There seem to be a lot of comments regarding the simplification of the model, so I'll address those here: -Yes, the cutout is big. If a big cutout makes a small difference that is less then a diffuser, what difference would you expect a small cutout to make? Also, if the drag is reduced by a big cutout, the trend will be that it will still be reduced by a small cutout, just less. Same goes for the downforce trend. -Yes, the floor is smooth. I would be very surprised if you saw different trends with a more realistic floor. -Yes, many off the shelf diffusers are terrible, but if you're the type of person who's going to remove their bumper for added performance, you're probably the type prepared to make a crude diffuser out of sheet metal that basically copies my design here. Try to remember I make these videos in my spare time guys, I simply don't have time to make hyper-realistic floor models for fun. I do this stuff for a living, and if you are really interested in specific models then you can always go to www.jkfaero.com and pay me for more specific modelling, but I still need to put food on my plate and fuel in my racecar, just like everyone else here. I would have thought that this video would've helped those making the decision on whether they want to remove their bumper for performance reasons or not. Thanks for watching! Kyle
Thank you Kyle and sorry if I came across too strongly. I think my main point was simply that the cut-away scenario is probably overestimating the effect and the difference to the diffusor is even larger. Seeing that turbulence (due to underbody roughness) affects flow-detachment I would assume some impact on the numbers, but probably more for the diffusor, as that kind of seems more reliant on a laminar-ish flow?
wait, so your telling us that jkf Aero is you? your saying that you, in fact, can get us a better calculation of aerodynamics by request? so, if we are fair, and trade our time at work (some of our pay) for your time for work, then we can have a actual aerospace engineer automotive specialist attack calculations and tests from different angles (styles and ways ect.) to form a hypothesis and then a theory and then prove a design that will definitely be top of the line in efficiency and performance all while having evidence to back it up when someone wants to talk smack about our cars? Krickey, it feels good to be a nerd!
You're explaining technical details to a diverse, general audience very well Ky. There are many subtle points which I liked on this one, such as the large pressure gradient generated by both the removed bumper as well the diffuser, but that only the diffuser is able to capture this pressure peak. This effectively allows for the generation of downforce on the surface of the diffuser. Keep it up mate.
Hi Kyle, Firstly thank you for making this video as I've found the subject of cut bumpers and diffusers comes up during many track side garage conversations. I know you said that your model is simplified and I appreciate that you cannot capture everything in a simulation however I noticed that it has a completely smooth flat floor. You can see by the flow lines that this is resulting in laminar flow ahead of the bumper/diffuser region. Typically people looking at cutting their bumper are not looking to do so on vehicles with a flat floor. Would you be able to run this simulation again with underfloor details that more accurately represent a real car? I would be really interested to know if the same lift and drag trend is found with a turbulent underbody air flow.
What if you do both? Also the reason most of us cut bumpers isn’t downforce. But the parachute effect when we remove the factory exhaust systems. Most factory systems have a huge unit to fill the under car space to keep flow reasonable smooth past it self. When we go to aftermarket exhausts usually just tubing there is lots of open space in front of the bumper and the bumper just catches wind coming from below the chassis. So we cut the bumper to get rid of parachuting/drag not downforce. I had a 2’x4’ gap that just caught air when I removed my stock exhaust under my Z.
This seems common with tracked z's, though I'm running a Z1 single exit with a bumper tuck. I'm not sure if that improves the parachuting effect though
Hey Kyle, what if we cut the rear bumper in a way that retains the side edges so as to mimic a diffuser with two blades - one on the leftmost edge of the car and one on the rightmost edge - in other words, just cutting the bottom section of the rear face off of the rear bumper. Would that be significantly better than a fully-chopped rear bumper like the one you have done in this simulation?
Every west coast Miata just lost it`s rear bumper lol Great video and I am only half kidding. Would you be interested in doing a more realistic test if a model with an under body and applicable ride-height would be provided to you? I certainly could do that.
