I'm actually reposting a good topic question that was brought up by someone in another forum I frequent. I figured you guys would have special expertise in this subject:
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I was just about to post this in the '07 Houston ALMS thread. But I think that it deserves it's own topic, as opposed to reviving a long dead one.
As we all know, the Audi R10 has a massive wheelbase compared to the R8(117.3in+/2980mm+ for the R10 vs 107.9in/2740mm for the R8), but what does that mean? Well, it implies that the R10's engine is long and heavy. It's estimated that the R10's engine is about 29-30 inches long, vs 20.5 for the R8. But the Ford Windsor SVO engine that Panoz and other teams used is as long, and those were used in cars with wheelbases slightly longer than the R8's. And the McLaren F1's BMW V12 is about as long and heavy, and it was used in a car that had a 107 inch wheelbase-shorter than the R8's.
This isn't really news, but this is. Guess the wheelbase of the Porsche RS Spyder. If you guessed 108-110 in., you'd be way off. The RS Spyder's wheelbase is over 114 inches! Not all that much shorter than the R10's! And the RS Spyder did just fine.
Read post #53 and 58 here: http://www.ten-tenth...t=100010&page=4
It basically says that the wheelbase lenghts of the R10 are dictated by the ACO's and IMSA's areo rules more than anything else. The R8 was 4640 mm long, with 900mm front overhang(from tip of front diffuser to front wheel center line), and 1010mm rear overhang(from end of tail/rear wing to rear wheel centerline). The R10 is estimated to have a 920mm front overhang, and a 750mm rear overhang(the ACO/IMSA maximums are 1000mm front and 750mm rear), and for comparison, the RS Spyder is supposidly running at the ACO/IMSA max for overhangs.
If the R8 had the R10's overhangs, it's length would shorten to 4480mm, which isn't good for aerodyamics. The results would be a lot of drag(due to running more downforce to improve stability), or a difficult to drive car.
Just though I'd make a comment as to why the R10 is so big demensionally, and that weight is the man factor in the LMP1 vs LMP2 debate.
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My thought was that aerodynamics, insofar as drag was concerned, was not so much an issue, but rather one of static weight distribution (where the engine and CoG sits in relation between the two axles) and polar moment of inertia, as well as the dynamic counterparts of weight distribution and polar moment (where the downforce of front- and rear wings at the car's extremities come into play).
Can someone shed more light on this, where I've got my reasoning wrong or incomplete, etc.? My post in reply, below:
A couple of little things to add. The overall dimensions and overhangs of the LMP cars are, as already stated, governed by the ACO. Within this framework, the car manufacturer can select the wheelbase and other dimensions.
I believe the main reason for the extremely long wheelbase of the R10 has two main facets. One, as also already stated, is due to the length of the engine. But I believe this is secondary, because the vehicle designer has to some extents a choice of either moving the passenger compartment forward, or use a transaxle that allows the engine to be moved back relative to the rear wheel axis. The primary reason for the long wheelbase is because, if the engine is at a given location relative to the rear wheel axis, then changing the wheelbase affects both the weight distribution and the polar moment of inertia of the car. In this case, lengthening the wheelbase causes the weight distribution to bias more to the rear, and polar moment increases. This may seem counterintuitive; usually you want a balanced weight distribution and low polar moments. A high polar moment makes the car feel more stable but less manoeverable. However, vehicle dynamics is a far more complicated science than these oversimplications do justice. F1 cars have seemed to converge to a point where as of 2000 in Peter Wright's Ferrari F2000 book, he suggests an ideal static weight distribution of about 60 percent rear/40 front.
I fail to see how wheelbase affects aerodynamics, insofar as drag is concerned, but it certainly affects, again, force distribution and polar moment. Ideally, you'd put your front and rear wings at the extremities of the car length and have the max amount of overhang, since the distance is the lever arm length from which the downforce acts and is reacted by the normal force at the wheels. That's why maximum overall lengths and overhangs are limited, or else teams might go with short wheelbases and incredible overhangs to take advantage of this.
Paul van Valkenburgh covers this subject in great detail in his book, Race Car Engineering and Mechanics. I need to give it a read again and get back to this thread if I should make any corrections to what I've said above or can shed any further light on this.
Since the R10 apparently interests you so much, have you already read this?
