Effect of increased rotating mass on front vs rear wheels
08-14-2013, 04:01 PM
#1
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Effect of increased rotating mass on front vs rear wheels
I know rotating mass in the drivetrain makes a difference in the power that will reach the ground. I figure the driveshaft is part of it, axles, clutch, flywheel, rear wheels..
Does weight reduction on the front wheels make less of a difference in a cars acceleration than weight reduction on the rear? (assuming no traction issues) rear are actually connected to the engine
I suppose there is also wind drag created by wider front tires.
Does weight reduction on the front wheels make less of a difference in a cars acceleration than weight reduction on the rear? (assuming no traction issues) rear are actually connected to the engine
I suppose there is also wind drag created by wider front tires.
Last edited by Andy13186; 08-14-2013 at 09:52 PM.
08-14-2013, 07:17 PM
#2
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Sorry if I sound like a techy geek, but ...
As for as acceleration goes, it makes no difference where you add mass. Mass added farther from the center of rotation takes more energy to accelerate, so 20" wheels of the same weight as 18s make you car cooler, but slower. I'll take the faster car every time!
Reducing unsprung mass (wheels, brakes, some of the springs and drive shaft) allows you to tune your suspension for better response. In this case wheel diameter doesn't matter, but lighter is (potentially) better. With that massive stick at the back of our cars, it will be very hard to find a wheel light enough to affect suspension back there. $600 of koni yellows will do way more than $5000 of forged ultralight wheels. I bought the konis.
If you're building a drag car don't forget to go for smaller diameter and thinner brakes. The Brembos weigh a ton (for good reason)! If you're building a road racer you could try a two piece rotor with aluminum hats. They're expensive though.
As for as acceleration goes, it makes no difference where you add mass. Mass added farther from the center of rotation takes more energy to accelerate, so 20" wheels of the same weight as 18s make you car cooler, but slower. I'll take the faster car every time!
Reducing unsprung mass (wheels, brakes, some of the springs and drive shaft) allows you to tune your suspension for better response. In this case wheel diameter doesn't matter, but lighter is (potentially) better. With that massive stick at the back of our cars, it will be very hard to find a wheel light enough to affect suspension back there. $600 of koni yellows will do way more than $5000 of forged ultralight wheels. I bought the konis.
If you're building a drag car don't forget to go for smaller diameter and thinner brakes. The Brembos weigh a ton (for good reason)! If you're building a road racer you could try a two piece rotor with aluminum hats. They're expensive though.
08-15-2013, 10:21 AM
#4
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Once any of these variables changes, your results will change. Reduce rotating mass and keep the level of traction you have (assuming you have adequate traction) and performance will improve slightly. Increase traction and you will see even more improvement.
For example, when testing our Focus ST we went significantly quicker with a wheel and tire package that had similar traction to stock, but had a rotating mass of 5 pounds less per wheel/tire together even though the wheel was larger (19 inch vs 18 inch). The difference was around 2 tenths of a second in the 1/4 mile.
08-15-2013, 12:20 PM
#5
From my personal real world experience, this is not completely correct. Two wheel packages of the same weight are going to have the same performance as long as the tire attached to them provides the same traction AND is the same overall diameter, your comment about mass added farther from the center of rotation doesn't seem to take into account the weight of tire as well, and as long as the profile is kept right and your overall diameter is the same, then the mass at a given distance from the center of rotation is not changing so dramatically that it makes a significant difference. So a 20 inch wheel would have to have a lower profile tire to keep the diameter the same, and as long as the tire used provides equivalent traction, your performance would remain similar.
Once any of these variables changes, your results will change. Reduce rotating mass and keep the level of traction you have (assuming you have adequate traction) and performance will improve slightly. Increase traction and you will see even more improvement.
For example, when testing our Focus ST we went significantly quicker with a wheel and tire package that had similar traction to stock, but had a rotating mass of 5 pounds less per wheel/tire together even though the wheel was larger (19 inch vs 18 inch). The difference was around 2 tenths of a second in the 1/4 mile.
Once any of these variables changes, your results will change. Reduce rotating mass and keep the level of traction you have (assuming you have adequate traction) and performance will improve slightly. Increase traction and you will see even more improvement.
