Suspension Height & Fuel Economy?

[mpgomatic]

Digging for Data ...

Does lowering a Jetta or Golf have an effect on highway fuel economy?

I'm working on a little project car proposal for this year's SEMA show. Simple build. Big crazy idea.

TIA!

 

[flatlanded]

I would say yes, a little in the aero department. Less air under the car the better, and it would punch a slightly smaller hole in the air too.

Low with a smooth bottom and some sort of rear/front diffusers is the way to go!

 

[mpgomatic]

Thanks, Flatlanded ... that's what I'm thinking ...

 

[shizzler]

I would say yes, a little in the aero department. Less air under the car the better, and it would punch a slightly smaller hole in the air too.

Low with a smooth bottom and some sort of rear/front diffusers is the way to go!

I concur. Completely.

What exactly is your "big crazy idea", mpgomatic?

 

[mpgomatic]

Cool. It's a three-stage thing, Shizzler. I can give you 1 & 2 at the moment ... #3 is still a bit of the unknown.

1. Simple bolt-on mods: focused on lowering rolling resistance, increasing aerodynamic efficiency & reducing weight.

2. Mechanical tweaks: optimized top gear ratio, perhaps some mild performance mods. Nothing wild.

Not looking to reinvent the wheel or perform magic ... just want to put together the pieces that have been proven to work and document the process with video segments.

 

[shizzler]

Right on. Aside from the stage 3 mystery, that's exactly what I've tried to do with my daily driver.

Weight is a tough one though. I pulled ~70 lbs out (but that's including my aftermarket subwoofer & amp) without any real noticeable effect, but then I hit a wall. Battery downsizing, rear middle seat belt, spare tire and possibly A/C removal were next on the list.

 

[grizzlydiesel]

mpgomatic - will be waiting with great interest, expecting a build thread when its done.

 

[mpgomatic]

Thanks, Shizzler. We're looking at lightweight wheels, possibly some carbon fiber body pieces. Gotta find a mule first ...

It's all about documenting the build, Grizzly. If we can pull this off, there will be lots of video.

 

[morpheus]

mpgomatic, Something to think about....

Weight is divided into sprung and unsprung.

Simply put, if you replace one fender with a carbon fiber duplicate, and save 2 lbs., you're good to go. BUT........

If you change one steel rim for an alloy one, and save 2 lbs. of weight, the actual vehicle weight savings is tripled to 6 lbs. Not to mention any resulting suspension performance gain.

Aero gains at legal(?) highway speeds, are a bit harder to perform. You don't state what body style you'll be using, so that makes suggestions harder.

First thing on the agenda will be frontal area. Lowering car reduces frontal area, so will narrower tires. Moon discs on all rims work real good in smothing turbulance around wheels. Another area to investigate is turbulance inside the wheel wells, as the spinning tires create a lot of that. Try to maintain a level attitude when lowering, as a nose down(raked) vehicle looks neat, but defeats your purpose. To prove this, ride down the road at 60mph, with your hand out the window, facing the front. Keep it level and note the amount of resistance that position is developing. Slowly lower the front(leading edge) of your hand and note what happens. Surprise, very little drop goes a long way !

Look at the vehicle from the front, including underneath and on the top. If it sticks out.... take it off ! Roof rack-mirrors-antennas-front tag frame(cause in Joisey you have them)... plug/fill in driving/fog light openings, etc.

This should keep you busy over this hot weekend, enjoy!

I hope this helps, as i'm new to this forum, and this is my first post.

Which swamp in Jersey are you referring to, as we have many :>) i'm down south, across from Philly.

Alan

 

[mpgomatic]

Yep, Morpheus! Want to go with the lightest weight alloys ... Enkei might get the nod. Been thinking about Moon discs, too ... old school hot rod stuff works for me.

Weight's the issue around town, not nearly as much out on the open road ... and that's where we're focusing.

Haven't decided on which car to use. A lot depends on what we can beg or borrow. A Golf or Jetta would be sweet. A 335d would be insane.

Did I mention we have no budget? bwahahha

Great advice on the aero stuff. I spent a bit of time hanging out with the Mustang engineers a few weeks back at Bristol ... lots of little tweaks add up. If you look the nose and underneath the '11 Mustang there are some very subtle touches.

http://www.youtube.com/watch?v=d-y25rX87vo

Funny thing ... I was doing the hand aero test just yesterday ... that's fascinated me since I was a wee one. We were on the way back home to central Jersey from the Shore, my kid was driving through the Pines (first time), and I was riding shotgun.

