07/05/09 As work continues on building the chassis and other things, I thought it might be interesting to reflect on the dp4 design process. The latest chassis revisions in particular, along with actually having several people sit in the bodywork mockup, have enabled positioning the driver a lot lower (lower still now than the last time I posted about it). The top of the rollbar went from 50" to 43" above ground and the overall car went from a little goofy to reasonably sleek.
As is often the case, the more we dig into the details the more there is to do. But things are definitely moving forward.
07/16/09 An amusing comparison - we've been moving things around the shop in preparation for fabricating the dp4 chassis and just happen to have a classic Mini Cooper sitting there for a few days. Seeing the dp4 shell next to the Mini next to the truck is kind of neat.
The dp4 is about a foot shorter than the Mini, in both wheelbase and overall length. Just a tad wider though :)
07/21/09 Some more incremental progress - the suspension arms came back from plating. I know, we were hoping to further along at this point but things ARE happening. On all fronts.
More to come.
By the way, Palatov Motorsport is now a dealer for Tilton (clutches, brakes, etc) and Aurora (bearings, rodends). E-mail me if you're looking to buy their products and I'll gladly give you a quote.
08/06/09 The chassis jig is finally being built. We've had a lot of delays and distractions (we did get a LOT of other things done so it hasn't been entirely unproductive) but now it's crunch time. The layout table is really paying off here - having a flat, level, SOLID surface with accurately placed tapped holes is awesome for this kind of stuff.
It may not look like much now but that will be changing rapidly. A lot of the other components are already done so the hope is that the car will come together in short order. We'll see.
08/11/09 Work continues. The chassis jig is moving along.
I've also picked up a set of Hoosier slicks and got them installed on a set of black wheels. These will be more appropriate shoes for this car than the R-compounds I had on the dp1 prototype.
08/14/09 (updated 08/15) The chassis is quickly starting to look like something...
It's a compact device. Should be pretty stout though and there are ample crush zones outside the 'survival cell'.
On another topic, I've been looking all over for a suitable steering rack. The Stiletto one I used on the dp1 prototype, while cheap and available, leaves quite a few things to be desired. It developed a good deal of play very quickly and is not well suited to the off-axis forces that are the consequence of the tight packaging in this car. The Titan racks are expensive and very long lead time in the custom size I would need, the Woodward racks are heavy, expensive and are known to develop play as well. So I'm left with designing and building my own. From the sound of it, there may be a market for a good, reasonably priced light car rack out there. My target is as follows: 12" wide (means about 14" between tierod pivots, extendable somewhat if need be with longer end eyes). 3.75" max total travel. 2.0" travel per turn (so 1.8 turns lock to lock, this could be limited with snaprings). It will be adjustable for backlash with an eccentric adjuster to eliminate play. Of course sealed bearings, precision shaft/bushings/gears, high strength heat treated steel rack and pinion. Aluminum housing. So far it's looking like about 3.5 lbs and I'm hoping to be able to sell these for under $400. Email me if that's of interest. I'll have pictures available in a few days.
UPDATE: The initial design of the rack is pretty much done, now it needs to go out for quotes. There may be some minor tweaks from vendor feedback but I'm basically happy with it. It took quite a few revisions to get the travel range, full bearing support, adjustment range and simplicity (the raw rack and pinion picture is an early version, there have been lots of detail changes since then but it illustrates the concept well enough).
The mounting is by clamps on the cylindrical portion of the housing (not with the bolts at the bottom like Stiletto racks, in this case the bolts just hold the housing together). The final numbers: rack width 12", housing width 7-3/4", total travel 3" total turns 1.5, total weight 3.25 lbs. Backlash is eccentrically adjustable. Boots are optional, depending on how exposed the rack is they may or may not be needed. I'm only planning one pinion size because the overall steering ratio is easily changed by varying steering arm length - at least in the applications that I'm targeting which is formula cars and sports racers. For reference, a 3" steering arm will give a 9:1 overall steering ratio (30 degrees at full lock), a 3.5" steering arm will give a 11.6:1 ratio (23 degrees at full lock) and so on. These are the two values i'm planning on for the dp4 and dp1, the former for autocross and the latter for track.
