the over running clutch to run slightly slower than the front tires. The ratio difference is about 20%. Now when the rear wheels slip at 20%, the Hilliards engage and power is transfered to the front tires.

Thank you, really great explanation of 4x4.

I usually have good results from searching this web site but I have come up with zilch about what the rear and front axle gear ratios really are.

Anyone have this info?

 

good reading jerrzr.....

 

so for the one guy who was asking about putting smaller tires on the front and bigger on the back, I wouldnt mess with it..... you dont want to mess with the factory diff/4x4 gearing setup or you will mess with the operation of your awd hillard clutch

very nice! thanks guys, the dealer almost got me all the way here with the questions I was asking when we bought our S, but he left me thinking this was a elecroneumatic clutch with check balls that ramp up when the magnet is powered... like modern AWD/4x4hi/low t cases

 

I believe that martymoe posted that at one time. Search "different ring gears"

 

Found it! Looks like 3.7 rear/3.85 front

Thanks

 

I have a patent pending on my design, which provides posilock for the AWD. Giving full control to the driver instead of relying on the computer. The computer that measures axle speeds to activate or de-activate the AWD is actually the speedometer. KC.

 

Hmm, how will that work, when the final link is the mechanical overdriving of the central hillard. The electrical on / off will not work unless you change front and rear gear ratios so that the front can be overdriven the whole time and then just turn switch on and off.

 

Speedo is just to make sure engine RPM are below 3500 before it will switch in the AWD electrical current.

 

Cant explain, "Patent Pending" all I can say is, I have it installed on my RZR and it works great. Once the legal stuff clears I will share okay.. KC.

 

3500 RPM is the threshold that the speedo computer needs to be under for the intial switch into 4wd mode, once in AWD mode the rpm does not matter, the mechanical overdriving is what engages and disengages the front wheels ( armature is energised allthe time once in AWD mode). Yes it is a safety so that you can't initial drop into AWD at high RPM and break stuff ( doesn't always work that way especially when jumping)

 

In otherwords, don't flip the "AWD" switch if the rear wheels are spinning.

If you do flip "AWD" when they are spinning, it can damage the front drive assembly.

The computer will not allow the front to engage at all if the engine is above 3500 rpms when the "AWD" switch is flipped.

I do stand to be corrected, though.

 

The 3500 RPM thing.. What exactly does this mean? Does it mean that past a certain speed, it's impossible to be in 4x4, or does it mean I just need to slack the peddle a bit to engage 4x4 while I'm moving at higher speed? When driving in 4x4 I'm pretty sure it's on at RPM's higher than 3500, so I suppose it's just some kind of safety to prevent damage if it was engaged at higher speeds? I'm confused about all this.

The ECU won't engage the magnet unless you are below 3500 RPM. I believe it is to keep from stressing out the armature plate. You'll notice that if you engage AWD above 3500 rpm, the indicator will NOT light. Once you slow down below 3500 rpm, the indicator lights and you're good.

My understanding is once the magnet is engaged and the indicator is on, you are in AWD at any speed.