This heat is nuts. We have a car to shoot this weekend, but I'm staying inside today, cutting video.

 

[kwong7]

As far as ride height goes, I don't think it matters too much. My front is raised about .5" from stock while the rear is lowered about .5" from stock (H&R Dunebuggy coilovers). The bigger factors with drag would include, the underside, the windshield, the side mirrors, front grill, and air diffuser.

When my lower engine cover was drooping down, my aerodynamics were horrible and my fuel economy took an 8% dive. The car felt less stable and lot less powerful at speeds >40MPH. After correcting the skid plate, my car feels much better and my fuel economy has returned to normal.

 

[dremd]

go hang out at ecomodder especially in the aero section.

If going all out, make it look like the aerocivic (silhouette) and you will be great.

 

[mpgomatic]

Aye, there are lots of little things that can catch the wind the wrong way. We'll probably swap in a metal skid plate when we drop it, for the sake of the oil pan.

As far as ride height goes, I don't think it matters too much. My front is raised about .5" from stock while the rear is lowered about .5" from stock (H&R Dunebuggy coilovers). The bigger factors with drag would include, the underside, the windshield, the side mirrors, front grill, and air diffuser.

When my lower engine cover was drooping down, my aerodynamics were horrible and my fuel economy took an 8% dive. The car felt less stable and lot less powerful at speeds >40MPH. After correcting the skid plate, my car feels much better and my fuel economy has returned to normal.

 

[SRPumpelly]

Typically if a race car wants top speed like at places like Fontana in IRL they will raise the car and lower it when they want more downforce in places like Iowa or Richmond.

The biggest difference with those cars is the underwing and the minimum rear wing angle of which the Jetta has neither so there are other considerations but in general forcing the air into a smaller gap under the car will cause drag and the extra pressure below will send more air over the top than typical. Both wil have the affect of adding drag.

Watch IRL cars on the straights at Indy. They move to the center of the track to keep the air that has to go around the side of the car from getting squeezed for just that reason, not because they want to make the straights longer.

Raise it up!

 

[dremd]

Raising the car WILL increase drag.
Increased frontal area alone is plenty in a road car to make this a certainty.

On an IRL car that has a CD > 1 that is an entirely different story. Taking the wing farther away from the ground and getting it away from ground effect "could" lower CD at higher speeds.

Do NOT ever compare open wheel high downforce race cars to street cars.

 

[shizzler]

Yeah sorry SRPumper, that's wrong. You said the key piece of info right in your own post though, "forcing the air into a smaller gap". This implies that you are directing the same amount of air under the car no matter what the ride height is. Except that in reality you have LESS air going under the car if your ride height it lower. Furthermore lowering should be done in conjunction with a front air dam / splitter to allow even less air underneath anyway.

Less air under the car and lower frontal area = good.

 

[SRPumpelly]

I asked a buddy that knows and he said:

The answer is..........depends! But I would say 90% of the time, lower is better MPG. Reason is simply frontal area. When you raise the car it looks like a bigger car to the air. Air does not flow under the car very well and tends to somewhat pack up due to the uneven surface and the boundary layer. So as you raise the car the object the air has to go around becomes larger, just as if you had made the car bigger. Now for the exceptions:
1) You raise the car really high (like a foot) so the air can now more easily flow under it and not pack up under it.
2) The shape of the car is such that raising it relives a unique pressure blockage specific to that shape.
3) Raising the car helps the underside draw air thru it better reducing the blockage effect and promoting better flow....think diffuser

Most road cars do not have any of these 3 possibilities.

So I guess you guys are right. I was thinking possibility #1 was achievable at much lower heights that he suggests.

 

[IndigoBlueWagon]

Kosei makes some light and inexpensive rims, I have K1s on my wagon although they're no longer available in 15" size for A4s. Motegi Traklites are very light, I have a 16" set that weigh 12 lbs. each. But they're pricey.

The people who have gotten the best fuel economy in TDIs (Lug_Nut and BRUSSELS BELGIUM come to mind) have optimized their setups so they run at the torque peak, between 1700 and 2000 RPM. Some have used taller tires to get more MPH at that RPM, others just drive slowly.

If you use a .658 5th gear (available for the TDI transmission) you'll get about 60 MPH at 1800 RPM.