08/16/09 A bit more refinement on the rack design. It's actually fairly elegant, if I do say so myself.
The last picture is a quick comparison with the Stiletto rack (also made by a wide variety of vendors for dune buggies, junior dragsters, etc.). My design is significantly more stout and a little wider. The major differences however are a provision for eccentric adjustment and a much more stable and flexible mounting scheme, allowing mounting the rack at any angle.
Cost is the only unknown now. Should have that in a week or so.
08/18/09 As the chassis is coming together and after staring at it in the jigged state I've decided to change and simplify a few things. In this regard delays are actually useful - they provide more time to contemplate alternatives. In this case the issue had been primarily with inboard brakes. The main advantage is the fact that unsprung weight is lowered considerably (by about 8-9 lbs per corner). The disadvantages are many and packaging is one of them. I just didn't like the solution I came up with for the suspension and diff/brake packaging.
So I finally decided to bite the bullet and redesign the car for outboard brakes. MUCH cleaner now, better in RWD form and only requires minor mods to a couple of the parts that have been already made. The current suspension arms are beefy enough to handle brake torque (wasn't the case on the dp1 prototype). The brakes are 10.25" vented rotors with 4-piston Wilwood calipers front and rear. Probably overkill for a car this light but the resulting low temps mean that I can likely use BP-10 compound pads and have them last basically forever. Low maintenance is good.
The suspension boxes are the same front and rear (except for steering rack) and just bolt up to the frame as complete units. Makes it very easy for servicing and repairs if need be. SolidWorks says the basic frame is 78 lbs so depending on how light I can make some other things the target of 650 lb dry weight seems doable in RWD form (and would only be about 50 lbs more in AWD).
08/20/09 Doing some refinement on suspension boxes prior to having them made. It's actuallycoming together rather nicely.
With just the raw flat panels the weight started out at 40 lbs, it's down to 22 lbs now and when I'm done the target is for under 15 lbs per box. that would make the total frame about 100 lbs which is workable. By comparison the larger and somewhat overbuilt dp1 chassis, with rollbars, is about 130 lbs. The overall weight targets for each car are 650 and 850 lbs respectively and while initially I expect to be a bit on the heavy side on both, subsequent iterations and improvements should bring that down.
I've been getting many emails from people contemplating designing and building their own cars. My advice is always to start by building a kit first, like I did with the Westie, because it's a great learning experience. And of course the dp4 will be available as a kit... :) Full kits (minus engine/wiring/wheels/tires) will start at $19K and I'm also considering offering kits without bodywork for quite a bit less than that. But for those rugged individualists who just have to do their own thing, I will be selling uprights and suspension bits. There will be two 'grades' of uprights - one for ultralight race cars under 1,000 lbs (basically dp1/dp4 parts) and one for streetable cars of up to 2,000 lbs and over 600 hp (basically dp2/dp3 parts, an evolution of the modular design I've posted on the Atom page). The kits will be complete with bearings, drive flanges, and optionally CV joints and axles. Stay tuned for details, they are getting finalized as this is written.
08/27/09 (updated 08/28) The frame is now being welded for real. I better get the suspension boxes finished and parts on order!
The welds are as complete as can be done while still in the fixture, the rest of it will have to be done when we pop it out. Maybe tomorrow.
The chassis is now out of the fixture so we can complete the welding. after that is all done the brackets that attach the suspension boxes will be welded on last, to make sure the suspension is correctly aligned front to rear and eliminate effects of any warpage during welding of the chassis itself. The frame does weigh about 70 lbs, just lifting it - will put it on the scales later. Rollbars are still to be fabricated and attached.
Plenty of room in there - even though the car is very small it should accommodate 95%+ of the people out there.
Just remembered that I have a picture of the dp1 prototype frame in approximately this stage of construction - from December of 2004. Time flies... But I guess I don't change much :)