Where to find info/discussion on aerodynamics

[midzt]

Surely thats becuase the rolling resistance of a car is much larger than a bike?

[chriswood_7]

Could it not just be down to the efficiency of the engine at the different engine speeds?

That would be my guess, you would need to engine data to see..

[chriswood_7]

If you rode push and even mountain bikes a lot, then you'd know just how wrong that is. Even the slipstream effect makes a significant difference at around 15-20mph. As a kid we used to do a lot of biking, and have slipstreaming races down hills. Rolling resistance does make a difference of course, but at anything beyond 20mph it's totally unrecognisable from the other forces acting upon the object unless you're using equipment to measure those forces.

Amateur aerodynamicist here too.

Yeah but on a bike your CD is about 1.5. The human body is not the most "aerodynamic" of bodies, on a car (well a road going car anyway) it generally becomes greater than rolling resistance at about 80kph (IIRC)

[kalniel]

Could it not just be down to the efficiency of the engine at the different engine speeds?

Could be - guess those speeds aren't ones they use for emissions tests The engine feels the same though, certainly economy doesn't get any better at um.. even higher 'overtaking speeds'

It is probably just a combination of normal increased drag plus decreased efficiency from the engine.

[chriswood_7]

IIRC one the engine curves max fuel economy occurs around the same as maximum torque, which id imagine yours being around 2krpm? If thats the case, driving at 70-75 would be around 3500rpm, maybe more.

My recommendation - Drive at 60 and save fuel LOL

[Vimeous]

If we're talking pure downforce vs drag then the Veyron is a fabulous example.
In order to reach the highest speed you have manually force the spoiler to remain down to reduce the drag effect. As you get faster the aero of the car is designed to generate enough downforce on its own (which will increase with speed if designed well) and minimise lift.
It's a delicate balancing act.

For normal cars 'adding bits' will only work to an aero advantage if done with knowledge of what you're using them to fix. If you want better fuel consumption underbody modifications and narrower panel gaps make a big difference but if you get the former wrong you'll generate lift as well and make the handling very 'entertaining at higher speeds.
The best way to influence fuel economy in your own car is to investigate harder, narrower tyres that reduce rolling resitance but retain grip. Also investigate if adding cruise control will help as it may control the throttle more consistantly and waste less fuel.

Of course a Disco and S-Type with a similar engine will still return very different fuel consumption at a constant speed. I wonder how close a 2WD, 1700kg Disco would get to the S-Type when little BUT the aero is involved?

[midzt]

IIRC one the engine curves max fuel economy occurs around the same as maximum torque, which id imagine yours being around 2krpm? If thats the case, driving at 70-75 would be around 3500rpm, maybe more.

My recommendation - Drive at 60 and save fuel LOL

Not sure if this is the same as what you are talking about, prob not, but when we did an engine test, the shape of the Torque curve and BMEP were exactly the same.

[chriswood_7]

I think you might be right, but you can relate brake specific fuel consumption to torque and BMEP IIRC.

Im sure that the BSFC was at its lowest when torque peaked, I must get my notes/books out and have a look LOL

[chriswood_7]

Im back loaded with information! Trawling through my books here I have found something that similarly resembles what we are talking about, in turbocharged engines... this MAY be different for NA engines.

Basically, it shows that your lowest BSFC occurs around maximum torque and when boost pressure reaches its maximum at its lowest possible engine speed (i.e. when for example it first reaches 0.6bar boost before valves start to regulate pressure in the turbo)

Ahem, I think we might be going into too much detail here. Basically stick bits to your car, but you run the risk of creating drag, lift and of course them falling off LOL

[midzt]

Yeah, i think it turned out that BMEP and Torque were basically the same but multipled by some other stuff. Don't think the BSFC changed much after about 2500 rpm.

I think Chriswoods final conclusions good!

[chriswood_7]

I remember once, a young lad I went to College with had an old Toyota. It had a Subaru style "Scoop" on the bonnet which was glued on and had no holes in the bonnet so it acted as a huge "wind-brake". Driving to the coast one day, he went head on into a gust of wind at 80mph and ripped the scoop off the bonnet, scratched all his roof and took loads of paint off where it had been glued on.

That was fairly amusing, but I would have been slightly peed off if I was behind him and something hit my car.

Moral of the story, dont stick bits to your car that could come off hitting someone elses car. Two conclusions of this could be A) this will make others watching laugh out loud at your expense and B) if you hit the wrong persons car with a flailing aerofoil he may follow you to the next junction, get out and biff you.

[digit]

I thought a lot of the time exterior modifications were bolted down? I can't see why people would glue bits to there car tbh...

[Guy]

The usually are, but there are a few excpetions, on a scoob estate the higher of the 2 rear spoliers is glued on. And cant say ive heard of any cases of it detaching.

[chriswood_7]

LOL it was a J reg 1.6 Corolla LX (or something similar),

[kalniel]

IIRC one the engine curves max fuel economy occurs around the same as maximum torque, which id imagine yours being around 2krpm? If thats the case, driving at 70-75 would be around 3500rpm, maybe more.

My recommendation - Drive at 60 and save fuel LOL

According to the book, max torque actually occurs from 1800 through to 2800 rpm (flat throughout this range). 70 is only about 2700rpm.

[kalniel]

Im back loaded with information! Trawling through my books here I have found something that similarly resembles what we are talking about, in turbocharged engines... this MAY be different for NA engines.

Basically, it shows that your lowest BSFC occurs around maximum torque and when boost pressure reaches its maximum at its lowest possible engine speed (i.e. when for example it first reaches 0.6bar boost before valves start to regulate pressure in the turbo)

This sounds plausible - best economy would then be at nearer 1800rpm, which is just below 60 and fits my findings.

Ahem, I think we might be going into too much detail here.

No way, the more detail the better.