Quantifying the aerodynamic impact of lowering a car? (drag coefficient Cd)
 

I'm aware that lowering a car can improve the fineness (Length to height) ration, and also drops the cD of the car..

Its also not normally an economically viable thing to do..

However...

The golf's suspension is shot- very little damping up front, and the bottom arms are clanking. It will need replaced sooner rather than later.

there are two options (both roughly similar in cost..)

1. Replace with standard parts (dampers/springs/arms)

2. Replace with a coilover kit to allow for a 40mm drop


Would the impact of lowering provide any tangible savings that I could use to justify the work?

or am I better just going with original/pattern parts?

My Jetta was lowered with a similar setup. It rides terrible but gives good mileage. I can not quantify the change as it was like this when I got the car.

In the post "fast n furious movie" world of automotive resale direct advantage to you and raises value. You will get better economy but it probbably wont be so amazing that you will be reimbursed the project cost ever.
It may make ride harsher as mentioned above andhas penalties with ground clearance and perhaps tire clearance.
But it looks cool and I say why not if its near same cost!
Especially if you are sure the parts are required no matter what.

Ironic that you used the word impact when talking about lowering a car.

Thanks-at least this suggest there wont be a disadvantage:)

Believe you me, if waterbeds had wheels and engines they'd handle and ride better!

as you say, its not really an "improvement", more a maintenance issue



:D good spot:thumbup:

Bumpstops add unnecessary weight:rolleyes:

I chuckled at that too. :)

Does the 96 Golf TD have the magnesium oil pan like the TDI? I put an aluminum skid plate on my Golf and resisted the urge to lower. I continued with a coroplast belly pan to the rear bumper instead. There are some downsides to lowering but IMO it can make a car look good. I think it is safe to assume that if you raise your suspension to mount giant tires you will lose mpg but I am not sure the reverse will gain you anything measurable.

lowering to reduce drag
 

Reduced height springs are used primarily to lower the vertical center of gravity in race cars to improve handling in circuit racers Not reduce undercarriage airflow.

Undercarriage air flow is reduced somewhat but that is better achieved by air dams and chin spoilers in the frontal area.
Drag reduction is better achieved by the use of undercarriage slipstream modifications by using bolt on flat panels of a suitable materials.These can be attached in such a way as to reduce he under carriage air mass ( road clearance ) by the use of stand offs. this requires no modification the suspension or ride.

The cleaner aerodynamics also contribute to lower overall drag for better performance and gas millage. If the panels contact the ground they are a cheaper sacrificial donor than your exhaust system or other expensive components.

Frosty 944

Since the cost is a wash (you'll spend the money either way, for a new stock suspension or lowered), I am strongly in favor of lowering. I've seen widely-varying estimates on Cd change when a vehicle is lowered, from -.01 per inch to -.08 claimed by Ford on the Fusion 999 hydrogen Bonneville car, which was lowered several inches (but not 8!)--but all sources agree, lowering a car has a beneficial effect on drag. The primary benefit, I think, is not in the (tiny) reduction in frontal area or airflow under the car, but the improvement in fineness ratio, as you mention. I put coilovers on my car a year and a half ago, dropped it just until there's no gap between the top of the tire and fender, and haven't looked back.

I honestly do not know re the sump- Think it may be steel.

The roads I use can be easily passed using the 328, and thats MUCH lower than I'd be going with the golf..the cheaper coilover kits prefer to have a bit of height to retain damping- 40-50mm drop max

i'm intending to fit a front airdam in addition to lowering the car- to get the majority of benefits of a bellypan, without the additional work, but you make valid points, especially if I was going to replace stock suspension that was in good order..

Thanks for the report oun your experience- I dont imagine I'll go as low as you suggest, more likely leave 1-2cm above the tyre for clearance reasons.

The addition of a front air dam should make up the "lost" drop!

You haven`t told us how much more the new springs would cost. Lowering does improve mpg but I doubt it would pay for itself. If you go for it, I strongly recommend using progressive or soft springs, so that you do not ruin the already not not so smooth ride quality. It can never ever pay for itself if it ruins your back, even if the signs will show in the distant future, it`s just not worth it to risk sacrificing your health.

lowering
 

Lowering,by itself,may not achieve the lowest Cd.Low drag concept cars as the Probe-IV not only lowered the nose,but also raised the tail for out-of-town driving,configuring the 'rake' of the car for overall lowest drag.This is something which a full-scale wind tunnel would be handy for ascertaining.
I'd be inclined to maintain a 'factory' suspension.

New springs= £80

New Shocks another £80 for fronts only.