 

[mpgomatic]

Great stuff, IndigoBlueWagon! The goal is to do an apples-to-apples MPG comparison. We test a new car for highway mileage each week over the same course at identical speeds (68 MPH). We'll run this in a similar manner, checking the fuel-efficiency differences with each major change (springs/LRR tires/.658 gearing).

First order of business is finding a worthy 2000-2004 test mule. Working on lining up sponsors.

I was thinking Enkeis ... the Kosei and Motegi wheels look sweet.

Kosei makes some light and inexpensive rims, I have K1s on my wagon although they're no longer available in 15" size for A4s. Motegi Traklites are very light, I have a 16" set that weigh 12 lbs. each. But they're pricey.

The people who have gotten the best fuel economy in TDIs (Lug_Nut and BRUSSELS BELGIUM come to mind) have optimized their setups so they run at the torque peak, between 1700 and 2000 RPM. Some have used taller tires to get more MPH at that RPM, others just drive slowly.

If you use a .658 5th gear (available for the TDI transmission) you'll get about 60 MPH at 1800 RPM.

 

[dremd]

Moon Disc + lightest wheels possible (maybe steel??) also don't forget LRR tires.

 

[mpgomatic]

Definitely LRR tires. A Moon Disc experiment is most certainly in order. 8)

Moon Disc + lightest wheels possible (maybe steel??) also don't forget LRR tires.

 

[coalminer16]

Your best mod and cheapest is your own foot and taking out things you don't need to carry in the car. I run without the rear seats when possible. I need the height so I added 2 inches (don't own a truck and live in any area with few snow plows). Light rims with no holes in the rims. Some people just tape the rims to smooth them out. Covering the wheel wells for better air flow helps. Blocking of some of the grill as the engine is dirty aerodynamics. Making the underside of the car smooth. There is a way to have fold in mirrors. Some take the pass mirror off but you do need the drivers to be legal. Lowering does help but not much. But none of them make much difference. It is a little here and there that add up.

 

[shizzler]

Well said, coalminer.

 

[mpgomatic]

Aye, Coalminer. I'm out on the road week-after-week, testing new cars with a light foot.

I need to find some photos of what's working for folks in the grille and underneath the car.

Aftermarket side-mirror cameras would be wicked cool.

Your best mod and cheapest is your own foot and taking out things you don't need to carry in the car. I run without the rear seats when possible. I need the height so I added 2 inches (don't own a truck and live in any area with few snow plows). Light rims with no holes in the rims. Some people just tape the rims to smooth them out. Covering the wheel wells for better air flow helps. Blocking of some of the grill as the engine is dirty aerodynamics. Making the underside of the car smooth. There is a way to have fold in mirrors. Some take the pass mirror off but you do need the drivers to be legal. Lowering does help but not much. But none of them make much difference. It is a little here and there that add up.

 

[coalminer16]

Some good aluminum that is worked around your rear axle and exhaust. You have to make sure your axle can have full range of movement and you have to be careful the exhaust doesn't get boxed in enough to overheat the surrounding things and cause damage. Even adding a few panels would help underneath. The rear near the trunk is the worse part of the open cavities. If you work it out right you could glide through snow better too. I haven't see many attempt the paneling underneath as it is a lot of work and fabricating and it has to hold on good enough to no rip off and damage things ripping out. The grill is easy. Cheapest way would be just clear tape. Would be a good way to find out how much you could block off and then custom fab a blocked off grill from there. I tow enough I won't ever do that but if you never tow you could see some result. Till I had a kid I had the rear seats out as well as the larger rear seat weights a lot as well. Every pound cut helps but don't go to extreme. Some people have taken the AC and power steering out saving both engine HP and weight. Heck if you really want to save more weight keep the small back seat in the back for a passenger and take the front pass seat out. That really would take some weight out. Lots of creative ideas but where do you stop and when does the payoff add up to pay for the mod you did? Also for anyone out there wanting carbon fiber I looked into that at once. You would never pay it off in fuel savings as the cost is a lot on top of the fact that most mass produced ones are almost as heavy as the tin work. They use to much epoxy making it heavy. They are many for looks unless you shell out more money for custom ones and then you have to save even more money on the mod not making it worth it.

 

[BridelessRacing]

Go crawl under mud race 4x4 trucks there is a real lesson to be learned to smothing the bottom. Mud is lots thicker than air. But the princaples are the same on a 60 mph mud truck.

 

[BridelessRacing]

http://www.aptera.com/ a teardrop is the most aerodynamic