APR Performance makes a beautiful carbon fiber diffuser for the 2005 - 2009 Mustang. This panel extends almost to the axle underneath the vehicle. It also has three large screened vents in the upper rear surface to allow the air that goes over the top of the diffuser a place to exit. The strakes are very large, so much so, that when I had to have the car put on a flat bed truck due to the trans going out, I had to remove the diffuser so it would not get damaged when transitioning from flat ground to the angled bed of the truck. I don't know if it makes a huge difference or not, but APR does say it is wind tunnel designed. On the upper surface, there are four large slots where the strakes are located. When I removed it for towing, they were filled with debris, leafs, pine needles, dirt and mud. So obviously there is air flow there. I chose to fill these in while the diffuser was off. Cut foam to fill the channels, then smoothed them over with black RTV, and made them very smooth using 3M seal tools. At least, if I have to remove it again, the channels will not be billed with muck. Maybe the filled channels make the diffuser work a slight bit more efficiently? shop.aprperformance.com/index.php?route=product/product&path=72_128_80_163&product_id=71
Good job, nice and interesting video... I seen in some cars something like “fake air escapes” in the rear bumper, and always wondered if opening those areas and creating some sort of air flows in that area could make some kind of difference.
The modeling for the stock unmodified condition seems a bit in error. The rear bumper does not fill in the area between the floor pan and the lower edge of the rear bumper. You show that as a solid without any void. The rear bumper cover is simply a vertical wall blocking the view from behind the car, and that large open area is open to the ground beneath the vehicle. I would be interested to see the modeling if you compared the removed rear bumper model to that same model with a vertical wall at the rear edge where that bumper cover is, and the open cavity catching air and causing turbulence.
^This, his model is showing a car that is flat on bottom, and very few cars come out of the factory like that. Most bumpers/rear clips whatever you want to call it are just a body panel covering the actual crash beam. And with the shape of them, they are terrible for aero as it's essentially a parachute trapping air. Which is one of the reasons for getting a diffuser, to fill in the gap between the bumper and the body so that air doesn't get trapped. Hold an empty cup out the window going down the road with the open end forward...that's basically your rear bumper.
You mentioned creating a vortex at the end of the diffuser. Would there be any benefit to adding vg's between the strakes on the flat of the diffuser at the rear of the diffuser?
Great video! Would love to see more of these CFD videos. What do you think about cars with a diffuser/cut bumper but non flat under tray like the Miata has in real life? I would guess both will still be somewhat effective at reducing rear end lift created by most stock rear bumpers on cars without under trays, but not quite as effective overall as one with a flat floor? Thanks!
What about all the "diffusers" on common street cars these days, that are basically just fins they put on the back of bumpers? (New focus RS one of many examples). When I first saw them I thought they were a ridiculous fake non functional fashion element. However, they seem to be getting more and more popular. Do they function at all?
Yes i would think so any diffuser is better than none, does it create as big of flow or vortexes as a proper racing built one on any racing car but it would do something like this test not as much as an all out diffuser but some down force would be generated.
An observation I made a while ago was that cars with diffusers but no full floor pan left quite a bit of space between said diffuser and bumper. This makes sense to me as any air going over the diffuser leading edge needs a place to exit otherwise the whole thing becomes a race inspired parachute....but does it actually work that way?
KYLE.ENGINEERS, can you use a diffuser purely as a drag reduction device, like for a drag racing car with plenty of mechanical grip, and tune most of the downforce out of it, or would it be more practical to just use a cut bumper since it already doesn't produce much downforce?
I don't fully understand why a diffuser 'sucks' more air in under the car. It was my understanding that air cannot support tension, and that the velocity under the car was to do with the total pressure of the free-stream. Why then, would a downstream diffuser increase the mass flow under the car?
I feel the "notch" in front of the rear bumper is typically larger than in your base config, but also there is the fuel tank, the rear suspension, the exhaust filling that "notch", so you'd never have the room for such a large and 90° cut-off. Which on the one hand would cause less effect but also less turbulence/separation...I am not qualified to say what effect it has, but the geometry you present seems not wholly relevant? Okay, if we compare high-end cars with re-routed exhaust and fuel cell, then sure, you have a lot of room to play with. But those high-end cars would always run a diffusor of sorts anyway, not a cut bumper. For anything less extreme, look at this Miata underbody: forum.miata.net/vb/showthread.php?t=587365 The final muffler/silencer would probably not be used, so that's probably about the potential for the cut-out. Also, what effect has the dirty air generated by the front of most cars for the flow across the rear?