New cheap coilovers £155 (front and rear)

Duly noted- this being the week before payday, I'm in no position to pay for the bits to alter anything!

interesting re the probe- maybe a front only drop would have similar outcomes?

Careful with the cheap coilovers...I have heard stories where they break and/or leak fairly quick..often made with inferior materials...especially cautious if made in China.

Why not some Eibach, H&R or other proven name brand that while lowering the chassis they actually are designed to improve the handling dynamics of the vehicle. Helps with maintaining momentum and keeps more control on windy highways, not to mention can add a little fun back into the driving experience. Also can work with factory shock absorbers with a minimal impact on life expectancy.

I would go with some eibach linear springs not progressive. They come inane different spring rates and are cheap, atleast state side. Just make sure u have the proper Id of the spring u want to fit onto the coil over and don't get cheap stuff like mensioned before

When I was a poor student I decided that my new 73 Toyota could use some lowering. I pulled the front coils and cut a full turn off the coils. I was interested in fuel economy as gas was sometimes over 50 cents but was mainly interested in handling.

Reducing a coil will increase the spring rate. I have done it many times on off road motorcycles. If you want to maintain ride height you have to put in a spacer.

If you have 10 turns on a front coil and it is 10" long then removing one turn will increase the spring rate by 10/9ths and reduce ride height by one inch.

Usually stock springs are way cheaper than aftermarket.

front only drop
 

Too bad for us that Ford never published a drag breakdown of the Probe as it transitioned from 'urban' to 'highway' configuration.
Dropping only the nose would,in essence,effectively raise the boot,taking some bite out of the rear contour.
I suspect that it would lower the drag.I just don't know how to predict a numerical value for it.
If you had access to a closed course,along with instant mpg readout capability,you could substitute rigid links where the shocks go,fixing the ride height at your new target,then drive it only as long as it took to get your new mpg reading.If you liked the number,then it might justify a proper modification.
If the numbers didn't pan out,you'd be out very little time and money.

I ran with cheapcheapcheap coilovers on my Rabbit for several years and never had a single problem. The ride was rough, though not unbearable. Handling was dramatically improved. Grease the sleeves if you drive in the winter.

You might be able to find some used H&R's or similar for much less money. Look on vwvortex, local VW forums or your local Craigslist type site.

Otherwise the popular cheap brands are Vmaxx, Raceland and Rokkor.

==============================

If anyone in the Michigan area has a Scangauge they'd be willing to lend for a weekend I would very much like to plug it into a car with air suspension (I could probably find someone on MIVE) to get real-time readouts on MPG as a function of ride height.

real-time
 

Man,that would be a great contribution.We could also look at mpg as a function of steady road velocity and begin constructing mpg vs speed tables.
Hope that falls together!:thumbup:

This article from Hot Rod magazine is enlightening:
Car Aerodynamics - Hot Rod Magazine

Using wind tunnel testing, they were able to reduce their already modified Camaro from .229 Cd to .201 and a similar, nearly stock Camaro from .497 Cd to .292.

I really can not appreciate "Hot Rod" as a source of first party aerodynamic theory.

:D

darcane, thanks for the article!

The top 5 do seem to roughly align with the info in the 65+ efficiency Mods thread here- I imagine the numbers for impact of Cd will be inflated, but eevery little helps.

re cheap coilovers/lowering kits etc.

Replacing the springs would mean only being able to do 1/2 the job- Eibachs would eat right into my damper budget.

spending £4-500 on suspension for a £400 car is not a plan (at the moment- we'll see how the savings pan out!)

I'd only buy TUV approved cheap Coilover kits- even if I get 2 years out them, that'd be 60k miles:eek:

Thats silly! Do You believe they cheated thier own wind tunnel test? It will mostly be true even for the staunch purist ecomodders.

I will say this again there are 2 different types eco modders here the
:turtle: drivers no offense who get every last inch of thier fuel eco possible.
and Eco modding drivers like myself who will sometimes be cought holding up traffic while attempting to test extreme gain of new mod. but roll normally at or just under a speed regular traffic. And pull 5-15% better than sticker empg.
We need both kinds, but Not every mod or Idea has similar effect when speed is increased. My dakota r/t lost mpg with air dam because it is running at and + speed of normal traffic. worked ok for my KIA.

I took it as "Dry" humour;)

Fortunately, you don't have to. Eaker is the guy running the wind tunnel and is the "first party" source...

But suit yourself.

civility chaps!

