# Unsprung weight or LRR



## Kelmark (Oct 26, 2009)

On an EV where 70% of the driving is stop and go traffic in town; wouldn't un-sprung weight be more important than LRR tires? Un-sprung weight is roughly 1:10 ratio, (cutting 1 lb un-sprung is like taking ten pounds off the car). So this would require less energy each time you accelerate. Granted having both would be best but I was just wondering which would be most effective. What are your thoughts?


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## tomofreno (Mar 3, 2009)

I don't know what "unsprung" weight is, but rolling resistance is proportional to vehicle weight, so I would guess a 10% reduction in weight would have the same effect as using tires with 10% lower rolling resistance.


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## Kelmark (Oct 26, 2009)

tomofreno said:


> I don't know what "unsprung" weight is, but rolling resistance is proportional to vehicle weight, so I would guess a 10% reduction in weight would have the same effect as using tires with 10% lower rolling resistance.


I am referring specifically to rotational mass. IE tire, rim, brake rotor, and flywheel.

“Unsprung weight includes the mass of components such as the wheel axles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight.” From Wikipedia


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## DavidDymaxion (Dec 1, 2008)

Rotating weight is important, not unsprung.

If you truly do lots of stop-and-go driving, a pound off the flywheel is like taking as much as 5 lbs off the car in 1st gear, and about 3 lbs off in 2nd gear. For the wheels, taking off a kg is like taking 2 kg off the car.

That's the good news, but the bad news is this is expensive weight to lose. A racing flywheel might cost $300 to $600, a lightweight pressure plate again that much, and racing wheels can cost $500 to $2000. The best benefit for the cost is having your flywheel cut down and removing the starter gear.

So take the case of my conversion, the racing flywheel/pressure plate saved about 10 lbs, so for accelerating the car acts like it is about 1% lighter. Yes, that is maybe 0.5% more range in stop-and-go driving -- it'll then drive about 300 meters further on a charge. Often it would be less, because it is rare your whole time is spent stop-and-go.

So if you have the money, it's a good thing. If dollars count, though, cramming in another battery, lightweight seats, lrr tires, etc. might be more bang for your buck.


Kelmark said:


> I am referring specifically to rotational mass. IE tire, rim, brake rotor, and flywheel.
> 
> “Unsprung weight includes the mass of components such as the wheel axles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight.” From Wikipedia


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## automd (Feb 5, 2010)

DavidDymaxion said:


> Rotating weight is important, not unsprung.
> 
> If you truly do lots of stop-and-go driving, a pound off the flywheel is like taking as much as 5 lbs off the car in 1st gear, and about 3 lbs off in 2nd gear. For the wheels, taking off a kg is like taking 2 kg off the car.
> 
> ...


I agree. My friend was able to save his money. He also decided to have he's flywheel cut down and removing the starter gear.


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## sunworksco (Sep 8, 2008)

Think of each corner of your suspension and wheel as a hammer.The lighter the hammer ,the more control you will have swinging it.Your steering wheel will be easier to move with more suspension feedback as well which equates to better handling.
There is also an economic payback for cutting overall weight becuase the lighter the electric car , the smaller the battery pack.


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## JRP3 (Mar 7, 2008)

DavidDymaxion said:


> That's the good news, but the bad news is this is expensive weight to lose.


Going clutchless is free and saves a lot of weight


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## Kelmark (Oct 26, 2009)

JRP3 said:


> Going clutchless is free and saves a lot of weight


I decided to retain the clutch because I heard the VW transaxle can be difficult to shift quickly without a clutch. I am trying to get the best 0-60 possible; I do not want to waste time fumbling with the gears.

Good news I found a lightweight steel flywheel that costs about $10 more than stock. It is not the lightest but I will be removing the ring gear and I have a friend that has a lathe that will trim a little more off. I also went with a stock clutch as I won't be slipping the clutch. 

I will post more details and pics in my build site as I get further along.


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## DavidDymaxion (Dec 1, 2008)

True... but I'm concerned with all the "wait" clutchless shifting adds when trying to get to highway speed!









JRP3 said:


> Going clutchless is free and saves a lot of weight


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## JRP3 (Mar 7, 2008)

Is that unsprung "wait"?


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## dtbaker (Jan 5, 2008)

not sprung, but IS rotational.... 

But really guys, unless you are drag racing the difference in getting a flywheel up to speed, or a lightened flywheel, or even NO flywheel is probably barely measurable. With no flywheel, it will be slower (if shifting) because of the wait for syncros be perhaps require slightly less energy.

From what I've been reading in the Eccomodder, MUCH larger gains are to be made with simple cheap aero improvements like wheel skirts in the back or maybe smooth hubcaps rather than open-spoke wheels.