For example, when testing our Focus ST we went significantly quicker with a wheel and tire package that had similar traction to stock, but had a rotating mass of 5 pounds less per wheel/tire together even though the wheel was larger (19 inch vs 18 inch). The difference was around 2 tenths of a second in the 1/4 mile.
Wonderful answer. ANd thank you.
I always say its easier to tweak final ratios with wheel/tire diameter than it is to swap ring and pinion for a quick swap for track, autocross or daily driver configurations.
08-15-2013, 12:37 PM
#6
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My new front tires are 8 lb heavier each than my currents.. seems excessive but they are 20 mm wider and also .7 inch taller and have 1/32nd more tread depth.
I am wondering if adding 8 lb per tire heavier rear tires would hurt the cars accelleration performance more than adding 8 lb heavier per tire to the front tires - assuming no traction issues, since the rears are technically connnected to the engine and the fronts are just rolling?
Maybe they both subtract the same amount of performance due to having the same rolling resistance increase
I could see the increased weight in the front tires having a bigger effect on braking, since the front wheels are main braking wheels and increasing rotating mass there must make it harder to slow the actual wheels
If that is true though, i think rear tire weight would have a bigger effect on accelleration.
I am wondering if adding 8 lb per tire heavier rear tires would hurt the cars accelleration performance more than adding 8 lb heavier per tire to the front tires - assuming no traction issues, since the rears are technically connnected to the engine and the fronts are just rolling?
Maybe they both subtract the same amount of performance due to having the same rolling resistance increase
I could see the increased weight in the front tires having a bigger effect on braking, since the front wheels are main braking wheels and increasing rotating mass there must make it harder to slow the actual wheels
If that is true though, i think rear tire weight would have a bigger effect on accelleration.
Last edited by Andy13186; 08-15-2013 at 01:00 PM.
08-15-2013, 04:24 PM
#7
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I am somehow being drawn into an argument about physics, for which there is no argument! Anyway, I won't intentionally insult anyone, especially Gus, since he probably has 10x my experience,
There's just no getting around the fact that when you move the rotational center of mass out, the wheel takes more power to accelerate. Coming back to Andy's (OP) question, It's likely your car will be measurably slower unless the new tires are way stickier than the old ones. Are you comparing your new tires to the OE pirellis? Btw, your wheels are awesome! I'd love a set of Forgestars.
All of the car's mass has to be accelerated (or decelerated), so it doesn't matter which end has the heavy wheels. However, more rubber at the back means more traction = better acceleration. Same for the front, except of course it's decel.
I'm impressed with the focus example, because that's not the norm. They must have been some pretty awesome wheels and tires! I'd wager the guys seriously competing in the focus are on 17s. The mini cooper s is always faster with the "base" 16" wheel than with the "upgrade" 17s or 18s. There are plenty of guys who've put 18" track wheels on their FRS/BRZ only to find its track pace is essentially the same because the car just won't accelerate as hard. Faster in the corners, but slower on the straights.
There's just no getting around the fact that when you move the rotational center of mass out, the wheel takes more power to accelerate. Coming back to Andy's (OP) question, It's likely your car will be measurably slower unless the new tires are way stickier than the old ones. Are you comparing your new tires to the OE pirellis? Btw, your wheels are awesome! I'd love a set of Forgestars.
All of the car's mass has to be accelerated (or decelerated), so it doesn't matter which end has the heavy wheels. However, more rubber at the back means more traction = better acceleration. Same for the front, except of course it's decel.
I'm impressed with the focus example, because that's not the norm. They must have been some pretty awesome wheels and tires! I'd wager the guys seriously competing in the focus are on 17s. The mini cooper s is always faster with the "base" 16" wheel than with the "upgrade" 17s or 18s. There are plenty of guys who've put 18" track wheels on their FRS/BRZ only to find its track pace is essentially the same because the car just won't accelerate as hard. Faster in the corners, but slower on the straights.
08-15-2013, 04:44 PM
#8
The contact patch and the rubber compound at target temperature combined withe overall diameter of the tire on the wheel regardless of the wheel size determines the overall effectiveness when combined with rear axle and transmission gear ratios.
The type of tire is relative to the job at hand. Taller sidewalls on smaller wheels for a drag race application. Shorter stiffer sidewalls on a larger diameter wheel for better track lap times.