That’s the whole point. The lower speed you need it to work, the more aggressive it needs to be. It would create enormous drag at high speeds though. Since FS has a pretty limited top speed and they’re not wanting for power but traction, it’s fine to run very aggressive aero.
Its sept 2018 .Really need to talk about street speeds .When at what speed does an aero flat floor and diffuser actually start to create down force. We all should know by now covering the bottom of the car as much as practical to rid the bottom of large air pockets like under engine compartment from air dam to firewall. And an ajoining section Covering the trans tunnel ( on rear drive cars and fwd ).And the third section from end of trans tunnel to rear bumper to cover diff fuel tank to bumper Which should, by nature, have a natural up sweep too gain some diffuser-ish angle. Especialy if the front is racked lower than the rear. This would be a simple task for most enthusiast with little money expended. However if the speeds of its effectiveness , dont come on till 100 miles an hour. It would not be a street performance mod for 87% of builders.
This is an interesting topic and your explanation make perfect sense. However i remember somebody has removed rear bumper on an S2000 and the lap time on the track wasn't improved...
In this video, ruclips.net/video/9jNi_rCSjmg/видео.html at minute 3:40 you can see a way to avoid the muffler at the back of the rear axle, by placing it forward and reversing the flow. In this case, the path followed is complicated, but I guess is due to Category Regulations about the exhaust system length. But that´s the basic idea on how to avoid mixing the diffuser with the muffler...
the car with the cut bumper in your program is a terrible example of a cut bumper, I'm not saying what your saying is not true, but I'd like to see more realistic model.
Yes it does, read the caption @ 2:34 ... The models don't show an exact model, but you should get a general idea of what he's talking about. Actually, the inaccuracy of the cut bumper model here doesn't show the actual bad effects it has on the back of the vehicle. Bumpers were put in there for a reason, and one of those reasons is for smoother airflow (think older super car designs before rear diffusers were a norm) and cutting it disrupts this purpose unless you put a proper rear diffuser under there along with a flat floor.
There seem to be a lot of comments regarding the simplification of the model, so I'll address those here:
-Yes, the cutout is big. If a big cutout makes a small difference that is less then a diffuser, what difference would you expect a small cutout to make? Also, if the drag is reduced by a big cutout, the trend will be that it will still be reduced by a small cutout, just less. Same goes for the downforce trend.
-Yes, the floor is smooth. I would be very surprised if you saw different trends with a more realistic floor.
-Yes, many off the shelf diffusers are terrible, but if you're the type of person who's going to remove their bumper for added performance, you're probably the type prepared to make a crude diffuser out of sheet metal that basically copies my design here.
Try to remember I make these videos in my spare time guys, I simply don't have time to make hyper-realistic floor models for fun. I do this stuff for a living, and if you are really interested in specific models then you can always go to www.jkfaero.com and pay me for more specific modelling, but I still need to put food on my plate and fuel in my racecar, just like everyone else here. I would have thought that this video would've helped those making the decision on whether they want to remove their bumper for performance reasons or not.
Thanks for watching!
Kyle
Thanks already covered the answer to my question.
Thank you Kyle and sorry if I came across too strongly.
I think my main point was simply that the cut-away scenario is probably overestimating the effect and the difference to the diffusor is even larger.
Seeing that turbulence (due to underbody roughness) affects flow-detachment I would assume some impact on the numbers, but probably more for the diffusor, as that kind of seems more reliant on a laminar-ish flow?
wait, so your telling us that jkf Aero is you? your saying that you, in fact, can get us a better calculation of aerodynamics by request? so, if we are fair, and trade our time at work (some of our pay) for your time for work, then we can have a actual aerospace engineer automotive specialist attack calculations and tests from different angles (styles and ways ect.) to form a hypothesis and then a theory and then prove a design that will definitely be top of the line in efficiency and performance all while having evidence to back it up when someone wants to talk smack about our cars? Krickey, it feels good to be a nerd!
Don't worry, those who actually care see the video is beneficial. Those who are complaining are the spectators.
You're explaining technical details to a diverse, general audience very well Ky.
