it's a high pressure zone at speed, normally, screen angle makes a massive difference to drag, so the raised bonnet changes the approach
have already raised the rear of the bonnet - easy as it sits on 5 pins these days - lets hot air out of bay and also improves the screen angle like you say Have enclosed the underside of the engine now from the front valance back to the subframe with a slightly modded 14 undertray from a Seat Inca diesel van. Pukka result. Have also blocked off all unnessesary front bumper inlets/holes and made fences behind the front bumper to direct air through the rad core only and not anywhere else. Measured up for flat floor last night - 1400mm from sill to sill so 1500mm wide ABS sheet will do the job nicely. Are going to fasten thin wall ali box section down the length of each sill underneath and then bolt up the abs sheet with heat mat (from gas boiler suppliers....cheaper than Demon Thieves) added along exhaust run. Rear will have a diffuser that I am designing presently in line with mod prod sprint regs having fun with this
I've been considering all that type of stuff too for the rally car ,but I'm wondering is it worth the hassle? @Prof , what speed does one need to be doing to make it worthwhile?
rob the bonnet will only let hot air out at low speeds. tramps came about 1/3 way up the windscreen with rallying the speeds are lower so you'd need bigger aero aids if looking for any downforce, remember old cossie escorts etc
I used to run against a chap with a pug 205 at aintree (a fast track - 120mph) who had about 170hp and he used to go through the speedtrap at the same speed as me (with 240hp) - 121-123mph I was quicker than him in overall time (I assume more drive out of corners etc) for the lap but top speeds were the same He had taken a lot of care with slipperyness of the car - blocked holes, fared air intakes in and flat floored his car with a small rear diffuser so I reckon it makes a difference at faster places - half a second in 60 is a lot in sprinting
tipping the whole car forward is good,for aero, but will change suspension geometry relative to road surface
mine currently sits slightly rear down to get more front castor at the expense of top end speed - most sprint tracks are not fast top end, aintree is the exception (maybe anglesey also actually) so hopefully with the mods I can have my castor and speed......
Hey rob, have you seen the pics of the red mk1 golf from the berg cup series, flat floored and on the rear of the wide aches has aero holes, i can't find the link to pics now. Did you see the article in TRC regarding this very subject?? fwd cars they suggested lower front splitter, no rear spoiler, flat floor and the vents on the arches to let air flow out the front inner arches, where they felt there was pressure built up. I'll try dig the mag out over the weekend, it was a dam good read.
that sounds like a good read - what edition was it ? I have most of them I think this is largely what I was thinking of - problem with fitting a splitter on the front is securing it to the car well enough - if it actually generates any downforce it might come off the bumper.....3" forward of lower valance and 1" at the sides - 2" step behind the bumper to give the venturi effect and extend the whole lot to front wheels - recommended material is 5mm plywood but this will need supporting somehow not just fastening to the lower bumper edge - hmm http://www.pts106.com/FB Aerodynamics Splitter.htm Got some designs now for 'flat' floor that isn't flat at all.....run along car bottom, curve towards ground to go over axle (speeds air up = low pressure zone under rear axle = downforce here) then into 'tunnels' as exits car (slows air down again) http://www.symscape.com/blog/secrets_of_diffusers abs 'skirts' on car sides to hold the air in - to a min of 40mm from the ground for mod prod regs red berg cup golf here post 17 - side skirts and rear diffuser apparent http://www.clubgti.com/forum/showthread.php?t=172523
Still not had time to look for that mag or issue number rob, but saw this pic over on dubforce.net that i though is of intrest.
If they were nearer the centre, i would say possibly to provide some exhaust cooling with minimum effect on the airflow under the car? Probably cooling for something.
Undertray from Seat Inca Diesel van - now fitted - fastens to chassis rails in engine bay, rear subframe and engages with rear of front bumper with 'fingers' on front of tray
hi, this seems like a good little thread, I've just started doing hillclimbs and sprints in my mk2 and the developments getting quite addictive already, not quite as far along as you yet though rob! anyways, i did a bit of aero work for formula student back in the day, from what i remember the expansion ratio and ramp angle of the diffuser is vital. its all about compressing and then expanding the air smoothly. Too steep a diffuser angle and the airflow detaches and becomes turbulent, rendering it pretty useless. Most sources we found and our own simulations put the optimum ramp angle at something like 14 degrees. The effectiveness is also controlled by the expansion ratio, i.e. the max height of the diffuser. On a road car thats going to be near on impossible to get the right angle along with a worthwhile expansion ratio. I'm not saying not to bother though, a flat floor is definitely good and any diffuser is better than none but you've got to weigh up whether its worth the weight penalty. also, people neglect the front of the car, the venturi effect comes from the expansion AND the compression, the more air you can ecourage underneath the car smoothly the more effective the diffuser becomes, thats why modern formula racers have raised noses. All of this of course depends on the car and skirts being low enough to take advantage. personally I think the best gains are to be had from reducing drag at the front caused by cooling. I'd love to use a small but thick radiator with efficient ducting and blank everything else off. Part of the reason the raised bonnet hinges are so effective is because they release the positive pressure in the engine bay! phew....hope some of that helps cheers
mec82, you can expand the diffuser outwards too so maybe not 14 degree up but some combo of up and out could work
Mate of mine fitted a diesel under tray to his Corrado 16v, he said the under bonnet temps shot up, dont suppose itll affect you with the short sprints
that car on it's side, there is a similar pic of a bercup car that has rolled & shows it from underneath...... just finding it on here is the fun bit.
@ daz - I have a temp probe already in there to check the under bonnet temps - my exhaust is very well insulated with 4 layers of ceramic so hopefully that will help - and theoretically hot air rises.....rear edge of bonnet being lifted will help also - engine breathes through its own external ducting so power wont suffer from a small heat rise we will see @mec82 and prof - optimum diffuser angle is 4-14 degrees I have read - initial idea is just to try and smooth the air flow for less drag and also put side skirts on to try and reduce rear end lift at speed - diffuser front and rear will come though through trial and error ps - excellent book on car aero - competition car aerodynamics by simon mcbeath
I found this link that "luke w" put up before: http://www.blackmarketmotorsports.com/Forum/viewtopic.php?p=29&sid=899c57ad24d9173ba75dd8b70f7f4fbc & after a bit of searching i also found that pic i was on about:
