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MG Midget and Sprite Technical - Boot spoiler
|My new boot lid has just been delivered. The lid is a new steel frame, skinned in aluminium and incorporating an aluminium spoiler based on the design I tested in the full scale wind tunnel. It's seen here placed rather than fitted to my car for a photo before I get it painted. It weighs in about 230 grammes less than a steel lid without a spoiler but after painting I'm guessing the saving will be about zilch. Short of my unexpectedly having a large sum of spare money it will be several years/never before I can have it tested in the wind tunnel. However, in the light of the 4 tests I made with the 'mule' boot lid and spoiler its very likely to put 2 mph on the top speed and reduce lift by 30 lbs at the rear
I wonder what the effects will be at the out side ends
|Looking good and forward to hearing how it works in practice.|
|It will work about about the same as this one!|
|Daniel, that's a very elegant piece of work. I wonder if I could get away with a slightly smaller version that stays below the wing top edge to retain the silhouette and stay within 'Roadgoing Production' regs? I rather doubt it.|
More particularly, I like the video. Has anyone done any similar work comparing top down v soft top v factory hard-top? I have been told that, with it's relatively sharp trailing edge, the hard top is better aerodynamically than the soft top. The airflow seems to follow the soft top hood quite well so that it flows across the boot, would the hard top deprive the spoiler of the air flow it requires to be effective?
In my case, with a padded roll over bar, passenger head restraint and my big head in the way I feel sure that top down is the worst option and, though I've tried back to back runs with the hard and soft tops I couldn't detect any obvious difference.
|Very nice Daniel, are there any dimensions of this in the latest edition of your book?|
|I think you will find that a shorter (less high/tall) spoiler than mine will work, it will simply reduce drag less and reduce lift less.|
Apart from Healey and I guess MG, in period, mine is the only Sprite/midget tested in the full scale wind tunnel.
I wasn't able to test a hardtop. I'm guessing most won't be much better than a soft top and also guessing that even if they are aerodynamically 'cleaner' that will in turn generate more lift at the rear of the car, which itself can create more drag.
With the soft top down the drag increases to a large extent but lift also drops to a large extent. I didn't test top down without front and rear spoilers (it was a final test with the car in its final and optimum configuration) so only have limited comparative data.
Yes, dimensions, measurements and test data.
|I'd planned to put the new boot lid and spoiler in the boot when taking it to the paint shop but since it wouldn't fit (even with the spare wheel removed) I had to fit it to the car last night and put the original boot lid in the boot and swap them over at the paint shop. So unexpectedly the spoiler had its first road test earlier yesterday. I'd also fitted the 2nd grille blanking device. At motorway cruising speeds it felt like I was on a lighter/smaller throttle opening than I'm used to. At higher speeds (!) there was no longer any pitching down or nose down attitude of the car (that I could notice). The engine pulled well and for longer up to the point where I have to sit and wait and watch the tacho than was previously the case and then climbed the little bit higher than it did before (other than on down hill stretches). Engine temps were much the same if not the same as without the 2nd blanking plate and at times it was a fairly warm day. I'm hoping the new painted boot lid and spoiler will be on the car before the MG Car Club meeting at Silverstone.|
|A Midget boot lid fits in the boot ? ! That by its very nature seems impossible, I'm intrigued ,look forward to seeing it at Silverstone . Lots of modified race Midgets have spoilers presumably never tested in a tunnel presume just trial and error|
|A boot lid will fit through the boot lid aperture if you manouvre it in at an angle - but it takes care to do it without scraping it.|
|Just back from a weekend of hill-climbing at Loton Park - very happy as, although I didn't improve my PB I did manage to break the speed limit for the first time: 61mph through the speed trap thanks to new K&N filters!|
I had a very interesting chat with the scrutineers re. using aerodynamic aids on a road-going production car. The basic answer is, you can't, but that's just the class I run in, other, modified classes will allow them.
However, the conversation moved on, concluding that there would be no real benefit in a twisty, relatively low speed hill-climb track anyway but for sprints on a fast circuit like Castle Combe, it would probably be worthwhile.
Which brings me to the question: At what speed do aerodynamic aids such as spoilers and bibs/splitters begin to generate useful down-force and drag reduction on Spridgets?
|The speed at which spoilers will make a difference depends on the car itself. The lighter the car the greater the percentage lift will be for any given speed and so the more noticeable the effect. On one car+driver the lift might be 12% of the weight at that end of the car+driver while 10% might be typical weight of car+driver and 8% on a heavy car+driver. |
The faster the car goes the greater the drag and the greater the lift so the lighter car+driver should feel the benefit of spoilers sooner than a heavy car+driver.
The other thing is that while the driver might not feel the benefit of say, 10 or 15lbs less lift at the rear it might be possible to see that on the time of the run. It's decades since I've been to Loton but at a MIRA sprint I'd have thought the use of spoilers would prove very beneficial.
Putting an mph number on the benefits I'd say 60 mph upwards and being increasingly measurable at higher speeds to the point where a top speed is actually higher with spoilers than without (engine bhp dependant) but say 110 mph upwards. I do have a spreadsheet with various CDa and bhp figures in it but haven't done a lot with it yet and mostly it's for modelling top speed with various engine BHP.
|Would a set of fences on the end help even more? Reduce some vortecies created, and prevent air from spilling over the ends?|
|Interesting view of things by the scrutineers A few years back I had a Mazda MX6 turbo which came std with a useless looking rear spoiler. It had a cable type boot release which was quite tight to pull|
Anything over 80kph (50mph) you could pull the release easily as if it wasn't hooked to anything as the bootlid was being pushed down tight from this speed onwards
|Silhouette view attached - a shorter spoiler might within the rules and still make a worthwhile difference (I guess). |
Because this is a spoiler, rather than a wing (which has airflow over top and bottom surfaces) it doesn't need end plates. What should work, a future project, is to extend the spoiler to the full width of the rear of the car as recommended to me by the aerodynamicists at HORIBA-MIRA.
