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#1
Jason J Ball

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I've been doing my annual spring maintenance on my car (re-pack wheel bearings, grease brake sliders, change all fluids, etc...) As usual, my front wheel spin very free after re-pack (just hubs and rotors). So I put the car in neutral and spin the back, which should have more resistance. My question is how much resistance? I can rotate the either hub by hand, but its feels very heavy to me. I don't have a dial type torque wrench to get a number, but it feels like 15-20 ft/lbs. Does this seem normal?

 

Car is a 1.6L with a VLSD.


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#2
Jason J Ball

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I guess this is an odd question no one knows the answer to. Well I put everything back together and it doesn't seem to take anywhere near that much force to spin it with the tires on it. We'll see if there's a problem on Friday test day. 


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#3
Bench Racer

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Jason, sometimes when we little kids answer questions the big kids become critics. If you were to remove a rear axle the tire/wheel would/should spin freely like the front wheel/tire. Whatever differential you have in the car will require more torque to rotate by hand. It'll never rotate freely like the front. Have you eliminated the self adjuster within the calipers for the rear brakes? They self adjust and make the rear pads tighter to the rotors.


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#4
luvin_the_rings

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Jason, 

 

The front hubs should have significantly less rotating friction than the rears.  

 

Some things that contribute to rear friction (turning one wheel alone) would be 

 

-Wheel Bearing

-Dust seal on upright rubbing on axle (this can be pretty significant depending on the age of the seal)

-Dust seal on differential housing rubbing on axle. 

-Axle CV joint movement

-Both differential axle bearings

-VLSD viscous torque (varies with oil viscosity and temperature)

-VLSD Internal gear bearings

 

Also, there is significantly more inertia when turning the rear so it will feel like its harder to turn, but in reality it just takes more torque to accelerate the rear drive train identically to the front, so don't confuse inertia with friction. 

 

20 ft/lbs seems reasonable to me. 

 

Cheers, 

 

-Z



#5
SaulSpeedwell

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The frontrunner approach to this:  Go to a Dynojet, run the car to 100 mph in 4th and throw it into Neutral and let it "coast down".  Instead of looking at the 116 HP (125?!?!) curve you are used to, you will see a -8HP to -15HP flattish line.  This is the horsepower your car is "absorbing" from the giant spinning DynoJet flywheel that you spun up to 100 mph.  So, that # is a representation of your drivetrain drag from the flywheel back. 

 

In my experience, -9HP  is Excellent, and achievable perfectly legally.  Get rid of brake drag, keep old floppy CVs and free-spinning bearings as long as possible, and have a well "broken in" (or blueprinted) trans and diff. 

 

We spend $6000 for 6HP at the crank, but there is 6HP behind the flywheel that many are not paying attention to.  If your rear brakes are dragging, you might have a $70 caliper eating up 5-10HP. 


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#6
Jason J Ball

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I was rotating the hubs by hand without the Calipers in place, although I have deleted the E-brake and auto adjusters. The calipers do float freely. Unfortunately my local shop went big time and moved to Charlotte to build the new MX-5 Cup cars so I no longer have access to a Dyno.  


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