Rotational Weight

[GeoTracker90]

Just wondering how big of an issue rotational weight is on fuel economy. I was doing a spreadsheet with information on various brands and models of 235/85R16 tires and noticed that the tire weights went from 40 lbs to 51 lbs. That makes the heaviest tires 125% the weight of the lightest. To me that seems significant.

With the way gas prices are going I'm trying to take such things into consideration before spending money on tires and also whels. I'm not sure of the weight difference between your average steel wheel and an aluminum version, but I would guess that there would be some.

Does anyone have any direct experience with this?

Mike

 

[madizell]

Only to the extent that saving weight on tires and wheels makes a difference you can detect in performance in terms of acceleration and braking, not to mention suspension cycle times. That there is a difference to be found with lower rotational weight, I do not doubt. Whether it has ever been quantified in a general sense, I don't know. Someone good with math and physics could make a work equation out of the amount of HP needed to move a pound of weight at a given distance from the axis of the wheel. Since such an equation would be an expression of HP, work over distance, as well as torque, work over time, I would expect it to be complicated. Most commonly, I suspect, one finds out these things the first time they take their vehicle to a dyno shop, where circumference and weight of the tires/wheels has to be plugged into the machine in order to compute rear wheel HP.

 

[GeoTracker90]

OK, here is my dumb follow up question in regards to the chassis dyno.

If you take a 51 lb tire with an OD of 31" mounted on a steel wheel and test the vehicle for HP and torque and then run the same test with a 40 lb tire with an OD of 31" mounted on a lighter aluminum wheel will the chassis dyno spit out the same HP and torque numbers?

I guess that would be a simple test to perform if someone had the money for two sets of wheels and tires and access to a chassis dyno. Might not be a good determination of potential fuel economy differences but it would be interesting none the less.

Mike

 

[IggyB]

Since I'm looking to get some 235/85-16''s for my Frontier, can you post the weights of the tires that you have found? Not many sites have the weight available.

 

[Grim Reaper]

That size is going to have a LOT of "E" Rated tires and the extra stiff carcass will add weight over a C rated tire.

The 235x75x15 on Steel factory 15x7 Toyota rims that were on my truck was around 61lb as a reference.

I will try to get a weight on a Michelin in the size you are asking about on a steel rim. I would guess the combo to be about 72lb. Its my spare. I scrapped the 4 rims on the truck in favor of aluminum to cut down weight. Trimmed about 8lb. a wheel


Now think about the rims.

I have 33x12.5'sx15 on a set of Aluminum rims and they weigh in at 68lb.
The lower the unsprung weight the better. The better the brakes work, the better the suspension works the better the acceleration.

 

[GeoTracker90]

Since I'm looking to get some 235/85-16''s for my Frontier, can you post the weights of the tires that you have found? Not many sites have the weight available.

I guess that I can do that for a fellow Albertan. (I grew up in Cardston, now living in Rexburg, ID.) Here is the spreadsheet that I started. I believe that all of these tires are load range E.

Hope that helps.

Mike

 

[GeoTracker90]

That size is going to have a LOT of "E" Rated tires and the extra stiff carcass will add weight over a C rated tire.

The 235x75x15 on Steel factory 15x7 Toyota rims that were on my truck was around 61lb as a reference.

I will try to get a weight on a Michelin in the size you are asking about on a steel rim. I would guess the combo to be about 72lb. Its my spare. I scrapped the 4 rims on the truck in favor of aluminum to cut down weight. Trimmed about 8lb. a wheel


Now think about the rims.

I have 33x12.5'sx15 on a set of Aluminum rims and they weigh in at 68lb.
The lower the unsprung weight the better. The better the brakes work, the better the suspension works the better the acceleration.

So with the lower weight did you see any kind of increase in fuel economy? I guess that might not be a fair question because of the different tire sizes involved.

 

[ntsqd]

There's a saying in road racing that one pound of unsprung removed is equal to 5 HP. How correct that is I do not know, but they apparently feel it's worth paying a lot of attention to.

Since chassis dynos do not usually measure transient response the weight of the wheel & tire should only make a difference in loading the tire to best possible traction on the roller drum. If they did measure transient response then the lightest wheel & tire combo would be the best option.

FWIW Torque is the measure of ability to do work. HP is the rate that the work can be done.
High HP = work done really fast.
High torque = lots of work that can be done.
Big rig truck engines don't have a lot of HP given their huge quantity of Torque, which is why they don't go over steep grades very fast but they do haul a huge amount of weight over those grades.

 

[Grim Reaper]

So with the lower weight did you see any kind of increase in fuel economy? I guess that might not be a fair question because of the different tire sizes involved.

I changed to many things all at once to tell.
The weight is up and the weight is on a larger circumference.
Now that I am regeared to 4.88 I get anywhere from 22-24mpg on the hwy in flat areas. DD in stop and go on my short 3 mile commute I get 17-18 in the summer and 16-17 in the winter.

 

[madizell]

Since chassis dynos do not usually measure transient response the weight of the wheel & tire should only make a difference in loading the tire to best possible traction on the roller drum. If they did measure transient response then the lightest wheel & tire combo would be the best option.

Chassis dynos measure output over the entire available rpm range, which necessarily involves a degree of "transient" response in the drive line in its entirety. This includes the drag of transmission, drive shafts, axles, and the mass of the wheels and tires as well as rolling resistance. You are apparently thinking of transient load as an up and down movement. Here, the issue is placing mass in motion rotationally, not controlling unsprung weight. The mass of wheel and tire have an immediate effect on the ability of any engine to put that mass into motion, and the lighter the mass to move, the easier it is. Also, the closer that mass is to the axis of rotation, the less force needed to move it. Tall, heavy tires are the hardest to move, which not surprisingly is the result seen every time we change to tall, heavy tires. Our ET's fall off even when gearing is compensated.

 

[ntsqd]

Transient response is the system's ability to respond to a stimulus, not the steady state achieved after the stimuli. I'm defining the system to be either the engine or the vehicle depending on which dyno type we are talking about, and the stimuli to be a change in throttle setting.

In the case of an engine dyno the inertia involved is pretty much only the engine itself. Waterbrakes are usually better about low inertia than Eddy Currents, though they can have flash boiling problems in transient testing.
Such is not the case in a chassis dyno, the roller alone has large Moment of Inertia. Though I'm sure it's done somewhere (probably OE emissions testing), it is much less common, indicative, or effective to see acceleration testing done on a chassis dyno than it is on an engine dyno. Tire slip is too big of an unknown. Just because it doesn't slip under steady-state load doesn't mean it won't slip momentarily in transition.

None of which has anything to do with the original question.

 

[GeoTracker90]

So it wouldn't hurt to go with a lighter wheel / tire package, even if for no other reason than to take it easier on the suspension and to have a little better braking and acceleration. Any fuel ecconomy benefit might just be icing on the cake. Right?

 

[Grim Reaper]

So it wouldn't hurt to go with a lighter wheel / tire package, even if for no other reason than to take it easier on the suspension and to have a little better braking and acceleration. Any fuel ecconomy benefit might just be icing on the cake. Right?

You got it.

 

[keepontruckin]

So your ideal combo is a lightweight tire that has low rolling resistance.

 

[madizell]

Fuel economy tests back in the 60's were performed using rock hard bald tires for just that reason. Rotational mass is a parasite that consumes power otherwise available to move you forward. Rolling resistance simply generates waste heat.