Diffuser for rear of Griffith
Discussion
Excuse my ignoranance,but isnt a difusser spose to keep the air running straight for lateral stability,We have put a alloy splitter running from the diff back(where the chassis turns up) from a flat alloy undertray and although I haven't driven the car,the difference is apparent
GreensV8 could you put in laymans terms how a difusser could create lift.
GreensV8 could you put in laymans terms how a difusser could create lift.
GreenV8S said:
The splitter is well worth having of course, and a pretty safe bet because it will always work if you make it big enough. I suspect you're getting massive gains from the big bonnet vent you have and these probably outweigh the other features by a large margin. That area of the bonnet is one of the lowest pressure areas on the car, and connecting it to the underbonnet area means you reduce the suction above the bonnet, reduce the pressure below it, and this will carry back to a smaller degree over the whole floorpan. You only need a small pressure difference over a big area like that to do something useful. Even with that sort of thing you will be lucky if you manage to overcome the lift let alone produce net downforce, and you will need to be well into three figure speeds before the benefit is enough to matter. And if you do start to get significant forces you need to worry about the balance as well as the amount, many of the bigger easier wins that produce downforce at the front can result in increased lift at the rear. For a road car I think the performance benefits of aero mods like this are negligeable and the only reason to do it is for the looks.
Overcoming lift was the main goal and every pound of lift that is removed is effectively a pound of downforce gained. As for three figure speeds. Not far off. The car used to go a bit light at around the 90ish level and that has gone. It is rock steady at silly speeds. The rear has a big spoiler/gurney flap on the boot lid which does a pretty good job of keeping it balanced. Learnt that the hard way when the boot lid was removed at Goodwood at around 145. The catch hadn't fully clicked and the lid was ripped off. The car without the boot was quite scary. So a Nascar type gurney flap was added and balance was restored.
Yes the car is a competition car and I do agree that for road use, the benefits are not that apparent and are debateable whether they are worth it. There is also a big pschyological factor as well but doing blind testing is a bit difficult.
People are going to start shouting.................
The basic problem with the Griff and Chim is the shape of the car, you need round edges at the front and sharp edges at the back of the car to disturb the airflow. The easiest way to stabilzer the them is to add a vertical piece of material 30mm tall across the whole width of the front of the vehicle infront of the wheels and then some thing on the boot to stop the air going down the rear of the car and underneath which causes the rear end lift.
I have seen aero figures for a Griff that show its rear end lift at medium to high speed to be very very bad (I think it was the worst vehicle they had tested). The chim was a little better but not much.
The basic problem with the Griff and Chim is the shape of the car, you need round edges at the front and sharp edges at the back of the car to disturb the airflow. The easiest way to stabilzer the them is to add a vertical piece of material 30mm tall across the whole width of the front of the vehicle infront of the wheels and then some thing on the boot to stop the air going down the rear of the car and underneath which causes the rear end lift.
I have seen aero figures for a Griff that show its rear end lift at medium to high speed to be very very bad (I think it was the worst vehicle they had tested). The chim was a little better but not much.
sjpalmer1 said:
...then some thing on the boot to stop the air going down the rear of the car and underneath which causes the rear end lift.
I have seen aero figures for a Griff that show its rear end lift at medium to high speed to be very very bad (I think it was the worst vehicle they had tested). The chim was a little better but not much.
I have seen aero figures for a Griff that show its rear end lift at medium to high speed to be very very bad (I think it was the worst vehicle they had tested). The chim was a little better but not much.
Think I saw those too, 0.344 as the drag coefficient?
Agree on the boot spoiler although I can't imagine how it would work without messing the looks up totally
The drap coeff can be reduced by the addition of the front upstand. This prevents / reduces the amount of air hitting the front tyres. The tyres can account for upto 20% of the overall drag.
By adding a very small lip spoiler to the rear will reduce the rear end lift, should be alot easier to make work. For a rear diffuser you will have to look at the whole underside of the vehicle and manage the air properly. Even the very sills influence how the air travels underneath the vehicle.
By adding a very small lip spoiler to the rear will reduce the rear end lift, should be alot easier to make work. For a rear diffuser you will have to look at the whole underside of the vehicle and manage the air properly. Even the very sills influence how the air travels underneath the vehicle.
This isn't terribly scientific but I do know that the rear of a Griff produces plenty of lift in the negative pressure area *above* the boot too.
Pulling out of a German service station without properly closing the boot catch saw it rise fully up like an air brake at about 100mph - and it takes quite some effort to do that as the single gas strut is not balanced as in modern hatchback tailgates.
Whilst this might not have looked very impressive to the SLK I had deafened by steaming past, it makes me wonder if the top of the bootlid is a better place to start than the rear undertray? This is my favourite styling area on the car so I wouldn't touch it anyway!
dumbfunk
Pulling out of a German service station without properly closing the boot catch saw it rise fully up like an air brake at about 100mph - and it takes quite some effort to do that as the single gas strut is not balanced as in modern hatchback tailgates.
Whilst this might not have looked very impressive to the SLK I had deafened by steaming past, it makes me wonder if the top of the bootlid is a better place to start than the rear undertray? This is my favourite styling area on the car so I wouldn't touch it anyway!
dumbfunk
Although the Griff, Chim etc all look fantastic I beleive that they all suffer from the same problem, They were designed with 'form' first instead of 'function'.
If they had have been done the other way round they would work properly. Its much easier to make something that works look good than make something that looks good work.
If they had have been done the other way round they would work properly. Its much easier to make something that works look good than make something that looks good work.
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