Aerodynamics: What can help with high speed stability?

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cantfindausername
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Aerodynamics: What can help with high speed stability?

Post by cantfindausername »

Good morning all,

I'm after some advice and also peoples opinions on off the shelf and custom aerodynamic aids.

I've altered the gearing on the car, and with the rev limit I've specified it should be geared to aprx 206-208 mph. Obviously aerodynamics will come into play in a big way at these sorts of speeds.

The cars (Toyota MR2 mk2) in general have always become very light at the front at anything over 130. Toyota added "speed flaps" to the later revisions to aid in high speed stability. These are small plastic flaps that divert air around the front wheels.

Ok, so the question is, what can I do to the car that would aid in keeping it stable at high speeds? I have seen people add canards, some have taken the flat bottom approach, diffusers, bigger wings etc.

I plan on competing with the car at events like Ten of the Best where there is a top speed run.

Does anyone know of a wind tunnel that can be hired?

Thanks,
Anthony
Julian
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Re: Aerodynamics: What can help with high speed stability?

Post by Julian »

First of all I suggest you do some reading - a subscription to race car engineering (or access to back issues) is a good place to start. One of their regular authors (who's name escapes me right now) is renown for his articles on the subject and also has a few books to his name, particularly on this very subject.

The problem you face is that you are talking about a symptom rather than a cause. High speed instability can be caused by a huge arrays of problems, most of which are tiny details on the car.

You can make any number of changes but without empirical testing you will never really be able to tell what is working and what isn't, that said it isn't impossible to solve this without access to a wind-tunnel.

First up you need to establish where the instability is, at the front or at the rear? Most cars of this era and size tend to suffer more at the rear due to the short roof but Toyota tackled this right from the start of the Mk1 so turbulance behind the cabin is significantly reduced. Troubles at the front can result from a number of details - poor airflow through the radiator, over the nose, under the car, etc. It could even be as simple as the windscreen wipers projecting into the air flow (not an issue on Mk2s that I am aware of).

If you want some idea of where the turbulance is occuring you are going to have to resort to the time proven method of tuft testing. Taping strands of string or wool to the car aligned with where you expect the flow to be prevalent. Get the car up to speed and get an accomplice to video the area as close up as possible. Keeping the tuft colour a good contrast to the body helps. Some of the tufts will lie pretty much flat as expected, some will go in directions you didn't expect and some will flap around like unacceptable. The ones that are really concerning are the ones flapping around, this indicates some fairly significant turbulance. You then try smoothing the flow (plastic card and duck tape is your friend), it's very random and without experience hard to predict (and even then it is far from easy). The modifications do not generally need to be large to make the difference and just remember to change one thing at a time as each will have a knock-on effect - even working back to front (problems toward the rear of the car can alter what is happening at the front).

The alternatives are computer modelling to try and find the problem areas without the need for real life testing (suitable testing facilities do tend to be hard to find) or to book time on a wind tunnel but that is both expensive and much harder than it sounds. Most of the commercially available wind tunnels are only operable to more mundane speeds (typically between 70mph and 100mph) which is not really going to help you out.

The real problem in all this is that so far we have only looked at testing the upper surfaces of the car, the underbody remains a pretty much unknown quantity. Computer modelling tends to be the only place you can really explore this unless you can strap a camera under the car and use the tuft test in that way. Not quite so infeasible as it used to be but it can be misleading.

Smoothing the flow under the car is not likely to do any harm, nor is lowering the car slightly to reduce the volume of space in the gap between floorpan and ground. Anything protruding into the airflow will likely disrupt it but most of it is unavoidable. Lotus used to tackle this with a bolt on undertray (affectionately referred to as a cheese grater) , it did a good job of controlling air flow although it dates back quite a long way now and there are better ways to do this when designing a car - not necessarily applicable to post-production modifications though.

Having said all the above I would hazard a guess at the problem though (only because my own car has similar traits). First of all too much air is getting under the car to start with and can't get out in a nice easy manner - worse still it is probably being trapped by something further back forcing the nose to lift. Too little flow over the nose doesn't help but smoothing things out can help. A lower front spoiler is a bit crude but might help to prove the point (rather than act as a final solution). Lowering the front of the car (relative to the back) can help in the matter but you need to change this in small increments to find the sweet spot as you can create instability at the rear if you go too far.
cantfindausername
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Re: Aerodynamics: What can help with high speed stability?

