Rotational Inertia - Effects on Horsepower

[@EngineeringExplained]

Thanks! I certainly will. The next two videos are basically math based (the next one mainly just words). Basically I just teach myself what I'm interested in, and then relay what I learn on my channel. Glad you guys enjoy it!

[@TheMotownhustler]

Thanks Professor. I'm 66 retired and drag racing for a hobby. I appreciate the information. 👍🏾😎

[@EngineeringExplained]

A heavier flywheel will not improve acceleration at higher vehicle speeds; it would however reduce engine braking, so you'd have more inertia to overcome when stopping the vehicle, while in gear.

[@IsmaelMartinezPR]

Another example is an ice skater doing circles as the move their arms in the spin faster. Great series you have a great future as a teacher any day.

[@jchafee1]

Nice connection. I always wondered. Thank you.

[@EngineeringExplained]

No worries, the formulas and technical aspects certainly aren't for everyone. I will have plenty more videos which are more "traditional" and simply explain how things work. That said, I like to be thorough, and topics like these are needed to tell the whole story. In a few weeks I'll have a detailed exhaust story (basics, catalytic converters, mufflers, o2 sensors) and shortly after I hope to start getting into hybrids/electrics. The math will be minimal!

[@EngineeringExplained]

You simply have to provide enough power to power the engine continuously, this likely wouldn't change much with a different flywheel. Worse mileage in the city, roughly equivalent on the highway. A heavier flywheel could potentially provide a small benefit in drag racing, storing more "free" (doesn't take up time) energy to use once the light turns green.

[@EngineeringExplained]

Well if off-road vehicles never planned on going over 10 mph or so, ever, then 1 gear may be acceptable. But it's always nice to have a low gear set also for moving very slowly with maximum torque. You just want the versatility of multiple gears, in part so that it can also be used on the road or anywhere.

[@n.s2833]

its important to know that flywheel mass is calculated in ic engines to store kinetic energy permanently during combution strok and giving it back in the following stroks to give the engine smoth tourqe curve between stroks

[@MercedesAMGsRULE]

I love this video. I have a Mini Cooper with the 'run flat' tyres on. I changed the tyres and weighed the old ones Vrs the new Non runflats. There is a huge 3.6kg per tyre difference due to the stiff tyre wall that the runflat has. Of course then I had it on the dyno, and the difference was 1.9 horsepower, (obviously it was dynoed on the front wheels so if I include the rear wheels I can double this figure) so the total Hp increase is 3.8. Also helps handing, it's comfort, quieter and more fuel efficient. The tyres are cheaper too! It was the Pirelli Runflats Vrs Continental Premium contact 2. I just carry an inflation kit in the boot if required.

[@joonyaboy]

Tires should be the first performance enhancement.

[@EngineeringExplained]

Thanks, glad to hear this! And I have no plans of stopping!

[@EngineeringExplained]

The answer to "why" is what I was attempting to explain in the analogy of the different bike wheels. Speed and velocity are the same here, as there's no directional change. Though both wheels move the same distance (and all the mass moves the same distance linearly) the rotational mass moves a greater distance in the first wheel.

[@EngineeringExplained]

There are no simple disciplines of engineering, your knowledge is sound. :)

[@MisterBinx]

About 4:10 is where it really clicked for me. Great explanation thanks.

[@EngineeringExplained]

That's a great question, and something I try to discuss with professionals when I get the chance. I believe it does mean this, but I'm not certain the effects it may have on stability at higher speeds. Also, it depends what your pivot point is. If I had a track at my disposal I would play around with weight locations to see the effects. Abandoned airports are a tad pricey these days.

[@EngineeringExplained]

Lifetime eh?? Sounds good to me! As long as you're out there I'll keep making videos.

[@williamdutoit9585]

This explains well why cars tend to spin their front wheels when shifting from 1st to 2nd, It's all that inertia that is "stored" in the engines high speed. when shifting to second the engine has to rotate slower but it doesn't want to causing a peak in torque at the wheels resulting in loss of traction. This also explains why my motorcycle doesn't wheelie in 1st but shifting fast to 2nd lifts the front wheel momentarily.

