Tag Archives: thinking about building cars

Take cover, I’m thinking again

I’m thinking about the Sprint-T again, except I’m “thinking” with a bunch of catalogs and web pages open at various rear axle kits. I’m comparing prices and features for the Ford 9″ kits compared to the two quick change options. Now if budget was not a concern, like if I won the lottery, then my choice would be the 8″ ring gear quick change with the magnesium centersection polished in clear powdercoat, with magnesium bells and aluminum tubes, and the aluminum locker differential. This is the best balance of weight and performance. There are other options that are lighter, but at a cost to performance. There are options that have a slight potential for better performance, but at a major weight penalty.

And then there are the less expensive (can’t really call them “cheap”) options in the Ford 9″ housings. The main thing against these is because they are made from steel they weigh more, in some cases a lot more. But they are way less expensive, $1K±, compared to the $4K± of the “no budget” option in the first paragraph. This is a hybrid option, a race housing, hubs, and brakes with a “performance street” centersection, ring gear, and differential. Now for both of these the aluminum spool is lighter and cheaper than the differential, which means if I’m not seriously considering it there has to be a performance cost. Yes there is a loss in performance in the intended use with a spool, locking the rear wheels together makes the car understeer in tight turns that are part and parcel of autocross. And understeer is SLOW in autocross.

Now how much performance difference is there between the $4K rear axle, and the $1K rear axle? Maybe a few hundredths of a second, maybe nothing, depending on how smooth the track is. The rougher the track surface is, the better the $4K axle compares to the $1K one. And of course there’s the difference between a polished quickchange and a painted fabricated housing, the polished quickchange wins every time. But this isn’t a show car, it has a mission. Looks are important, but looks that don’t improve performance are expensive nonsense.

(Aside about that last word, it took forever to come up with “nonsense” because for some reason my brain wouldn’t go there, I got “ostentatious”, and “frippery”, but not “nonsense” until I started browsing thesauri online. I even knew the word started with the letter “n”, but going from “n” to “nonsense”… too far until I found the right thesaurus)

Anywho, there is another thing the $4K axle does is allow adjusting the final drive ratio from race to race and also for highway cruising between events. I mentioned this in a previous post, but it bears repeating that with the quick change I’m not stuck with a 4 or 5 speed overdrive automatic, I can get a 2 or 3 speed that might be lighter or cheaper. In other words if I spend an extra $3K± on the axle I can save a couple hundred on the transmission, or what’s really important, I can save maybe 150 pounds in the weight of the transmission. In actuality the weight savings are more like 60-70 pounds as Powerglides are about that much lighter than 4l60es unless I really need the 4l80e in which case then we are looking at over 100 pounds weight savings. Now 100 pounds less weight is a big chunk out of a car that will weigh less than a ton with driver and race fuel load. If I got my sums right, we are talking about under 1800 pounds on the starting line with driver and race fuel load using an aluminum block engine and PG transmission.

Also the $4K axle is maybe 20 pounds of unsprung weight lighter than the $1K axle. Now 20 pounds in a car that weighs less than a ton is not insignificant, but 20 pounds unsprung weight? That’s a bunch, particularly in a car that has as bad a sprung/unsprung ratio as the Sprint-T. And one of the things that has a major effect on handling on rough pavement is having a high sprung/unsprung ratio, unsprung weight is bad for keeping tires firmly in contact with the ground.

Thinking about the Sprint-T again

Not a big think, because it’s a small but important thing. What I was thinking about was what to do about the top of the frame?

Just in case you missed it, there will be a structural bellypan welded to everything on the bottom of the frame for aero and structural purposes. The problem is that leaves an open space to catch water and debris that will lead to rust spots as the frame ages. Possible solutions include moving the pan to the top of the bottom frame members which has the advantage of making the body mount easier, but which requires mounting the engine 1.5″(38.1mm) higher and also leaves a nasty lower area as far as aero is concerned. Plus that just moves the rust out area someplace harder to inspect.

Anywho, what I was thinking about was using some of the HDPE plastic I have for the fenders and hood to cover the exposed gaps in the frame. Then as I was thinking about the bellypan on top of the frame it occurred to me that it would be much easier to make access panels from HDPE than from steel or aluminum, and I wouldn’t need to paint it. And if I did the HDPE on the bottom I could just hotknife the hatches from the HDPE and be able to reuse the cut piece as the hatch because a hotknife has a very small kerf. For the Mini Sprint-T it wouldn’t make any difference because I would do the same thing for either one, glue a piece of 0.01″ styrene to the frame between the tubes. Visually it wouldn’t be any different on the model, a flat surface outside the body is a flat surface outside the body. Flat is flat, model or 1:1 scale. The main difference is if I put the solid bellypan above the rails I would paint it to match the body color of Omaha Orange aka Schoolbus Yellow, and the HDPE is a different kind of yellow. On the bottom the difference would be the steel pan would be black and the HDPE would be the same yellow as the top, because it’s cheaper that way. Or I could go with the black HDPE because it ain’t that much more expensive, and I’m really not quite that big a tightwad. Or getting back to cheaper, the flanges and bits to mount the HDPE plastic might be enough to make the part of the frame that goes under the body smooth might be enough to not need to weld the pan to the frame for stiffness, and I could just go with HDPE top and bottom and save a few ounces. Not to mention HDPE is cheaper than steel for the moment because of the tariffs. We make HDPE here in the US so no tariffs.