Post by cantfindausername »

Thanks very much for the reply. Some food for thought there. I think I will raise the rear of the car as I seem to remember is lowered further than the front, so I think some tweaking could be made here for starts.

I will look into this tuft testing. Sounds interesting! I have a bullet cam so I could set that up for recording results too.

There is a flat bottom kit available for the car. But I'd probaly look at making my own up as it is only available in the US. Importing it would be pricey!

I think I might also have to look at getting the exhaust made custom. The one on the car hangs a lot. I was thinking of a side exit one but guess that could have effects on dynamics aswell no?

Thanks again for the reply.
Ant
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Re: Aerodynamics: What can help with high speed stability?

Post by Julian »

Not sure where you would put a side exit exhaust on a mid-engined car but if you want some examples of how to tidy it up I would suggest looking at the TVR Sagaris. If you move the exhaust in front of the rear wheels you have a whole new set of problems but a simple cutout through the bodywork allows you to let the exhaust out without intruding into the airflow or disrupting the existing flow along the side of the car (too much).
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Re: Aerodynamics: What can help with high speed stability?

Post by Guy Croft »

Very interesting Julian.

I read years ago about diffusing ex gas at the rear of the car to reduce drag. Venting into a large chamber with a slotted grille on it or similar. Do you know anything about diffusers for this purpose? Might be a misnomer, I've lost the book unfortunately, or lent it out, which usually amounts to the same thing...

GC
cantfindausername
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Re: Aerodynamics: What can help with high speed stability?

Post by cantfindausername »

By side exit, I mean either coming out of a vent panel (or there abouts), or going nascar style (long and thin exit) just under the sill.

Interesting you bring up diffusers. This is something else I'm wanting to incorporate into the car. The intercooler is being mounted in the boot with 2x 11" pusher fans pushing are through the core. I'm not sure how they've finally installed it but I had thought about exhausting the air from the fans into a diffuser type arrangement to keep it neat at the rear end.

I think one main problem I'll have at the rear in terms of flat bottoming etc, is that the rear subframe is rather large and protruding from memory.

Here are a few pics of it as I dropped it off to Phill.

Showing how low the exhaust sits - hence thoughts about custom
Image

Bottom of engine looks to be relatively inline with front portion of chassis floor
Image

Rear subframe, not too out of line from the bottom of the engine, but the boot floor is raised considerably.
Image

I think with a non rear exiting exhaust or, possibly as one that exits the rear bumper just behind the wheels, then a diffuser could be installed successfully, and also to incorporate the intercooler "air exhaust" method. Thoughts?
Julian
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Re: Aerodynamics: What can help with high speed stability?

Post by Julian »

I'm with you on changing that exhaust. It probably isn't doing you too many favours to start with although I do appreciate the need to try and keep the noise down.

With the exhaust sized as it is I am guessing this is an import turbo engine - if so then your main concern should be to just keep the exhaust as simple as possible. A twin exit like that may be aesthetically pleasing but doesn't really achieve anything (in fact it can made things worse in some situations). You have a quite significantly raised boot floor as you have already pointed out so I would go with a silencer mounted across the width and exiting along the shortest route to the outside world (just behind the wheel), it does introduce some convolutions along the way as you need to send the pipe over to one side and then back again but it does allow you to take advantage of the space you've got without everything hanging down like that.

A diffuser would be ideal but could prove a tight squeeze. I'm surprised there isn't more available direct from Japan for this. The MR-2 GT Turbo was a very popular tuners car although obviously a bit outdated now since they haven't been made for a good few years.

