Street Car smooth underbody?

[Kambu]

Will street cars benefit from a completely smooth underbody?
It seems logical, that a smooth underbody would reduce drag, but what about downforce?
Will a Diffuser and a smooth underbody give any noticeable Downforce gains, on normal "Sports car" ride heights?

I have a rather highly modified Toyota MR2 (1994). That Is a bit of hobby car of mine, and am thinking about manufacturing
a smooth underbody with a diffuser for it.

[flynfrog]

The smooth under body will do more to drop drag. It would create some down-force assuming you had some rake. Sports car ride heights is pretty vague but the closer you get to the ground the better it will work until it begins to choke.

[Jersey Tom]

Downforce isn't exactly a high priority for street cars.

[Kambu]

Ofcourse it isnt a high priority. I am interested to know, If there is any point in building an underbody,for a car that is driven on trackdays frequently, but is still high enough to clear speed bumps etc.

[superdread]

Current street cars mostly have flat underbodies with a cutout for the exhaust system (cooling). That started in the 80s (if I'm not mistaken, the Mercedes W124 being one of the first).
The rear undersides have morphed more and more into "diffuser-like" shapes (very rounded for the air to expand smoothly, but not to create a low pressure to suck air from under the floor). Also the fronts got little splitters to force more air under the floor.

Sports cars tend to have real diffusers with strakes and all that, to create rather drag cheap downforce.

[flynfrog]

Ofcourse it isnt a high priority. I am interested to know, If there is any point in building an underbody,for a car that is driven on trackdays frequently, but is still high enough to clear speed bumps etc.

if you can clear a speed bump with it your DF is going to be next to nothing.

[Cam]

wouldn't a sculptured underbody be better than a flat one? If you had a choice and no regs (being a street car), that'll be the best option?? I think most teams would love to get rid of the flat bottoms.

Having got rid of sliding skirts, the next challenge to F1 aerodynamicist came at the start of 1983, when sculptured under-body tunnels were prohibited. As from the start of that season, flat bottoms were required for F1 cars from the trailing edge of the front wheels, to the leading edge of the rear wheels.

Racecar aerodynamicists found that without side skirts it was still possible to induce downforce by sculpting the underbody of a car into 2 tunnels either side of the engine-gearbox assembly. The tunnels ideally start close to the middle of the car, where the maximum downforce will be generated, and then gradually rake upwards (between 4-14 degrees) towards the rear of the car. The overall effect is similar to a venturi in that the air is first accelerated by gradually decreasing the cross-sectional area and then decelerated back to its original speed and pressure by gradually increasing the cross-sectional area. At the highest velocity (smallest cross section) the lowest pressure is produced according to Bernoulli's principle.

Edit: added extra info

[stez90]

Sculpted underbody with side skirts and enough ground clearance for speed bump can't work together.. IMHO it's better to focus on drag reduction..

[superdread]

Sculpted underbody with side skirts and enough ground clearance for speed bump can't work together.. IMHO it's better to focus on drag reduction..

They could, the diffuser (or Venturi Tunnels) has to be large against the gap to the side (maybe twice as high).

The main problem is that a car usually has stuff right on the floor (engine, tank, ...) and you would have to move that for a sculpted floor. So a flat floor, a pronounced splitter on the front, maybe a front diffuser if there is a path to vent it (wheels?) and a rear diffuser should be the way to go (consider bodywork rules of your country, splitting pedestrians into two and all that).

[Ted68]

The 1994 Ferrari F355 was the first road car I can recall that had a flat underbody with venturis engineered in for downforce.



Colin Chapman's 1959 Lotus Elite was a fiberglass monocoque car with a flat underbody and achieved an astounding for it's time, and still good today, cd of .29.

[Just_a_fan]

A flat floor would be good for drag reduction. You could make it generate some downforce too - probably enough to counter the lift forces generated by the upper body shape. You would need to be careful not to introduce an aero imbalance though; it would be quite easy to end up reducing overall lift at one end of the car and not the other. This would result in some interesting handling traits.

Large amounts of downforce isn't a great idea on a road car for three reasons:

1. When downforce is interupted the grip drops instantly which means you're likely to crash if you're cornering on grip provided by downforce and then the downforce is lost for any reason.

2. Large amounts of downforce adds drag although floor derived downforce is generally less draggy than that derived from wings/spoilers. Large amounts of downforce also require stiff suspension to prevent bottoming out at speed etc. Not great on the public road really.

3. A fully enclosed floor means that you need to introduce cooling ducts elsewhere. Don't forget that on a road car, the air that goes in through the radiator generally exits in to the space below the car. This isn't a deal breaker but it needs a lot of work to prevent overheating (even on a MR2 although that would be easier because of the engine position). Also, a fully enclosed floor might reduce suspension / steering articulation.

Designing a flat floor that generated downforce would be quite involved. You could just slap on some alloy panels to give a smooth surface and that would cut drag for sure. Would it generate a whole heap of other issues? Almost certainly. Worth it? Only you know that...

I would think that you'd need to put a flat floor (without any diffuser etc) on and see how it went. You could then add a diffuser (keep it reasonably shallow though - if you put on a steep diffuser it will likely experience flow separation and be no use anyay). You'll probably need a front splitter and/or dive planes too to try to balance the lift reduction / downforce figures. You might look at the Porsche 911 - I think it has a sculpted underside in front of the front wheels to help generate some downforce. Here is a picture of the nose section of the flying 1999 Merc CLR from LeMans. You can see how a diffuser has been created to try to generate front end downforce to balance the car:


It will require a lot of trial and error but should be quite fun and fulfilling to try out.

[superdread]

http://blogs.insideline.com/straightlin ... 1-gts.html
On the second picture you can see a little tunnel, but that's manly for front brake cooling.

The CLR is not a very good role model in the area of stable aerodynamics. Problematic is that a car, that can be used on the road, has rather soft suspension, so you should not have a straight splitter line in the front (height differences in the splitter when one wheel drives over a kerb can easily cause one side of the underfloor to loose effectiveness resulting in sudden direction changes sideways or upwards).

One solution could be what LMP did to counter the flying problem, skirting the front diffuser (or at least the floor) to the sides with some panes (or whatever they are called):


Make them out of wood (as some racing teams do) and you can slide above speed bumps (serves them right).

[marcush.]

the elise has a near flat bottom ,but this does not work very well ,as there is a big cross and bad detail on the bottom of the chassis .
I worked a bit on those putting flat sandwich panels under the car creating an impressive flat area from Car nose to Rear axle.
Without a doubt it added massive front downforce at speed transforming the steering towards heavy crisp and planted when before the car was all over the place at high speed.
Unfortunatelly no rideheight measuring on the fly back then ,but:
The front splitter was stiff enough you could stand on it (I was under 70kg back then) ride height was around 85mm front axle and 100 mm at the rear of the chassis .
Roughly at 200km/h the car started scraping the front edge of the floor....so i neeeded to increase the spring rate (with bump rubbers).

[Kambu]

A bit offtopic, but whats the reason for raising the front of the underbody on the prototype car?
Isnt it "The lower the better". Is it just regulations, or is there another reason for it? Maintaining grip when going sideways?
I wouldnt really worry about achieving too much downforce on the mr2, I just dont see it happening.

[AlpineF1]

hi

i have a question look at this picture

what are those fins at the side of the exhausts are they meant to be diffusers?? on a fiat i know this is kinda off topic but i seen them ages ago and i havent stopped wondering since