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wardy18

[misc_work] Project 205 Diffuser

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wardy18

I've always wanted to custom make a proper rear undertray/diffuser for the Hillclimb 205, has anyone else ever had a go at one?!

 

It's coming along nicely at the moment, fixings brackets have been made and the trays dimensions and shapes have been determined. First off its going to be cut from 1.2mm sheet ally (the one with tin in it so its got more spring and wont dent as easy) then tig welded together complete.

 

I could leave it here but ive got the fibreglassing bug at the moment with making more of my wide arch kits to sell (including custom wide rear arches to match the fronts) so im then going to make a f/glass mould of the diffuser so i can then re-produce further f/glass diffuser reducing the weight from the ally one.

 

Ill take some shots tomorrow on the progress.

 

Exciting stuff :)

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Henry Yorke

Rear diffusers are more effective if you have deep side skirts so the air is channelled out from the front to the rear, so creating more ground effect

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wardy18

Am looking into some cheap sideskirts which I can adapt to be suitable!

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mmt

Sounds great.

 

Pics please :)

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Cameron

Have you also got a front splitter and flat floor? Fitting a rear diffuser alone can be counter productive, you don't need the extra rear end grip but more importantly adding downforce at the rear can reduce tyre load at the front and increase understeer.

 

If you're fitting a front splitter, flat floor AND a rear diffuser, that would be very interesting indeed! :)

Edited by Cameron

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wardy18

Yes I have a large and deep front splitter with adjustable struts to adjust the tip angle closer or higher from the ground also run a large undertray from the underside of this splitter back to the subframe and floor pan underside, am goin to get some side skirts and put some extra channelling fins on the inside of these so the air is channelled to the rear diffuser!

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Cameron

Are you talking about vertical side skirts? A complete flat floor would make more difference to be honest, you can extend the floor out past the sill to reduce the amount of air spilling out the sides, vertical skirts may well help but they could be a pain as you lose ground clearance. It's also worth considering whether you get more downforce by having the floor lower to the ground or by having vertical skirts, I suspect you'll get way more by having the floor closer to the ground i.e. faster moving air & lower pressure than reducing a tiny bit of spill at the edges.

 

Do you have any cooling trouble with your splitter / undertray? It's something I've been thinking about for a while, but just makes me wonder where the hot air can escape and whether it decreases cooling efficiency.

 

Anyway, very interested to see how this turns out! :)

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wardy18

The only trouble with a complete undertray is still having to keep a channel open for the exhaust system, my undertrays stop and start at the front and back of the floor pan tub remembering that there's a big gaping space where the STD fuel used to sit, this has been covered over by the rear undertray/diffuser.

 

Also don't want to add too much excess weight.

 

Ride height isn't an issue for me in hillclimbing and sprinting.

 

Not sure how you mean extend the floor tray past the sills, how would you finish the edge which protrudes from the sills? Actually if I were to fit some side skirts I could then fit a undertray along the floor pan underside and attach to the bottom of the side skirts??

Edited by wardy18

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Cameron

Ah, yeah you're making it out of sheet metal so it could be difficult to finish the edges. I think your last idea would be best then, if you had some side skirts then you could attach your flat floor to this. I'm not sure the exhaust tunnel is much of a problem, you'll get air flowing down there from the engine bay so it should help keep it from getting too hot as long as you have an exit at the back of the car so the air can flow out of it.

 

I wouldn't worry too much about the weight of it, if you're making it out of sheet ally then it won't weigh much, but even so the weight is about as low down as you can possibly get.

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tri_longer

Copious amounts of pictures on this as I'm really interested in this for mine.

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wardy18

Well once mine is made up from ally then I will be making a mould in fibreglass from it, obviously the inner fins cannot be moulded but these can be made up separately and bonded on! I'll be happy to make up these diffusers upon request once they are up to a standard I'm happy with!

 

Pics coming soon

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wardy18

Progress so far, unfortunaly you cannot take pictures of measuring and thats 90% of the process so far.

 

You can see how i've cut the rear bumper and valance to suit the diffuser upsweeps.

 

The diffuser will be on 2 levels, the sides upsweep will be lower down than the centre upsweep purely because the centre unsweep will come up and join to the bottom of the bumper whereas the side upsweeps will join to the bottom of the valance underneath the fog lights

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Cameron

Is that the angle you're going to make it? If so, it looks a little steep!

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wardy18

Yes more or less that'll be the angle, i didnt wanna make it too shallow because the car is so low that all you would see from behind is 4 fins and you wouldnt bne able to see the upsweep at all, think it would have looked a little silly

 

Thought i had a happy medium, do you think it should be a shallower curve?!

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Vili
Yes more or less that'll be the angle, i didnt wanna make it too shallow because the car is so low that all you would see from behind is 4 fins and you wouldnt bne able to see the upsweep at all, think it would have looked a little silly

 

Thought i had a happy medium, do you think it should be a shallower curve?!

 

 

Well the scientific fact is that that is not going to work properly. Angle is way too steep. That sort of angle works with F1 cars because their rear spoiler directs the air flow behind the formula upwards. In a car the best compromise in diffuser angle is right about 7 degrees. If you put more angle the airflow will separate from "the roof" of the diffuser and cause turbulence i e drag but not down force.

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wardy18

At 7deg i would have to start the upsweet well back under the car to gain the height required surely?!

 

This diffuser is just as much to tidy up the rear end and box in and smooth out the underside as it is for downforce, i mean how much downforce can you really gain on a hillclimb tin top car where your average speed is approx 50-55mph?!

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Tom Fenton

You should extend the rear valance down to meet the diffuser, not the other way around. As said yours is too steep.

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wardy18

Hmm now you have got me thinking, all i need is a block of styrofoam and i can make my own rear valance to suit!!

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Cameron

Exactly what's been said, the angle you have there is too steep and the diffuser length is too short to be effective. You can have a maximum angle of around 10 degrees, but it also needs to extend further under the car, something like 1/4 of the car's length.

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wardy18

So in this case with 7deg lift should the upsweep part be straight or a curve like i have it?!

 

Does it make a difference?!

 

Would be very complicated creating only a 7deg lift with a curved diffuser......

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Tom Fenton

This is what we use on the Minis,

 

1carbon-rear-diffuser.jpg

 

Slightly more than 7 deg, but a compromise to clear the rear susp.

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Saveit

Sorry mate, but that just wont work. Youll end up with anything but downforce. The diffuser has to be at a 7 degree angle. Why make it that steep? Put on a complete rear bumper, and mount the diffuser to the underside of the bumpers edge. Ill add a photo of how we did on a 106 in a minute.

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Cameron

That's for a straight wedge shape, I'd try and keep it straight for the most part. You can go for a larger angle, but you start to gain drag over about 10 degrees. That doesn't really matter for your car though IMO as you're not reaching particularly high speeds, so if you can't package anything less than 10-15 degrees, don't worry about it too much.

 

Here's a good example of a hillclimb car using downforce..

 

gallery_6398_18_231374.jpg

 

150.jpg

 

Note the relatively shallow angle of the diffuser, it's a straight wedge too, also the side skirts in photo 2, that's what I was suggesting earlier. :D

Edited by Cameron

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wardy18

Ill get back on the measuring after work and see what i can come up with, dont really want the expense of buying new bumpers, if anything ill just extend the std valance somehow.

 

Thanks for the info, least im now on the right path

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Batfink
Ill get back on the measuring after work and see what i can come up with, dont really want the expense of buying new bumpers, if anything ill just extend the std valance somehow.

 

Thanks for the info, least im now on the right path

 

with that big hole you've cut in the back maybe a double diffuser is in order :lol:

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