[@EngineeringExplained]

I would say check out my videos on HP vs Torque. Specifically "Formula 1 vs NASCAR, Horsepower vs Torque" as this shows the difference of high revving engines to lower revving engines, with torque being the altering factor.

[@EngineeringExplained]

Basically it could be any rotating component which is out of balance. My first inclination would be the wheels/tires, as this is probably the easiest location for something to get out of balance. Even if there was just simply a clump of dirt stuck in one of the rims, it could cause a significant vibration at speed. First I'd just check to make sure everything is clean and how it should look; then perhaps take it somewhere to get the wheels balanced.

[@mackchannel6348]

Old content is still good content. This helped me understand much about my vehicle that I did not know. Thank you for taking the time to put out this video!

[@EngineeringExplained]

You'll likely want to get a different tire size to compensate for the lost 1" of the diameter. You'll definitely benefit from the change (assuming the weight of wheel + tire goes down, and rotational inertia goes down), but as to whether or not it will be noticeable is unknown.

[@francisgaliegue6645]

This video does not have enough views. I have never figured out what inertia really was until I saw this video... Excellent job. Congratulations!

[@shananagans5]

Great vid. Many people discount the importance of rotational inertia.They understand weight overall but they don't really understand how you are kind of doubly penalized by weight that also spins. One time I was buying lighter wheels & tires for my buggy (1,200lbs) & I said "this should make a huge difference." The guy said "it's only 40lbs you won't notice it" I said it's 40 less lbs of unsprung weight & 40lbs less rotating weight. He didn't understand & looked at me like I was a crazy woman

[@EngineeringExplained]

If you check out my video on flywheels, I explain why they have a certain mass. Regarding reducing rotational inertia, the real reason why it isn't done (to a greater extent) is cost. To get equally strong components, the materials are going to be expensive.

[@livebyreason]

It bothers me a little that you used the capital letter "M" for mass. :)

[@danb3e3]

We don't care

[@abangjago2935]

I too don't care...

[@Irish6538]

This is the first time the concept has been made clear to me. The explanation was great and I really understood it since being aware of it for 3 years now. 5 STARS!! Thank you. :)

[@EssensOrAccidens]

I can concur that most cylindrical components in an engine would be better off with a lower relational moment. In my opinion, the flywheel may be an exception. In a comment below EE mentions a key reason for a heavy flywheel: smooth power delivery. I want to watch the follow-up video treating traction, but it would at this point intuitively seem to me that too-abrupt power delivery would be a greater threat of traction compromise. Anyway, another excellent video! Thanks EE.

[@EngineeringExplained]

Lighter flywheels for more power, but you're power delivery will not be as smooth.

[@EngineeringExplained]

Good to hear!

[@SavageJim01]

Excellent review of MoI! Brings back memories of my highschool physics teacher using a pipe &rod of the same wt rolling down his inclined physics book on a desk! When I got to university to learn materials science, I had to use the Polar Moment of Inertia to calculate torsional stresses. I always thought back to examples like yours to remember that a pipe will take more torsional stress than a rod of the same area on the plane perpendicular to the moment vector.

[@fifthgear93]

Keep making videos about complicated things. Your videos have a lot of educational value. Make a video please, about the differences between diesel and petrol engines. Which is better for everyday use and why.

[@eraybulgaria]

Incredibly well explained, the drawings and logical follow up were on point!

[@jackrios4040]

This video convinced me to get lightweight pulleys and lightweight wheels lol great explanation 🙂

[@DENicholsAutoBravado]

Uh oh! Late last night, I was thinking about this, and now I want cross drilled brakes. For my car's design theoretically it won't help with braking much, but it'll reduce the rotational inertia that has to be overcome for acceleration. ....when I'm up for the mod, I'll be weighing it and new ones to be sure the difference I expect is actually there.

[@EngineeringExplained]

***** Umm.. yes they do. Your brake calipers remain stationary.

[@chincemagnet]

+DE “AutoBravado” Nichols You're not gonna lose much weight from a few drill holes.