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## DavidDymaxion (Dec 1, 2008)

Well, since the clock in my conversion is electric, and not a wind up clock powered by springs, I would have to say it is an unsprung "wait." 

More on topic: Push your car by hand in 1st gear. Push it again in 2nd gear. The difference in pushing while accelerating will be about the difference the car experiences keeping the clutch vs. going clutchless. (This assumes the armature weighs about twice the clutch + flywheel + pressure plate.)


JRP3 said:


> Is that unsprung "wait"?


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## sunworksco (Sep 8, 2008)

David,
Here are some Dymaxion photos.
The major flaw in the design was the rear steering control.
It needed to have a variable steering control that would steer the rear wheel at a slower speed the faster the Dymaxion is traveling,otherwise you could flip the car over at speed.The variable power steering could be easily accomplished today with electric power steering.


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## sunworksco (Sep 8, 2008)

There is no other vehicle like the Dymaxion turning on a dime!
Changing the steering from the rear to the front is like castrating a bull!


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## roflwaffle (Sep 9, 2008)

Kelmark said:


> On an EV where 70% of the driving is stop and go traffic in town; wouldn't un-sprung weight be more important than LRR tires? Un-sprung weight is roughly 1:10 ratio, (cutting 1 lb un-sprung is like taking ten pounds off the car). So this would require less energy each time you accelerate. Granted having both would be best but I was just wondering which would be most effective. What are your thoughts?


Realistically, for most cars, the unsprung weight has a ratio of ~1:1.5, not 1:10. RR otoh is pretty much linear, so going from something like snow tires to LRR tires would cut the energy needed for city driving in half give or take. Shell out the cash for some LRR tires, make sure the alignment is zeroed out, and don't worry about un-sprung weight.


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## JRP3 (Mar 7, 2008)

sunworksco said:


> There is no other vehicle like the Dymaxion turning on a dime!


I can turn on a dime and give you change


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## JRP3 (Mar 7, 2008)

roflwaffle said:


> Realistically, for most cars, the unsprung weight has a ratio of ~1:1.5, not 1:10. RR otoh is pretty much linear, so going from something like snow tires to LRR tires would cut the energy needed for city driving in half give or take.


 So a car getting 30 mpg in the city would get 60 with LRR tires? I don't think so.


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## roflwaffle (Sep 9, 2008)

JRP3 said:


> So a car getting 30 mpg in the city would get 60 with LRR tires? I don't think so.


I never said that. Engine efficiency drops with load (Most need to be regeared to take advantage of aero/RR efficiency improvements), so for most cars mileage wouldn't double even if the energy required per mile was cut in about half, and even at lower speeds aero has an impact. EVs are different, but I'm not sure by how much. Assuming the controller/motor combo behaved favorably wrt efficiency (Similar efficiency at different loads) and a drop in load, then a typical compact ~2500lb EV with a conservative driver going from snow tires (Crr=.016) to LRR tires (Crr=.008) at a ~25mph average in the city would see a a ~40% reduction in energy consumption per mile. Going from whatever the car's alignment is to a zero loaded (Very important!) alignment could also shave off another ~5+%) If the motor/controller doesn't behave favorably and efficiency drops as load drops, then the gain would be less just like w/ a conventional car.


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## maxvtol (Nov 11, 2009)

roflwaffle said:


> Realistically, for most cars, the unsprung weight has a ratio of ~1:1.5, not 1:10.


To be clear, like others have pointed out before, unsprung mass is the wheels, tires, suspension parts that go up and down with the bumps. It has nothing to do with rotational mass, it is just on the "unsprung" side of the suspension. The primary reason manufacturers try to keep the unsprung mass down is for better handling and control over bumps and ride comfort. 

The rotational mass of the tires, wheels, brakes, etc. may be on the order of 1:1.5, but the rotational mass of the flywheel in 1st gear can be 1:10 or more.


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## roflwaffle (Sep 9, 2008)

Assuming we run the motor up to 4000 rpm the difference in KE between a 10lb lightweight flywheel and a 20lb stock flywheel is ~3+Wh for every acceleration. I suppose if someone went from 0rpm to 4000rpm to 0rpm every mile it could add ~3+Wh/mile, but that's excessive even for stop and go city driving AFAIK. The 10:1 figure is referenced in terms of power, not energy efficiency. W/ an engine revving to 8k rpm, a car that's accelerating in whatever gear over 5s could see a ~2kW improvement by shedding ~10lbs of flywheel weight, which I'm guessing is the equivalent of removing ~100lbs for a typical 3000lb/100kW car, but the same thing does not apply in terms of energy consumption unless the car was constantly accelerating then decelerating at the maximal rates.


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