There are many subtle points which I liked on this one, such as the large pressure gradient generated
by both the removed bumper as well the diffuser, but that only the diffuser is able to capture this pressure
peak. This effectively allows for the generation of downforce on the surface of the diffuser.
Keep it up mate.
just sharing little bit,
there are some cars which the bumper is like small parachute. :)
Like the audi tt
Hi Kyle,
Firstly thank you for making this video as I've found the subject of cut bumpers and diffusers comes up during many track side garage conversations.
I know you said that your model is simplified and I appreciate that you cannot capture everything in a simulation however I noticed that it has a completely smooth flat floor.
You can see by the flow lines that this is resulting in laminar flow ahead of the bumper/diffuser region.
Typically people looking at cutting their bumper are not looking to do so on vehicles with a flat floor.
Would you be able to run this simulation again with underfloor details that more accurately represent a real car?
I would be really interested to know if the same lift and drag trend is found with a turbulent underbody air flow.
What if you do both?
Also the reason most of us cut bumpers isn’t downforce. But the parachute effect when we remove the factory exhaust systems. Most factory systems have a huge unit to fill the under car space to keep flow reasonable smooth past it self.
When we go to aftermarket exhausts usually just tubing there is lots of open space in front of the bumper and the bumper just catches wind coming from below the chassis. So we cut the bumper to get rid of parachuting/drag not downforce. I had a 2’x4’ gap that just caught air when I removed my stock exhaust under my Z.
This seems common with tracked z's, though I'm running a Z1 single exit with a bumper tuck. I'm not sure if that improves the parachuting effect though
Hey this is like really late but do you know if it would do anything on a 2021 Elantra? I just love how it looks
Hey Kyle, what if we cut the rear bumper in a way that retains the side edges so as to mimic a diffuser with two blades - one on the leftmost edge of the car and one on the rightmost edge - in other words, just cutting the bottom section of the rear face off of the rear bumper. Would that be significantly better than a fully-chopped rear bumper like the one you have done in this simulation?
Every west coast Miata just lost it`s rear bumper lol
Great video and I am only half kidding. Would you be interested in doing a more realistic test if a model with an under body and applicable ride-height would be provided to you? I certainly could do that.
Hey I have a 12" cross section wind tunnel. I wouldn't mind doing some testing for you.
APR Performance makes a beautiful carbon fiber diffuser for the 2005 - 2009 Mustang. This panel extends almost to the axle underneath the vehicle. It also has three large screened vents in the upper rear surface to allow the air that goes over the top of the diffuser a place to exit. The strakes are very large, so much so, that when I had to have the car put on a flat bed truck due to the trans going out, I had to remove the diffuser so it would not get damaged when transitioning from flat ground to the angled bed of the truck. I don't know if it makes a huge difference or not, but APR does say it is wind tunnel designed. On the upper surface, there are four large slots where the strakes are located. When I removed it for towing, they were filled with debris, leafs, pine needles, dirt and mud. So obviously there is air flow there. I chose to fill these in while the diffuser was off. Cut foam to fill the channels, then smoothed them over with black RTV, and made them very smooth using 3M seal tools. At least, if I have to remove it again, the channels will not be billed with muck. Maybe the filled channels make the diffuser work a slight bit more efficiently?
shop.aprperformance.com/index.php?route=product/product&path=72_128_80_163&product_id=71
Good job, nice and interesting video... I seen in some cars something like “fake air escapes” in the rear bumper, and always wondered if opening those areas and creating some sort of air flows in that area could make some kind of difference.
This video is excellent, exactly what I was looking for, so glad somebody cared enough to answer this question
Thanks a lot Kyle. I like the simple, flowing way you explain things.
Always very inyeresting to watch.
to be fair most off the shelf diffuser designs are crap.
do a vid on front splitters
And side skirt effect pls
The modeling for the stock unmodified condition seems a bit in error.
The rear bumper does not fill in the area between the floor pan and the lower edge of the rear bumper. You show that as a solid without any void.
The rear bumper cover is simply a vertical wall blocking the view from behind the car, and that large open area is open to the ground beneath the vehicle.
I would be interested to see the modeling if you compared the removed rear bumper model to that same model with a vertical wall at the rear edge where that bumper cover is, and the open cavity catching air and causing turbulence.