[@DENicholsAutoBravado]

chincemagnet Nope. But inertia is inertia. Hey, this is the least effective brake mod, so why not try it for weight? I'm thinking more about how lighter rotational items mean a lot more than overall weight. Still haven't done it. I'm so excited as to wait for newish rotors to wear out. Lol. Get the likelihood and time frame? Lol

[@danb3e3]

Good analogy talking about the distance the mass has to travel. Cheers!

[@EngineeringExplained]

Well it of course depends on multiple things, including the weight of the car and the current power. Regardless, every bit helps. Increase a 5 hp go-kart to 10 hp, and you will surely feel it. Increase a 300 hp car to 305 and it's unlikely you'll notice.

[@TilakRajVerma]

Thank you Mr. Professor. Such a great insight and so simply explained 👋👏

[@RBfarming]

You go into detail about why you would want less rotational inertia but made no argument as to why it's there in the first place. Absolutely fantastic channel though, watched every single video ;)

[@shananagans5]

I like you going into detail. The math isn't really my thing but,taking the time to actually work it out, lets me see how much difference it really makes. Granted I am ill & stuck at home with nothing better to do but I am sure several of your viewers are engineers or in school to become engineers. Those details are more important to them than most but the average person can understand the concept or principal without doing the math.Going into detail you to give both the simple & complex answer

[@EngineeringExplained]

Lightweight rotational components, and try to locate the mass towards the axis of rotation.

[@nolimeplease]

I just love the way you explain things. I have just been given a solid concept in physics that will be a tool I carry for years. Thanks!

[@trillionare7]

Hey man, I am thinking of getting 17 inch wheels weighting 16 lbs each, over stock 18 inch wheels which weights 23 lbs each. Size difference is 225/50 R17 vs 225/50-18. Will I benefit from the this mod in terms of gas mileage and acceleration? Thanks

[@EngineeringExplained]

No, it is a plus. If it were a minus the moment of inertia would decrease as the mass moved away from the axis of rotation, which is not the case.

[@marcholler4047]

So is a bigger rim and wider tire decrease horsepower due to weight?

[@destin842]

Pretty much, in drag/street racing Your goal is to be able to get traction on the smallest tire possible.

[@EngineeringExplained]

Well, fatter tires are going to have increased rotational inertia. But if you're traction limited then it could help acceleration.

[@PranavHosangadi]

Simple way of putting it: Moment of Inertia is to Torque as Mass is to Force. Since people are more used to thinking of masses and forces than MI and torque, it becomes really easy to remember.

[@ingeniousx2581]

I actually had a idea of using that principle in racing cars ....u contract the mass towards the centre after a car is about 50 miles an hour and its speed would increase by too much

[@rickitickidicki]

another concept flawlessly explained! I'm gettin' this! Thanks man, You!, You've got a gift you!

[@DENicholsAutoBravado]

Thanks for the great video. Here's my reaction when I found a practical application of this video's information. Wow. You have to read the description on the video link I'm sharing here. 3 grams on the pistons assembly equals 100 lbs. of torque at redline. Watch the following engine rebuild presentation. His goal isn't to reduce weight so much as to balance his pistons. It's better to have the piston heads balanced, but that takes instrumentation beyond his ability. Hyundai Assembly 4 - Balancing Rods This modification is completely within my ability. I want to do it first on my vehicle that isn't a daily driver.

[@EngineeringExplained]

Why don't you look it up before you tell me "check wherever I want." I check many sources before making my videos, and though I do make mistakes, this is not one of them. Come back to me with a source that says otherwise. All my engineering books have typos? Don't think so.

[@Encryptus1]

I recently bought a set of aluminium wheels to substitute the steel ones I had. I bought them at a junkyard, had them checked for "straightness" and then painted them myself. I spent two fuel tanks after the change and the mpg increased. The steelies were 6kg each (around 12lbs) and the aluminium ones are half that at 3kgs. It "added" 50 kms (30 miles) to my fuel consumption per tank, being one tank with the steelies 650kms. So, around 8% fuel savings 😎

[@jamesseaton7033]

great video and weird seeing the old content! glad you kept the whiteboard 😎

[@modestcody123]

That made a lot of sense man. Really good job!!

[@EngineeringExplained]

Thanks!

