04/18/09 Today being a Saturday and a nice day (calling for an Atom run of course) we only did a bit of work. Trimmed the test bodyshell and plopped it on top of the wheels to see what it would look like as a complete car (I still have a picture on my wall from when I did this with the dp1).
Ok, I'm motivated. This thing will be a blast. That is all.
04/29/09 Work is moving full speed ahead on the dp4. I've decided to go with a tube frame and molded composite underbody for the first release. This decision is driven by several factors - time to market, cost and SCCA homologation requirements to allow the car to race in DSR class (in RWD form only of course). It will be an interesting comparison with the composite dp1 chassis. So far Solidworks says 71 lbs for the frame but of course it is not complete and we're looking at closer to 100 lbs when done. Then add 30 or so pounds for the underbody. That will make it about even with the dp1 chassis which is roughly a foot longer but hopefully the dp4 approach will cost less.
I'm now on about 8th or 9th iteration of the chassis, some of which were complete re-designs. That's all part of the process.
Simultaneously with finishing the design we are preparing for its fabrication. We're looking at what fixtures and jigs will be necessary, among other things. One required step is finally leveling the table. For what we've been doing so far the fact that it wasn't perfectly level hasn't been an issue. Going forward it will matter. So we spent most of the day getting the thing set up - taking apart every leg, greasing the adjustment plates and screws, then putting it back together. In the process we took out a whole stack of shims. For some reason they were in every leg.
Now that it's done and once the same can be said of the design, we should be ready to build stuff. It'll be fun (and frustrating at times, I'm sure, if history is anything to go by :)
05/10/09 One benefit of countless iterations of chassis and underfloor is that I've managed to find an extra 3" of legroom (no small feat in a car with a 68.5" wheelbase!) so I was able to lower the driving position and get the driver's shoulders below the bodywork. This in turn allows lower and simpler rollbar, less drag, lower CG and generally sleeker looks. Time well spent then.
Here's a comparison of an earlier rendering and a current SolidWorks screen shot:
To test this in real life Tristan built a simple frame under the bodywork out of 2x4s and Larry loaned us a race seat, so we've had a number of people actually sit in the car. It's quite roomy.
As with everything I do there are quite a few experiments in this car, especially in terms or aero. Having learned that eyeball aerodynamics is akin to playing the lottery I'm in discussions to get some proper CFD done on the proposed undertray (well, there are actually 5 or 6 versions under consideration). I am also gathering advice and opinions from people far more experienced than myself. Hopefully I'll be able to arrive at a configuration that works reasonably well to start with and refine it from there.
Since in RWD form the car will be eligible for DSR competition, the effectiveness of the aero will determine whether it's just a nice low-cost entry level racer (a trackday car you can also race) or an all-time bargain :) The bar is set very high in that class with front-running Stohr cars being only 4-5 seconds off LMP1 times at PIR so I'm not underestimating what I'm up against. Of course whatever I end up with for the first attempt will not be set in stone and the overall chassis architecture allows a lot of opportunity for different aero configurations. So even if I don't hit it spot-on the first time there will be a path to move forward. But it would be cool if it were competitive right out of the box. Gives me something to shoot for.
06/14/09 Some of the preliminary CFD results are in, thanks to Richard at AlignedCAE, so I thought I'd post some initial observations.
With the asymmetrical nature of the dp4 and the proximity of the driver to the trailing edge, one would generally expect some aero consequences. It is interesting to see just how much effect there is though - on the driver side, the diffuser flow is not only stopped but reversed for a good portion of it due to the turbulence behind the cockpit (all images courtesy AlignedCAE).
The last image in particular shows the effectiveness of strakes - the center section of the car, between the two strakes, maintains low pressure a lot further back. The 'dual phase' diffuser that's present in the center only is also interesting in regards to the additional low pressure generated and shows its potential as a tuning tool.
This is only a first set of results, at 50 mph and zero pitch, but it already shows several areas that need work. I definitely have to figure out a way to reduce the effect of the cockpit on the airflow - the 'fairing' behind the driver is doing a lot more harm than good and if I were doing it again I'd leave it out. Now I need to figure out how to make it work for me and not against me. The data also shows that additional strakes in the diffuser could be quite beneficial.
It is pretty easy to see how the visualisation is helpful in anticipating and (hopefully) addressing any issues. If I had to figure out what's going on without CFD to help me it would be a lot more difficult, a guessing game at best. Finding solutions and optimizing the design is still up to me (although there are some numerical optimization techniques available, they are outside the scope of what I'm doing here). This at least gives me an idea of what needs to be done, where and why.
There will be more posts as work progresses...
06/24/09 We are now building the chassis. Suspension arms are first because the fixtures are already done and it's an easy part on which to sort out some processes and test the tools.
Speaking of tools, the second picture shows the DeWalt 18" cold saw - the thing is awesome. Fast, clean and accurate. Cuts through chromoly and stainless like it's nothing.
After we get the A-arms built the chassis fixturing and then the chassis itself are next. I'm thinking of trying black zinc coating on the arms instead of powdercoat - will be curious to see how that works out.
06/25/09 While we're working on the chassis and suspension, the first real dp4 body is now out of the mold at Contactscale. Here are a few pictures from Contactscale's new facility at Moffett Field, with the dp4 body sitting in front of Hangar One.
One of the world's smaller cars in front of one of the world's larger buildings. Kinda neat.
Work is also progressing on the underbody mold. More on that later.