^This, his model is showing a car that is flat on bottom, and very few cars come out of the factory like that. Most bumpers/rear clips whatever you want to call it are just a body panel covering the actual crash beam. And with the shape of them, they are terrible for aero as it's essentially a parachute trapping air. Which is one of the reasons for getting a diffuser, to fill in the gap between the bumper and the body so that air doesn't get trapped. Hold an empty cup out the window going down the road with the open end forward...that's basically your rear bumper.
You mentioned creating a vortex at the end of the diffuser. Would there be any benefit to adding vg's between the strakes on the flat of the diffuser at the rear of the diffuser?
Great video! Would love to see more of these CFD videos.
What do you think about cars with a diffuser/cut bumper but non flat under tray like the Miata has in real life? I would guess both will still be somewhat effective at reducing rear end lift created by most stock rear bumpers on cars without under trays, but not quite as effective overall as one with a flat floor?
Thanks!
What about all the "diffusers" on common street cars these days, that are basically just fins they put on the back of bumpers? (New focus RS one of many examples). When I first saw them I thought they were a ridiculous fake non functional fashion element. However, they seem to be getting more and more popular. Do they function at all?
Yes i would think so any diffuser is better than none, does it create as big of flow or vortexes as a proper racing built one on any racing car but it would do something like this test not as much as an all out diffuser but some down force would be generated.
Maybe they veeeeeeery slighty decrease fuel consumption . But mostly probably just for looks.
All show. Insignificant difference.
Holy crap what programs do you use
And how much did you get them?
Also PLEASE do a video on suspension geometry
can you please do some of these videos on s2000s
So sad that this guy ditched his youtube channel. I really love his explainations
Great work as always kyle. I love this new series of cfd analysis you have
could you test drilled hole bumper!?!?!?
Hey Kyle,
I race remote controlled cars.
could you explain damping?
oil viscosity Vs piston hole size.
pack etc.
That was awesome, thanks for taking the time to do that :)
What about a cut bumper and diffuser?
MeshLab + ParaView FTW. Nice video!
Does assuming a flat bottom floor/not having the turbulent flow from all the surface irregularities on the floor of the real car effect this much?
An observation I made a while ago was that cars with diffusers but no full floor pan left quite a bit of space between said diffuser and bumper. This makes sense to me as any air going over the diffuser leading edge needs a place to exit otherwise the whole thing becomes a race inspired parachute....but does it actually work that way?
What if you cut the bumper and left a diffuser attached to chassis, is there a greater advantage?
Hi Kyle!
Can you draw a bumper for Nissan e-nv200?
KYLE.ENGINEERS, can you use a diffuser purely as a drag reduction device, like for a drag racing car with plenty of mechanical grip, and tune most of the downforce out of it, or would it be more practical to just use a cut bumper since it already doesn't produce much downforce?
If the car has a hump for a spare tire down there would it still benefit from cutting the bumper?
Does the addition of a diffuser always correspond to a reduction in drag when compared to no diffuser, or does it vary case by case?
Can you do an analysis of the Ariel Aero-P Atom? Thanks.
What happens if you have no rear bumper and a diffuser like some of the rocket bunny kits? Would that be better or worse than having one or the other?
Very interesting. I’ve seen ridiculously large rear diffusers on 1st Gen Scion xBs and now I know why. 🤣
How about taking off the rear bumper and putting on a diffuser? I saw a guy do that...
question. are these kg's less drag and so on already multiplied by two (whole car) or does it count for half a car (your model)
I don't fully understand why a diffuser 'sucks' more air in under the car. It was my understanding that air cannot support tension, and that the velocity under the car was to do with the total pressure of the free-stream. Why then, would a downstream diffuser increase the mass flow under the car?
I like the new hair style 😂 great vids I really enjoy watching them!!
What software would you recomend for calculating and simulating drag and downforce from models drawn in CAD?
Ansys I suppose
I feel the "notch" in front of the rear bumper is typically larger than in your base config, but also there is the fuel tank, the rear suspension, the exhaust filling that "notch", so you'd never have the room for such a large and 90° cut-off. Which on the one hand would cause less effect but also less turbulence/separation...I am not qualified to say what effect it has, but the geometry you present seems not wholly relevant?