Well, this is the second post today, and there was a lot of stuff in the first post what with all the pictures, so I’m going to bliss out to my trance mix and get some meditation in.

Foot problems are keeping me off the computer

Can’t take too long this time. In a nutshell I have been spending too much time sitting down, which makes my feet swell, which then makes my feet hurt, and so far the only relief I have found is lying down and elevating my feet.

And while I’m lying down, I think. It’s almost a reflex by now, the feet come up and the brain goes into gear. Since I’m lying down during the day I have light to read by and I bring a Speedway catalog with me. This at least keeps my mind from wandering off on tangents instead of solving problems that need solving for the least amount of money possible.

Something else I should mention is I won a $25 gift certificate to Speedwaymotors.com that had a limited lifespan, and I wanted a shift handle so I got the one in the link. The gift certificate covered all but $0.38 after the handle, shipping, and sales tax. So I have the style shift handle I always wanted for $0.38 out of my pocket.

And I just faceplanted in the keyboard again, so I’m calling an end to this post and time for bed. I guess I shoulda said I haven’t been sleeping well to go with the foot problems.

I’m awake during the day but still thinking

Ordinarily “Still thinking” would have the caveat “Take cover” but not so much today as what I’m thinking about is another A/MOD SCCA car. Basically what I’m thinking about is an LS style engine and a Powerglide transmission smack in the middle of a 72″ wheelbase and offset enough to the right to balance a driver to the left so the total polar moment is as low as possible for good transient handling. The chassis would be a semi-monocoque tub with a tube roll structure and subframes to tie everything together and if I did my sums right I’m looking at a 1000 pound as-raced weight, or about 100 pounds over the minimum. Combine this with the power and torque from the LS architecture V8 and forward progress would basically be limited by available traction, which would be limited by tire compound choice and footprint which would be limited by tire and wheel size, which would be limited by what’s available and unsprung weight considerations. So, basically a beastly quick car that was more limited by human reactions than the laws of physics.

The aluminum block LS engines all weigh within a few pounds of 415 minus the engine driven accessories. When those are stripped to the configuration for running in this car, which is basically water pump and alternator, I need to add those in and we’re looking at 450 for the engine. I have been watching enough videos to know that what I need for this car is a mild cam, a long runner intake manifold, and either tri-y or long tube 4 into 1 headers to get enough low end power that also doesn’t fall off as the RPM rises to redline, which are all included in the 450 pound engine weight. So add in the 76 pounds of Powerglide and you have 526 pounds of powertrain to moment out for left-right balance with the roughly 200 pound driver. Basic math tells us the driver has to be about 2½ times as far to the left as the engine is to the right, add in the widths of the engine and the driver’s legs which have to go to the left of the engine and a little algebra gets us the driver has to go 27.5″ to the left of the engine which is slightly to the left of the left side of the car. Going back and letting the top part of the engine hang a little over the driver’s legs and we get 24″ left offset if the engine sits with the bellhousing on the centerline. Again this places the driver outside of the car. Going to the minimum width of the driver the driver centerline has to be at least 7″ left of the engine minimum width which is 8″ to the left side of the engine so driver is -15 moment arm or -3000 moment to the engine. Solving for x gives us 3000/526 which is 5.7″ right going to the level of precision limit of the driver’s seat mount. The driver sits 9.3″ to the left of center, 15″ left of engine centerline. But doing the moments we get a moment of -1860 for the driver and a moment of 2998 for the engine and transmission which means the driver is not far enough to the left and the engine is too far to the right. But if the driver is 14.991″ to the left of center then the moments are equal which means I set the equation up wrong.

Going back to first principles (526*x)+(200*y) = 0 and also y = x-15 because the driver must be 15″ left of the engine, minimum. So substitution gives us (526*x)+(200*(x-15))=0 gives x=500/121 and y=15-(500/121) making x=4.13″ and y=10.87″, and dividing out gives the exact 2.63 ratio in moment arms. Which means I got the formula right this time. My butt would be just to the left of the bellhousing of the Powerglide, and my legs would be under the left cylinder bank with the exhaust manifold radiating heat over them. Bad for a street car, but for this it would be warm but tolerable. Combining my shoulder width with the offset the narrowest the car could be is 2*(10.87+11.25)=44.24 which means my right arm would be on the right side of the car… And the car would be 44¼” plus the width of the tires wide and 72″ plus the diameter of the tires long, aka tee-niney, aka a V8 powered gokart.