During smoke visualisation (done after all the testing measurements have been done at higher wind speeds) the only vortex I saw was at the base of the softop rear 'screen' which is exactly where one of my old reference books shows it would be. I don't think you can clearly see it in the youtube clip.
"Because this is a spoiler, rather than a wing (which has airflow over top and bottom surfaces) it doesn't need end plates".
-- it would work better with end plates
And I havn't seen any mention of a Gurney lip
|I guess, it will still be producing a pressure differential though?|
Either way, Il trust you! Have you played about with moving it for and aft on the car? Just wondering what made you choose the good and not further back.
|following on from your airdam and spoiler design in the book daniel, i came up with a 3 piece aluminium design which is removable so I can tow the car to meetings. together with the rear spoiler the car was much more stable at speed and ran a new PB of 8.8 seconds @116mph last weekend at the strip.
|The rear spoiler is designed to reduce drag rather than increase downforce. Again it bolts on (and prevents the boot from being opened!). The main plane is horizontal, and of course the car has no windscreen so the airflow is less disturbed as it reaches the rear end.
|Sounds like a good result - what was the previous PB? |
Are you going to experiment with different heights or lip sections on the rear spoiler?
Also, have you experimented with grille blanking plates?
|Previous best was 12.1/112. Yes I started with 10 degrees up on the spoiler, but it was slow, so I will go down another 5. Based on some other reading the angle of the lip or turndown at the back edge is quite critical so will look at that also.
I thought about more blanking at the front, but the waste heat coming off the turbo is a concern,and i need full flow through the charge cooler. The engine intake is also in the grille aperture. By the end of a run, the exhaust manifold is glowing, and the air from the compressor is 95C, reduced to about 40 by the cooler.
|In terms of hood up/hood down I had 10mph extra on the back straight at Croft with the roof on. Effectively 10% quicker.|
|I think mine is an accidential set up that worked out well
On mine ive got the front spoiler, a large radiator that blocks off all the holes into the engine bay except thur the rad, a parish hard top that has a bit of tilt to it because of the soft top cover and its bubbled and the rear luggage rack that kind of acts like a rear wing
Its extremely easy to hit 100mph in a short era on a flat area of highway...and it WAS stable
Now with the front spoiler half missing from a hidden speed bump run in, it still gets to 100 mph but its a little more skiddish and as you say lighter floaterier in the front, but the rear still looks down and holds the road as near as i can tell
Do you run any wind deflector/screen at all
Your 116mph speed is starting to get into the area where I had the biggest surprise ever with no screen fitted
I had just lowered the front ride height about 20mm from previous and first full speed run down the back straight at our local circuit, full hook top gear as usual and just about to let off at the end and then poof my helmet sucked up so much the opening was above eye level--bit scarey for a moment or two---A little 50mm high deflector was enough to stop it
Interesting how much airflow is changed by body attack angle
|Unless your class regs prevent it I'd have thought the biggest gains would be in looking at an aero nose which would cut down the Cda to improve lift,drag and maybe shed some weight. The adoption of something like a Speedwell snout, together with faired headlights,aero screen,hard tonneau,head fairing and a flat floor could be a package of bits to make up a kind of low drag Sprite. In addition if you could try to maintain a nose down/tail high attitude the frontal area would drop even further.
|Hi wily, yes I used to have an aero screen on the car and once ran with it folded down-nearly blew my head off! Now have a little screen made from polycarbonate heat formed over a buck, the shape determined by trial and error, then forming a final version.
Hi FP I love the photoshop, car would look great and there are no class rules. However I have some practical limitations including a single garage which precludes using a flip front, the desire to keep the car road legal, and the huge waste heat I need to get rid of. I also wonder about a hard tonneau, at present I have a light framework supporting the standard one. Nevertheless it's food for thought. My present priority is a homemade launch system to gain some tenths.
Are you on fuel injection--
A friend here made a system that has a button on the steering wheel that spills off a heap of fuel pressure--he fiddled and fiddled till he hit on the right pressure but now he can leave the line on full throttle,get launched and then let the button off
It's a 400KW supercharged v8
The car itself has traction control std. but this system is quicker off the line than the control, he's got it now so that it has just minimal wheelspin but not breaking down like the trac.control does
Might be worth considering
Sorry about the thread drift Daniel
|Drifting back to aero! You could try louvres in the very tops of the wings - you'd lose engine bay heat without increasing drag or or lift and would probably reduce both.|
|Hi Willy, quick reply then I'll stop hijacking the thread. I need to slip the clutch off the line to build boost. The car runs a blow through carb and early mega jolt so no proper launch control. I'm fitting a brake bias valve in the clutch line so I can slow down the clutch take up, then bypass it with an electric line lock so I can launch with slip then switch to instant lockup once rolling. Hope that makes sense.
Daniel, back to aero. Presumably I would get more extraction from the top of the wing than the existing louvres in the wing sides or bonnet top? I could consider closing some of the holes in the front then.
Can you explain thst clutch idea your doing a little more in a broader scence.... i kind of get what yoir saying but not fully
its an interesting idea... i want to more fully understand it... esp the one way valves on the clutch line
Thanks for sharing it
|Hi Prop, it's a brake bias valve, normally used to reduce pressure to the rear brakes, but in this case to slow down the rate of clutch release and allow a controlled amount of slip. The line lock bypasses it for normal shifts.|
This thread was discussed between 12/05/2017 and 03/07/2017
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