Okay, if we compare high-end cars with re-routed exhaust and fuel cell, then sure, you have a lot of room to play with. But those high-end cars would always run a diffusor of sorts anyway, not a cut bumper.
For anything less extreme, look at this Miata underbody:
forum.miata.net/vb/showthread.php?t=587365
The final muffler/silencer would probably not be used, so that's probably about the potential for the cut-out.
Also, what effect has the dirty air generated by the front of most cars for the flow across the rear?
I don't really get the point of gigantic Aero packages on FSAE
cars. Do you think they are effective? if so how at low speeds?
That’s the whole point. The lower speed you need it to work, the more aggressive it needs to be. It would create enormous drag at high speeds though. Since FS has a pretty limited top speed and they’re not wanting for power but traction, it’s fine to run very aggressive aero.
does that apply on a car has no flat floor ??
Its sept 2018 .Really need to talk about street speeds .When at what speed does an aero flat floor and diffuser actually start to create down force.
We all should know by now covering the bottom of the car as much as practical to rid the bottom of large air pockets like under engine compartment from air dam to firewall. And an ajoining section
Covering the trans tunnel ( on rear drive cars and fwd ).And the third section from end of trans tunnel to rear bumper to cover diff fuel tank to bumper Which should, by nature, have a natural up sweep too gain some diffuser-ish angle. Especialy if the front is racked lower than the rear.
This would be a simple task for most enthusiast with little money expended. However if the speeds of its effectiveness , dont come on till 100 miles an hour.
It would not be a street performance mod for 87% of builders.
I’ve heard aero starts giving noticeable effect at 80km/h or 50mph already. I’m no engineer ofcourse.
The Best Bait ever Made i have Clicked
What if you do a half cut bumper with a diffuser?
You unlock all the downforces 😁
Which software have u used for CFD?
Kyle thanks for your videos. Well appreciated!
How much different a flat floor would make?
Alot 0ExG0
Thank you for your work!
This is an interesting topic and your explanation make perfect sense.
However i remember somebody has removed rear bumper on an S2000 and the lap time on the track wasn't improved...
WHY NOT BOTH?
Who leaves the stickers on their monitor?
Can you tell us what program you are using?
He uses OpenFOAM for the CFD calculations, and ParaView for the visualisations...
what software is that
Flawed test as the main issue is older cars where the rear bumper is a parachute.
Thanks for posting! I get asked this a lot. Subscribing
In this video, ruclips.net/video/9jNi_rCSjmg/видео.html at minute 3:40 you can see a way to avoid the muffler at the back of the rear axle, by placing it forward and reversing the flow. In this case, the path followed is complicated, but I guess is due to Category Regulations about the exhaust system length. But that´s the basic idea on how to avoid mixing the diffuser with the muffler...
A cut bumper and a rear diffuser do two different things.
6kg of downforce doesnt mean anything when the bumper you removed weighs more
1:26 your left eye o.0
This is a perfect example of engineers over simplifying on their calculations and resulting in an incorrect answer.
I own the spoiler on the is300 in the thumbnail :D
the car with the cut bumper in your program is a terrible example of a cut bumper, I'm not saying what your saying is not true, but I'd like to see more realistic model.
Please stfu
Fox Mense Ok, it's not that accurate...
But, it does give an idea.
Therrydicule a rear bumper can act as a literal parachute and trap air inside. The models came nowhere close to representing that
Yes it does, read the caption @ 2:34 ... The models don't show an exact model, but you should get a general idea of what he's talking about. Actually, the inaccuracy of the cut bumper model here doesn't show the actual bad effects it has on the back of the vehicle.
Bumpers were put in there for a reason, and one of those reasons is for smoother airflow (think older super car designs before rear diffusers were a norm) and cutting it disrupts this purpose unless you put a proper rear diffuser under there along with a flat floor.
merkantelismo actually, no. bumpers are put on for the exact reason they are called bumpers
Innacurate, most miatas don't have a a flat bottom
People do this? Seriously??? Might as well add stickers for extra speed and power!
Brilliant
Should add a 2" hole cut out like (ALL) the Honda civic guys like to do in the USA.
i11.tinypic.com/67sqhiq.jpg
To cutting my 350Z rear bumper it is 🫡