viscous clutch? Is it constant or variable? vs weight transfer

[ed hess]

viscous clutch? Is it constant or variable?

I was allways under the impression the viscous clutch was in constant engagment to the front drive axles.

Recently into into a discusion which made me wonder.

now as for tying front end downs to limit weight transfer vs letting it all transfer to help the rear hook so it doesnt spin anyway so that it doesnt matter what the front drive does. Comments?

or tie the front end down to limit weight transfer and all 4 get the power, if the drive is constant the power divided by 4 tires should hook better. comments?

or how about this, suppose the 40 to 60 power ratio i heard of in the past is acccurate and
suppose you had scales on all 4 corners and you set up the weight distribution to match the power distribution?

before anyone comments on this consider 2 things.

1 Make sure you know what your talking about and prove it.

2 think outside the box


myclone I'm counting on hearing your comments on this.

ed

 

[George Blake]

Re: viscous clutch? Is it constant or variable? vs weight transfer

http://auto.howstuffworks.com/differential9.htm

Read that first. See if it answers your question.
The split isn't constant. It's variable.

 

[myclone]

Re: viscous clutch? Is it constant or variable? vs weight transfer

myclone I'm counting on hearing your comments on this.

ed

ROFLMAO.. How'd you know I was sitting here reading the post over and over while working on a response.

Im getting WAY too predictable in my old age.

 

[gkrcr882]

Re: viscous clutch? Is it constant or variable? vs weight transfer

I'm just a rookie, so go easy on me, but here is what I think. From my understanding, the VC is a varible unit in the sense that if it was a fixed output all of the people who drove w/out a front propshaft would have consideable power loss due to the fact that power would be diverted to the nonexistant front propshaft, where as in a varible unit the VC would sense a higher shear load and trasnfer more power to the axle with the most traction. Personally I would allow some weight transfer since the majority of the power is being sent to the rear axle, and the rear axle has a limited-slip device whereas the front axle is an open unit. JMO:2cents:

 

[myclone]

Re: viscous clutch? Is it constant or variable? vs weight transfer

viscous clutch? Is it constant or variable?
now as for tying front end downs to limit weight transfer vs letting it all transfer to help the rear hook so it doesnt spin anyway so that it doesnt matter what the front drive does. Comments?

Not a simple answer(s) here IMO but I'll make a stab at it since Les called me out (thanks ya turd ).

Going by the link that George provided there must be some difference between the front/rear drive line speeds for the viscous coupling (vc) to transfer torque so on paper as long as the two drive lines are at the same speed one drive line is getting 100% while the other is lagging slightly and just along for the ride/absorbing power from parasitic losses. However, whats going on at the instant of hard accelleration from a standing start is where it gets complicated IMO not to mention the steady state minute amount of difference in front/rear drive line speed from tire wear, tire pressure, etc.

If you follow the theory of the link provided and apply it to a syty the rear tires MUST slip to some degree before the vc sends torque to the front. How much slip though? Would it be 1/1000th rotation of the rear drive line, 1/2 rotation, or somewhere in between? Since we're talking about a production line piece here Im inclined to think that it could be anywhere depending on the tolerance of the vc in each xfer case as in my truck may sping the rear tires 1/2 turn before the front drive line sees much torque load increase where as someone elses vc may only allow an imperceptable amount of rear tire spin before throwing tq at the front drive line.

Lots of variables in the above two paragraphs that Im not smart enough to calculate the affects of. None the less to sum it up IMO no matter what you will need to test your particular truck to see what it likes. More weight transfer with a tight vc may do nothing but spin the front tires harder and you'll prolly go slower where as more weight transfer with a sloppy vc may actually help your 60ft since the light front end is lagging behind the front in tq application and the front tires are more efficient at moving the vehicle forward instead of being over powered and just spinning. Make sense?

To validate the tight/loose vc Ive personally seen trucks that will barely (or no perceptable amount) spin the front tires on a hard launch while other trucks will spin the rears ~1/2 turn or so before the fronts will spin. My truck spins the rears a fair amount before the fronts which is a combo of a loose vc, too much boost, and WAY too much weight transfer...as in whacking the bump stops in the rear. Watch this for proof http://www.turbotimelounge.us/albums/myclone/aci.mpeg

or tie the front end down to limit weight transfer and all 4 get the power, if the drive is constant the power divided by 4 tires should hook better. comments?

With a loose vc and/or soft shocks Id say this would be the way to go.

or how about this, suppose the 40 to 60 power ratio i heard of in the past is acccurate and
suppose you had scales on all 4 corners and you set up the weight distribution to match the power distribution?

With a tight vc and no weight tranfer this would be best IMO. However keep in mind the weight on the 4 tires changes dramatically between static and dynamic (sitting on the scales vs launching). With a tight vc you could tune via the static weight numbers to give you the best 60ft times via experiementation but the best set up would not end up being a static number of 40% of the weight on the front and 60% of the weight on the rear.

1 Make sure you know what your talking about and prove it.

Even if I dont know what Im talking about I can EASILY make it look like I do by the shear amount of bs that Im capable of typing. That old saying of "if you cant impress them with knowledge baffle em with BS".

 

[George Blake]

Re: viscous clutch? Is it constant or variable? vs weight transfer

If you want something that will be consistent every time, your gonna have to go to a 4wd transfercase. That way there is no "shear" factor waiting for the silcone to heat up to make the plates start grabbing. What they do is leave a small amount of air space in the viscous unit itself to allow for expansion of the silicone as it heats. The amount of fluid varies from unit to unit I sure.

 

['JustDreamin']

Re: viscous clutch? Is it constant or variable? vs weight transfer

The only other way to make the viscous unit consistent (in my mind) is to force it into lockup through some form of starting line or in-the-pits procedure. Otherwise, it'd be luck of the draw as to how "locked" up the viscous unit is.

You'd need a way to apply brakes to one end (probably rears) and power to the other (fronts) but allow that end to spin. You'd have to do this hard & long enough to get the viscous unit to lock up (I think they call it "hump"). Not sure how long or hard that would be to do. Then it'd be like running a locked case (as apposed to a mostly unpredictable case).

Other solutions would be a 4wd tcase (like a part time case where speeds are locked together) or one of the On-Demand tcases (like the NV246) where there is a clutch that engages the front axle @ rear output speed (minus any slippage).

The problems you'd run into is the part time case gives up dry pavement driveability. If you try to turn while the front axle is engaged, it'll be a handful (due to the differences in front and rear axle speeds in corners). And the On-Demand case requires you to have to figure out how to properly control / engage the tcase clutch.

Don't know if that helps.

'JustDreamin'