Daily Archives: February 9, 2021

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.