[@EngineeringExplained]

Punching holes in brakes for the sole purpose of reducing inertia sounds a bit dangerous. You want a large surface area for good cooling. Sometimes they are slotted/drilled for cooling as well. But basically they are sized for the car.

[@karllued]

I recently bought lighter rims for my bicycle. Riding on these I have discovered some potential disadvantages. The extra weight around the outside of the rim adds some stability with a greater gyroscopic effect. The energy is not lost, but stored in the gyroscope. Yes, it takes longer to accelerate to maximum speed, but once you get there, you have more inertia stored in the wheels. The same bicycle with lighter wheels going into deep sand, will stop faster, possibly getting stuck, right? Also, as you pedal, you tend to track along the ground in a slight, "S", pattern, caused by uneven torque on the crank arms. This can be exaggerated with lighter wheels, decreasing overall forward motion. Also, since the bicycle doesn't have power steering, going through uneven terrain puts more torsional forces on your shoulders and arms, as the lighter rims being less gyroscopic can be more easily moved from a straight line track. This would also apply to any off road vehicle, any thoughts? Is lighter really better in all circumstances?

[@leebaidyanathan1]

The topics are getting better by the day :)

[@EngineeringExplained]

Check out my video on flywheels, which explains there purpose. They are useful for a smooth power delivery. And yes, you wouldn't want it smaller than the clutch.

[@BouncingHope]

Wow! I like this explanation of Moment of Inertia! Makes so much sense now. So would that mean, in linear terms, mass can be seen as an object's resistance to acceleration?

[@EngineeringExplained]

Yes, as it is the object's resistance to rotation.

[@amirarif6641]

let say for moped crankshaft if taken out a bit of metal on the outer diameter, does it helps the bike for long distance? i know it sure will improve response but incase topspeed will decrease in results, still can counter by sprocket am i right?

[@lewis7909]

no

[@KasaiHasuki]

Don't change on the account of one person. I love the nerdier videos. Do what makes you happy or nobody will be happy.

[@EngineeringExplained]

I doubt light weight pulley's are going to give you 5 hp more, but if you've seen the dyno that proves it then yeah it could be worth it.

[@EngineeringExplained]

I think it's honestly just the room that I film in. It's kinda cramped and I think it just echoes a bit around the room. I'll have to think of a solution.

[@mohammadenteshariun1629]

that is probably just a typo but the equation in red is I=1/2 M ( r1^2 - r2^2 ) there should be a minus instead of plus

[@alejmc]

For anybody that starts doing some laps and races as a hobby this channel suddenly gets 400% more valuable. Open a patreon and get my money

[@siedpe13]

I understand how this video explains benefits of a lighter flywheel. I was hoping this video would cover the purpose and benefits of a heavy flywheel. When replacing the clutch on my daily driver, I learned that yes, a light flywheel would increase my acceleration, but it would have a negative effect for stop and go driving as I regularly engage my clutch to inch forward. I think some information as to why vehicles use heavy flywheels to begin with, would be useful

[@theamericandemocracyausers513]

Assuming you actually DO want to spend power revving up rotating mass (mainly to absorb intermittent and grossly over-powered inputs), am I correct that the slower-to-go-downhill cylinder will also take longer to slow down at the bottom, protecting more of the energy it does pick up from gravity on the way down from friction losses by resisting the change in rotation that friction would otherwise try to impart?

[@EngineeringExplained]

Yes, it's hollow, and yes, it's a "+" sign. May not be entirely intuitive, but MOI increases as the mass is located further away from the center of rotation.

[@stephenkeighley7993]

I truly enjoy these videos. Doing a great job, please be sure to keep it up.

[@shananagans5]

Yes. Depending on the diameter & overall weights of course. If the diameter is larger a lighter hollow cylinder can have a higher moment of inertia than a slightly heavier cylinder with a smaller diameter. Think of a weight on the end of a single bicycle spoke. The closer the weight is to your hand the easier it is to move. Now put that spoke in a completed wheel. The farther the weight is from the axle the harder it is to make it spin.

[@PavelShevchuk]

Oh, changing wheels is so underrated! If you decrease overall diameter of wheels, you'll get more torque at expense of top speed and speedometer precision. If you fit narrower tires - you can decrease grip and have more fun with rwd/awd car. Watch "The Mercedes C63 AMG Experiment - /CHRIS HARRIS ON CARS" video to get the idea.