Diffusing exhaust gas along with the airstream has been one of those topics that seems to go in and out of favour. Out in the field I've seen very few examples of it being used and nothing to show if it is actually successful in improving the quality of recombined air flow at the back of the car. Pretty much everyone seems set on seperating the exhaust gas out but I suspect that has more to do with legal requirements since (even in racing) the exhaust has to extend beyond the floorpan of the car (and then it has to be the rear part of the floorpan) to prevent ingress of fumes into the cabin. The only time this isn't relevant is open top single seaters and sports cars.
cantfindausername
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Re: Aerodynamics: What can help with high speed stability?

Post by cantfindausername »

Interesting read.

Yes the car is a Japanese import.

I've just been looking around at the custom exhaust parts, and have a primary idea. On the last picture the flexi pipe (center section) comes out facing towards the left very slightly. As such I was thinking of a simple 3.5" pipe with silencer and tip exiting just behind the wheel.

One thing thats annoying me at the moment is that I dont' have access to the car to see if any of these options are really viable. I really am just exercising my mind at the minute.

The exhaust you've described is pretty much identical to the BLITZ Nur-Spec exhaust. A very popular model on this type of car. My only concern with anything going across the boot floor is that the intercooler is somewhere above it.
Rich Ellingham
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Re: Aerodynamics: What can help with high speed stability?

Post by Rich Ellingham »

If you can get the last 12 issues of race car engineering then you will learn a huge amount as they have done a whole series called aerobytes which has lead to much testing and feed back on Lotus Exige and saloon cars.

To summarise you want to stop air from going under the car - Front splitter/air dam

Reduce drag and turbulence from items under the car - flat bottom.

Reduce turbulence with the rejoining air - Diffuser

As Julian would note from this months RCE diffusers are not downforce creators, more drag reduction to aid the benefit of the smooth bottom.

BUT, and here's the main one - is you time spent more wisely somewhere else, how ofter will you see such speeds unless you are going for some top speed testing? I would say in the scheme of things this would be a future nice to have modification, and and one of those fabrication jobs which could take a lot of work. I'd look for a low front splitter and an spoiler for the rear with a notable aerofoil shape and adjustable angle.

For the exhaust - as I know your car is at G and G you could take it not so far away to Retford to OJZ engineering who made my exhaust before I took my car to Phil really are a high end quality manufacturuer well beyond much of the off the shelf items I have seen. Lance the owner could fabricate what you need in a single pipe and well packaged configuration. If you see my Tipo race car thread you can see the exhaust I had made. http://www.ojzengineering.co.uk/page.php

Rich
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Re: Aerodynamics: What can help with high speed stability?

Post by Julian »

You have a good point Rich but I disagree that it isn't so important. If your car only hits 180mph once in a month then it may seem like an extravagant expense but if the car suddenly loses control as a result it would be worth every penny to avoid that situation.

We have a similar problem on my Dallara - it goes light at the same sort of speeds but thanks to some modelling (both in the computer sense and the physical sense) it was proven to literally take off at 170mph. We've addressed the issue to reduce lift as far as we can within reason and reduced the effective speeds to make sure we don't stray outside the safe envelope - although a strong headwind at the wrong moment could still make the car unstable but it won't take off anymore.

Oh - and the author's name that I couldn't remember before is "Simon McBeath" - and I haven't read this months article yet but I am getting to it :D
cantfindausername
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Re: Aerodynamics: What can help with high speed stability?

Post by cantfindausername »

Thanks Rich. I will give them a looking over, but I'm not local to Lincoln. I'm 200miles south. But if they do the work I like then I'm willing to travel, hence why the car is a G&G.

You're right that the speeds won't be reached very often, but I will one day attempt a rediculous speed in the car. Obvioulsy safety fist, and the roll cage will need to be in, along with a custom CompBrake brake kit. After those are in though I think a serious amount of focus will be put into increasing the the aerodynamic efficiency of the car.

Thanks again,
Ant

edit: Just looked at his stuff, and there is some very nice work there! Will definately get in touch with them for a chat at some point!
Rich Ellingham
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Re: Aerodynamics: What can help with high speed stability?

Post by Rich Ellingham »

Its a fair point Julian, but as Ant has some more fundemental work prior to the car actually running I'd say put this a little later in the timeline as opposed to forgetting it. Unlike my car I think this MR2 could easily reach speeds where handling especially braking could become unpredictable.

Rich
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