Rrr I'M INTERNET MAD :D

No kidding. I lowered the same make/year/model twice. The first time was very successful, so the second time i said I want the same springs and a stiffer anti-roll bar. The guy that ordered the parts ordered stiffer springs with the stock anti-roll bar.

Now if I ride with my left elbow on the driver's door arm rest, I get pain in my left shoulder. Never happened before. :(

I mentioned it once before on another thread here, but we independently tested a couple of the suggestions from that article (lowering front, back, and total, and the massive verticle air dam), not wholeheartedly believing all of it initially, either. But as best as we could tell, it all seemed to be true (doing what they said DID improve coastdown times on our car).

They mentioned these things applying to "musclecars", and the car we were using (1971 Buick Riviera) was more that shape, too. So things might be a bit different with the later model cars that are designed with more emphasis on aerodynamics. But, as applied to "pre-aero" cars, I'm satisfied the article is right.

Ive lowered every vehicle Ive own. Sidekick, Scion XA, Insight, Geo Metro, Scion xb just to name a few. Ill buy springs or cut the stock springs. Worse to worse, cut lowering springs to get the drop you want. Nice thing if you can swing coil overs is both the shock and spring are adjustible, but you got longer payback.

I find I getupwards to 5 mo mpg and the handling is well worth it. You can curner better, steering seems crisper as well as braking.

OOps my apologies truly for putting heat where it may not belong.
:eek:

no worries- just arguing on the internet is a fruitless waste of time..

plenty common ground to agree upon instead:)

Just get the cheapest dampening adjustable coilovers available, keep them spun all the way up (not lowered) on a soft dampening setting, leave the helper spring and the bump stops in. They will ride very nice! And please buy a deep vr6 lip, make sure you get an oem one even if its used, the replicas shatter. I love my mklll vento jetta, i am just lowered a little with a vr lip.

Make sure you get dampening adjustable!

Please everyone do not cut springs! It ruins your strut shocks and can be dangerous.

You can cut springs, oem and lowered ones. Thats ow the pros get that nice lowered look with plus size wheels and tires. In those cases for something that low you need to cut or even remove the bump stops. My tanabe springs set for my insight said to cut all but an inch off the bump stops.

Sometimes lower springs sag. My sidekick has tires 3x larger than oem and hr lowering springs. The rear is saggins and I have to add an inch spacer in the rear to level it out and stop the fenders from hitting the tires on bumps. :eek:

Since it looks like you are more interest in performance than mpg, maybe try a forum for your car?

It is not a good idea to cut springs due to the fact that most have Vehicles that use progressive spring rates, I own the same chassis as the OP, which is why I chimed in on my setup.

The "Pros" don't use cut springs and level spacers, they run coilovers. Lowering isn't just for appearance it's for the sport :turtle: IE handling, wheel fitment and aerodynamics.

Of course one would have to shorten the bumpstops if you run a shorter spring and strut/shock, but most strut/shocks are not made to bottom out and will break.

There is nothing wrong with leveling spacers other than they shorten your strut/shocks stroke length. I have one inch spacers on my vw on the rear since I like to carry all of my luggage and tools, as well as a full tank.

Sounds like you need to roll/modify your fenders or have taller bumpstops.

I am assuming this was towards me, :rolleyes: if I was interested in performance, I would have not bought a vw nor would have joined ecomodder.com :D

Well, yeah, if you can get a coli over for your vehicle and afford it. Those typically come with adjustible shocks to boot. Most of us cant afford it or its not available. Tanabe was the only thing for my insight 3 years ago, unless you cut your springs. I havent seen many economy cars with progressive rate springs. :D Even then, you cut the coils from the tightly wound side and leave the looser coils alone.

Ive rolled my fenders, there is only an inch of less travel when the fender contacts when turning in the rear, an inch spacer will eliminate that. A good sway bar could also do it.

Ive had the bottom of my car hit the ground, but yet to blow a shock. I think they are better made like oil filters. I remember blowing cheap oil filters back in the day on VW Diesel cars.

Had a very productive day today:

Picked these up for £50

http://i663.photobucket.com/albums/u...psffe0bb83.jpg

Boge Gas turbo dampers, with lowering springs:thumbup:

they're off a mk2 golf Gti 16v, but fit the mk3. Should be about a 40mm drop- sensible and comfortable!

and those are progressive rate springs!!!!! They are average 200 bucks USD depending on where you buy them.

Now, disassemble them and remove the urthene bushing and cut that in half and reinstall. My tanabe instructions said to do so and it helps to reduce the whip lash effect if you bottom out, but still leaves enouugh to avoid a jarring bump if you have any ride quality issues.