[@EngineeringExplained]

I think I used all caps, simply to spite you! :)

[@Soffex]

wait, is that moment of inertia formula correct? Isn't it outer radius minus the inner radius? I'm pretty sure you subtract the inner radius since there is no material, hence no mass, in the center. i.e. 1/2*m*(ROuter^2) - 1/2*m*(RInner^2)

[@EngineeringExplained]

Yes, and the opposite, depending on the mass and dimensions.

[@tarkin1980apa]

This is a great video, but I don't think you made it clear why you can't just make the flywheel smaller and thicker (ie reducing the diameter while keeping the mass the same). Thanks for a great channel. I like these theoretical videos as I am interested in engines and egineering but don't really care much about cars :)

[@MrIrrepressible]

Thanks for the answers. Regarding the pulleys I meant that they free up an extra 4-5 hp to the wheels not that they add extra hp. 250 dollars for a set of 3. Still considering the purchase

[@EngMadison]

I think of the moment of inertia as sitting on a spinning chair with your arms out vs. arms in...which one is easier/faster at spinning? But I dropped out of Mechanical for Civil Engineering...so maybe I'm not the one to be giving advice. Great videos, watch them all the time!

[@Bluegogglesonadog]

Hey man, really nice vid. I just had one small "negative" comment i thought you might want to know. Lately I've been getting, i would'nt say bored, but not so enthused from your videos. I really liked the ones you did in the beginning, explaining how engines, and brakes and turbos and suspension, etc. work with very clear drawings. Although i liked this vid in particular, ever since you started explaining with formulas and stuff (basically your "new" videos) i've felt lost most of the time... *

[@alexi077]

You should Remake that Video. Please maka a Video what impact on Performance and mileage it has when saving mass on wheels. 0-100 times and consumption messuerments would be great!

[@ricardoferreira5597]

Hi there, Can you make a video explainig lightwheight flywheels? Are they worthy for a "non-race car", like my daily? Would the engine get better or lose power? Thank you, you have a great channel !

[@UkrainianVids]

heavier flywheels have the advantage to make your car go further distance when idle (example coasting to a red light). Also heavier flywheels add stability to pretty much all of your engine components. lightweight -> nice and fast sport car to impress everyone. heavier -> best lifespan of vehicle and fuel economy

[@ricardoferreira5597]

MTShell thank you. I'm rebuilding an engine, and I shake him up before it goes to the car. And as a "Lightweight" I mean give it a touch just to rev faster. Just a sprinkle. But i dont know yet. Thank you anyway

[@edlingja1]

If you drive a 4-cylinder, just gently blip the throttle before accelerating properly (auto). You can just drive the car harder if you drive stick. But it's all about reaction times for cornering & passing, or for stock racing, etc. If you want to DD with a light flywheel (standard), try to find a popular aftermarket one that has some previous customers.

[@williambob245]

Just to get this all straight. I have a 01 single cab z71, I'm planning to swap an 8.1L 8100 vortec from a 03 crew cab long bed. From factory that truck makes around 340hp and 450lbs of tq. So I should be able to expect slight a slight gain beach of the shorter drivetrain?

[@EngineeringExplained]

That's what my flywheel video is for :)

[@sorenmotors]

You mentioned that if we release those cylinders the solid cylinder is gonna reach the bottom faster. But when they reach the bottom which one is gonna stop sooner??? And if we release them at the same time from top of a hill which of them are gonna convert more distance???

[@sorenmotors]

And which of those cylinders would have more velocity when they reach the bottom of the hill???

[@-_addy_-]

Very good and informative video!

[@XWMaster]

I was wondering about 2-piece brake rotors. I see that they are on all SuperCars and when I was researching I saw that they are the best kind.. Why is that? Because they would have a much larger rotational inertia?. Or does the weight lose make up for it?

[@EngineeringExplained]

Well honestly I don't know too much about their engines; I don't follow the cars that much since they're pricey (to say the least) and solely luxury based.

