Should I dyno tests after installing lighter wheels?

[OICU812]

Lol some of the shit I can read here daily is entertaining for sure.

[industrial]

These are all just lightening mods. Engine power doesn't change. Every change of motion happens(accelerate, decelerate and change direction) quicker when you do these mods. The dyno doesn't really capture inertial losses since they are most significant when you first start accelerating and nobody drops the clutch on a dyno session or bothers to measure the milliseconds between transitional power application. Lighter everything makes a difference that's hard to quantify (dyno, lap times, other metrics) but easy to notice. I do think an extreme case like the one proposed by the op would be quantifiable on the track. 40 pounds of unsparing rotating mass will show up in lap times given the same tire compound and non-compromised rims.

[suaveflooder]

Originally Posted by Model Citizen

Are you doing this for science? Bragging rights? World peace?


Ive never dynoed a car ever. Why? Because as much as i like the idea of subjective numbers i like money better.

I'm starting to agree with you here. Other than checking for safety concerns (A/F) on a dyno, they tend to, for the most part anyway, just be numbers.

[8R6]

save your dyno-testing money and put it towards a driving class. driving improvement is the biggest, longest lasting mod over any other mod.

unless your goal is to impress car guys at a parking lot and become an internet celebrity. then in that case, driving is overrated.

[makesdrivingfunagain]

Originally Posted by OICU812

Lol some of the shit I can read here daily is entertaining for sure.

Well, if you were referring to my statements I am glad my stupidity (lacking data, quality information, experience etc) could be of a good form of entertainment Perhaps I should have asked a more informed question.


I thought there was a correlation of weight of rotational mass or in this case unsprung rotational mass to horsepower gain or loss. In other words is there a principal or a correct concept of adding weight or reducing weight to the drive wheels of an automobile and a direct relation to hp increase or decrease on a dyno. I guess that was the principal or theme for me to debunk in this post.


Thanks gentleman!!

[fatoni]

Originally Posted by D K

Going to heavier wheels is like adding weight in the car.
Might accelerate slower but engine power is the same.

D

Engine power will be the same but the dyno doesn't measure engine power so it doesn't matter

[stugray]

Originally Posted by makesdrivingfunagain

I thought there was a correlation of weight of rotational mass or in this case unsprung rotational mass to horsepower gain or loss.

I think you nailed the issue with that question right there.

You are confusing/combining two separate concepts into one.

The rotational mass of the entire drivetrain (engine, trans, driveshaft, diff, axels, rotors, wheels, tires) is what "could affect" the apparent hp of the car at the rear wheels.
Of all of those things the wheels only play a small part.
The tires contribute much more to the rotational inertia of the system because they are very dense and are arranged around the perimeter of the biggest rotating mass in the system.
In contrast the drive pulley on the engine is perhaps the smallest contributor.

The "un-sprung weight" refers to the mass of the suspension member that must move up and down to maintain contact with the road.
The lower the unsprung mass, the faster the suspension can respond. Hence the term "responsiveness" of a cars suspension.

So any mass you can shed from the wheel, or the tire, or the A-arms, linkages, rotors, calipers, etc. reduces unsprung mass.
You can see why F1 moved the brake rotors & calipers in-bound on the rear axels to get that mass away from the "un-sprung" part.

Originally Posted by makesdrivingfunagain

In other words is there a principal or a correct concept of adding weight or reducing weight to the drive wheels of an automobile and a direct relation to hp increase or decrease on a dyno.

Short answer - No.
The amount of mass you have to modify on the wheels will be a relatively small percentage of the overall rotational mass of the whole system.
Of course it helps if you have money to spend on pure racing wheels.

[Model Citizen]

Originally Posted by suaveflooder

I'm starting to agree with you here. Other than checking for safety concerns (A/F) on a dyno, they tend to, for the most part anyway, just be numbers.

Don't get me wrong, I have 0 issues with dyno tuning.

But dynoing for the sake of bragging rights is like paying to join mensa.

[makesdrivingfunagain]

Yeah, I understand the power to weight principal of a vehicle fairly well. However there is that mystery that seems to "hover over" certain areas such as unsprung weight, unsprung rotational mass (especially) and sprung rotational mass in improving the acceleration of an automobile.


well anyways, thanks for your help gentleman!!

[retoocs]

Basically unsprung mass typically affects suspension and handling.

Mass reduction in general affects acceleration.

Unsprung rotational mass is the best of both worlds.

Now, rotational mass is not all the same. If you were to take a wheel and remove 5 lbs from the center of the wheel (near the axle) and put the 5 lbs you took off back onto the car, you will see next to nothing in acceleration change. If you were to remove 5 lbs from rim of the wheel and put the 5 lbs on the car, you will see a slight improvement in acceleration. If you have a hard time understanding this, pick up a hammer by the handle and hold it straight out. Then pick up the hammer but hold it by the head and hold it straight out. See the difference?

Originally Posted by stugray

You can see why F1 moved the brake rotors & calipers in-bound on the rear axels to get that mass away from the "un-sprung" part.

That hasn't been done since the 1970's. The tradeoffs made it not worth the effort. Inboard brakes required beefier axles because brakes have more "horsepower" than the engine. It was also a safety issue because if you broke an axle, you lose your brakes.

[industrial]

Give this page a read.

http://stephenmason.com/cars/rotationalinertia.html

[Jive Turkey]

you're not changing the output of the engine, a dyno is redundant.