From a dig the overall weight of the vehicle will have adverse effects on the launch. Also the weight distribution on the vehicle will play a part. For most drag applications the weight is preferential towards the rear of the vehicle since inertia then momentum will benefit from a slight distribution over the area of traction.
from a roll the distribution for a 50/50 balance is much more favorable for a balanced and predictable car braking into an Apex then powering out of the Apex of a twist or turn.
And from a roll a taller diameter will have the top speed advantage over the smaller diameter with an equal amount of HP and torque pushing the wheels.
Tit for tat once you're rolling inertia and gravity factors are replaced by horsepower and aerodynamics.
Again applied science dictates the proper wheels and tires based on function.
Engineered centrifugal Single piece forged aluminum wheels will be lighter and stronger than a cosmetic mainstream Aluminum wheel.
rubber compounds and overall contact patch size will also determine the effectiveness of one tire choice over another.
It's actually all pretty straight forward.
The type of tire is relative to the job at hand. Taller sidewalls on smaller wheels for a drag race application. Shorter stiffer sidewalls on a larger diameter wheel for better track lap times.
From a dig the overall weight of the vehicle will have adverse effects on the launch. Also the weight distribution on the vehicle will play a part. For most drag applications the weight is preferential towards the rear of the vehicle since inertia then momentum will benefit from a slight distribution over the area of traction.
from a roll the distribution for a 50/50 balance is much more favorable for a balanced and predictable car braking into an Apex then powering out of the Apex of a twist or turn.
And from a roll a taller diameter will have the top speed advantage over the smaller diameter with an equal amount of HP and torque pushing the wheels.
Tit for tat once you're rolling inertia and gravity factors are replaced by horsepower and aerodynamics.
Again applied science dictates the proper wheels and tires based on function.
Engineered centrifugal Single piece forged aluminum wheels will be lighter and stronger than a cosmetic mainstream Aluminum wheel.
rubber compounds and overall contact patch size will also determine the effectiveness of one tire choice over another.
It's actually all pretty straight forward.
08-16-2013, 09:08 AM
#9
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Weight, several different rotational inertias, torque, axle gearing, tranny gear ratios, tire diameter, aero + rolling drag and several other factors are always present, just in different proportions at different speeds. This can be mathematically simulated and comparisons made, but this would be a lot of work if you're starting from scratch.
One of the factors that are part of the rotational inertia effects depends on whether we're looking at the wheels, or if we're looking at something whose rpm increases faster than the wheels (think flywheel/pressure plate/clutch assembly or fluid-filled torque converter and harmonic balancer here).
As far as rotational inertia effects of front vs rear tires/wheels goes, it'll be the same if the front and rear wheels and tires are all the same. Otherwise it'll be slightly different. Not by enough to matter in normal street driving, though.
Norm
One of the factors that are part of the rotational inertia effects depends on whether we're looking at the wheels, or if we're looking at something whose rpm increases faster than the wheels (think flywheel/pressure plate/clutch assembly or fluid-filled torque converter and harmonic balancer here).
As far as rotational inertia effects of front vs rear tires/wheels goes, it'll be the same if the front and rear wheels and tires are all the same. Otherwise it'll be slightly different. Not by enough to matter in normal street driving, though.
Norm
Last edited by Norm Peterson; 08-16-2013 at 09:12 AM.
08-16-2013, 05:17 PM
#10
Mr. Peterson... This is partly incorrect. The wheels affected with the inertia would be the ones where the torque is applied. For our RWD Mustangs this is of course the ones on the rear. The Front Wheels factor into the equation as additional resistance. The front wheels are an impact on forward momentum based on their rolling resistance and the mere fact that Mustang s197's by their very own design are front heavy, they load the front wheels . So, for drag applications it's optimal to apply countermeasures to this resistance by allowing front wheels to lift off the Tarmac. Thus applying less physics variables for the wheels with power to overcome.
On the road for a daily driver larger diameter tires effectively make for taller ratios and top speeds. Once rolling at high speeds the advantage goes to the taller ratios provided horse power and all else is the same between both vehicles.
On the road for a daily driver larger diameter tires effectively make for taller ratios and top speeds. Once rolling at high speeds the advantage goes to the taller ratios provided horse power and all else is the same between both vehicles.