[@nuk3man]

I have a couple of questions (I might have missed it in your video), *why* is it that objects with lesser moment of inertia would travel down the hill faster than an object of comparable dimensions but with higher MOI? I understand that it represents the resistance to change, but from where does this force of resistance arise? Also, do the two wheels have the same *speed* but different velocity? Isn't it higher in the smaller diameter one as a result of less distance travelled?

[@pedrovera97]

Anyone know if this can translate to preferring a spacer to get proper fitment, vs getting a wider wheel or lower offset? I would assume since the weight is being added to the very center it would be better compared to the additional weight coming from the wider wheel?

[@EngineeringExplained]

That's a good idea!

[@silentthunder1000]

I read about sprung vs I sprung weight in relation to weight reduction if you could explain that it would be much appreciated. Great videos. Thanks.

[@EngineeringExplained]

Sprung weight is weight that your suspension supports (the chassis, and everything that rests on it). Unsprung weight is not supported by your suspension (wheels, tires, brakes, etc.)

[@silentthunder1000]

I have read claims that every lb of unsprung weight saved is worth 2 lbs of sprung weight, sometimes more. Why is there a difference. Or is this not correct. Thanks.

[@DENicholsAutoBravado]

I'm probably thinking about something else, but are you talking about how springs weigh more while under pressure? The energy literally becomes an infentesimal addiontal weight. That's right, if you car is jacked up in the air it weighs a bit less then when it's on the ground. Potential energy has mass. Like E=MC^2 without a nuclear reaction.

[@JasperSkallow]

how do I calculate tyre weight vs circumference? short and wide vs tall and narrow. weight vs distance etc.

[@EngineeringExplained]

Nope, it's correct. May not seem entirely intuitive. But, if you were to subtract than the moment of inertia would decrease as weight moved away.

[@wyzmaro9774]

What about rotating mass in the bottom end? Would a lighter weight Crankshaft be more resistant to getting the inertia going to the drive train? (0-60) Or would a heavier crank be more likely to keep the rotating mass going?

[@shaneb2447]

on my '06 350z I used to have, every time I would launch it, it would just kinda sit there and burn some rubber a bit before going. After installing a lightweight clutch and flywheel (from factory they already have composite driveshafts) every time I launched it afterwards, it would chirp and then grip and launch the car forward, and I was able to shave off a notable amount of time off my 0-60. However, it was slightly annoying hearing all the chattering afterwards coming from the transmission.

[@samrattomar7552]

Great Jason, Again a very beautiful and effective video content on moment of inertia. I would request you to prepare a video on selecting/designing the AVM (ANTI VIBRATION MOUNT) for engines. I would like to understand as how do we define as what is the amount of frequency which we need to damp w.r.t engine or how to define the percentage of isolation for the chassis from the vibrations of engine. I would request you to prepare a content on it Best Regards! Samrat Tomar

[@alexander2093]

Great video I'm currently lifting my truck and am choosing between 3 sets of tires..each have slightly different height and weight. Would you be kind enough to do the math for me to see which one has the least rotational mass and more suitable for fuel savings... My math is very weak, your help would be greatly appreciated Thank you, Alexander

[@Bluegogglesonadog]

*... maybe look into koenigsegg, they do some pretty innovative stuff. I could give you a link. But anyway that's just my point of view. Please keep on making videos, we always learn. cheers!

[@kevinjeeves1634]

What would be the 'hauled equivalent' of rotational mass? Ie power to spin one extra pound vs hauling that same pound?

[@gubercc411]

Try this experiment! Get someone to spin you in a chair while you hold your legs straight out. After they get you spinning, bring your legs in. Your rotational speed will increase! Then, start spinning with your legs tucked in under your chair. Once you're spinning, stretch your legs out. You will slow down! So in essence, the further out the mass is from the axis of rotation, the more force it takes to rotate the mass at the same velocity as something closer with equal mass. :)

[@neko_789]

Would you prefer a lightweight flywheel or a stock flywheel on a car that has a lot of rev hang like a subaru? Would the lightweight flywheel make the rev hang worse? Thanks.

[@chancemonagon9688]

Even though I’m going back to a video you made six years ago I am still wondering if this hurts the lifespan of the engine and why?