I am considering building a rear wing for the 121 to help reduce the oversteer issues I have currently. In sprint events it feels like the rear wheels have McDonalds trays under them, its that loose, and especially under deceleration. You can see it in the video, sprints start at 2:20 of this video: http://www.youtube.com/watch!v=YbmDWwuv4LI
I even spin out on a relatively straight piece of road which Im annoyed at myself for doing (even skimmed the wall haha) but I don't like the car doing that, it shouldn't be that loose at the rear.
One option is putting a wing at the rear, optimised for downforce at speeds of around 70km/h to 120km/h. Does anyone have any pointers which will get me started in designing a rear wing for it! How much downforce do you think I will need! There is about 200kg on the rear axle currently sitting at a standstill but of course this is greatly reducing to a matter of grams if heavy braking is applied and obviously enough reduction to cause oversteer while cornering without gas being pumped in.
The design of wing I am considering spans the width of the roof, and closely relates to the design of a general purpose airfoil for a light aircraft, although upside down of course. Id like to have enough wing to be able to run it at a high angle of attack for the slow tracks but then reduce the angle for faster tracks, Ill do that in the mounting system but the wing will need to be appropriately sized to suit my range of requirements.
I dont really need it for gymkhana, I didnt experience any issues in that and oversteer is desirable for that anyway but going around the faster corners at around 80-100km/h with switch overs and weight transfers it was quite bad as you can see in the video.
morrisman1,
Jan 30, 6:25am
I should add, the wing can be about 1.2m wide and Id like to run it at about 10° Angle of attack for the driving, so that I can increase it to 15° which is just under the angle where it will stall, if I need more downforce
pollymay,
Jan 30, 6:38am
Sheet of iron like a ducktail on the roof with a curve and hinged at the front. Put braces down onto the hatch or something with lots of adjustment, you could use sleeved braces with grub screws or a bunch of things. You'll have huge area and for every bit you adjust it up you present significantly more surface area if it's deep enough.
Think like a sprint car. Only you can go from near flat or nothing to pretty much the max you'll ever be able to realistically use before it's too much drag and everything inbetween. It will probably look hideous though, there was an old holden at our club who was adamant his wing was improving his sprint times in his VK.
Behold my paint skills. Changing the depth and presented area will change things. Also you can put whatever curve suits into it that allows the hatch to open and it to lay as flat as you desire etc. You'd have to play around. However this is about as minimal effort as anything, doesn't need to be flash, just braced tin.
morrisman1,
Jan 30, 6:51am
might as well just lock the boot lid up! haha.
It will be relatively easy to make an airfoil, I have the gear but I am just not quite sure how large it will need to be to generate the downforce required. I only fly planes, not design them so that kinda thing is a bit beyond my knowledge
jason_247,
Jan 30, 6:57am
the car is light enough you might as well do the horrifically opposite thing of putting ballast in the back
morrisman1,
Jan 30, 6:59am
I think Ive just worked it out. To throw some math into it, the pa28 will support 975kg with a wing area of 15 square metres at a speed of 90 km/h. If I was to require 100kg of downforce at this same speed I would require 10% of this wing area, round figures which is 1.5 square metres. So if the wing I build is 1.2m in width then chord needs to be 1.25m deep. No good, too huge.
Perhaps If I make the wing wider, bring it out to 1.6m, the width of the vehicle then it would only need chord of 900mm. That is still ridiculous so perhaps I need to revise my downforce figure.
So coming to that point, what sort of downforce will I require to keep the car stuck to the ground satisfactorily!
morrisman1,
Jan 30, 7:01am
Thought about that but putting ballast will do two things: 1) Increase the level of grip the tyres provide 2) Increase the centripetal force required from the tyres, and hence increase the requirement for grip.
So adding weight will slow my acceleration down but the benefits to cornering will be questionable.
clark20,
Jan 30, 7:16am
Before jumping into the wing idea first make sure all the bushes are good, are your shocks adjustable ( rebound) and think about toe in/out esp when braking. Think about the front as well (how fast it dives) and anti roll bars , it all has to match.
pollymay,
Jan 30, 7:19am
The formula for downforce gets ugly fast, plus it would still be an estimate. Honestly I'd just try some different things till you settle on something. Not like there is years of research behind mazda 121 driving dynamics
The speedway guys don't get out the formulas when they change attack angles, they drive the car, test n tune, drive some more etc while some old guy that has done it for years says things they should try. You'll just have to be "the mazda 121 guy". On top of trying a different alignment I'd personally make a frame for the style that I suggested that has threaded studs or threaded holes in it on top of adjusting arms. Add and remove sheets + attack angle till you settle on something that works. There is no magic number someone can tell you. You may get ballpark suggestions though so yeah, wait for someone that is a bit better with wings I guess.
socram,
Jan 30, 7:52am
Agree.A wing is not the problem.As an amateur in these matters, I'd also be looking at suspension set up, (back and front), spring rates, anti roll bars too. Tyres and tyre pressures are also critical, warm or cold plus corner weighting. Front airdam would probably work better.Wouldn't even be dreaming about a rear wing, as for a wing to work, it is going to take a lot of horsepower to overcome the drag.
rayzor14,
Jan 30, 7:56am
As an alternate what about a difuser/venturi arrangement underneath the rear!
smac,
Jan 30, 8:08am
Y'lost me here. How is it you think downforce from ballast is any different from downforce from a wing! Or are you talking about during cornering!
I had thought it was generally accepted that the force available from a sensible sized wing was only helpful in a racing application at high speed. As you've indicated, at lower speeds (say less than 100+) the wing has to be huge.
Don't underestimate the ballast idea - it's not just about settling the rear, it's about evening out the corner weights and allowing the suspension to work as it was intended. I think you've removed glass, also trim, and a bigger engine.the cars balance will be shit at anything over 50kph (as you've found).
The only thing that will get you over really bad front to rear balance will be a wider car.or shorter wheel base.and for that you should have started with.oh I dunno.a mini ;)
morrisman1,
Jan 30, 8:17am
I currently have a new set of 165/70r12 tyres on the back, thats as large as I can fit currently. If I can get some of the semi slick A008r tyres on, at 185/60r14 how much difference is it likely to make, considering the weight on the wheels!
To fit those wheels on I will have to get some hubcentric spacers, probably 25mm or even wider
pollymay,
Jan 30, 8:20am
Requires high speed.
A wing will help, I'm just not sure how much. It would be biggish hence the looking ridiculous part. However you would compromise it at higher speeds. You can also try stagger the rubber to the back more as well as change alignment for a different setup
clark20,
Jan 30, 8:20am
That would make lots of difference. 165s!
morrisman1,
Jan 30, 8:51am
argh dammit, just found out that the wheels do fit without spacers, what the hell was I doing before that made me think that they didnt!
flashgordon_nz,
Jan 30, 8:57am
yep, as we talked about mate. your on the right track playing with the rear wheels. have a look at my photos thou, the rear wing on the rally starlet would be the go. but get a heavier wheel (unsprung weight) down on the ground. stiffen up the sway bar. useless bit of info, but i went from a 185/60r14 on a 14 inch 5 spoke ROH rim, over to a 15 inch motor form sector, running 205/50r15 - weight gain per corner was 4.7kg
hondalova,
Jan 30, 9:13am
do a "burt munro" trick, lead out of car batterys
unbeatabull,
Jan 30, 9:15am
If it's understeering on Deceleration (as per OP), what's happening is weight is transferring to the front to the extent there is no force left on the rear. How to fix! Stiffer shocks/springs in the front, more weight in the rear (lower the better, higher it is the more the weight will "Transfer" and will cause a similar problem to what you already have), a Wing on the back won't help much on Decel, if at all.
morrisman1,
Jan 30, 9:16am
do you mean to say oversteering on deceleration unbeatabull! Because I suffer from oversteer not understeer.
budgel,
Jan 30, 8:46pm
Is there any way you can lower the rear relative to the front!
Oversteer on deceleration can sometimes be overcome by narrowing the rear track.
bill-robinson,
Jan 30, 9:10pm
Mechanical grip is what you need. Lower the back ride height, move any thing you can to the rear,(battery, fuel cell, even the radiator if possible.) Stiffer front springs and lose some of the rear braking. Softer rear tyres. Forget wings at those speeds they just mask the real problems
mgmad,
Jan 30, 10:10pm
I agree with some of the above, look at your suspension setup first, then once that's sorted look at aero. Tyres would be the first step, get some good semi slicks (R888s are brilliant wet and dry) and then have another go.
It is possible to get significant downforce at low speed, but you need significant angles of attack, and the stall speed is quite low. Plus, there is heaps of drag. Ground effects can be very useful though, and simple to implement, but I would still be looking at making sure the suspension is sorted first.
unbeatabull,
Jan 31, 4:52am
Yes, sorry, I meant oversteering.
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