Making your first few modified cars for racing is a lot of fun. It's fun to learn how to disassemble a car, customize it, and then put it back together. Seeing a car you "made" is always a treat. But at some point you're going to want to start winning and the novelty of just seeing your custom car "work" won't be enough.
Here are some tips you can follow to make your fast cars even faster. Of course, there's no guarantee for any of this but none of it will hurt...or at least, it shouldn't. Your mileage will vary but keep trying different techniques until you get the results you like. Be patient and expect to make a lot of cars that don't win.
This article was adapted from an original thread from 2014.
Start with a solid foundation
A good place to start is by finding the fastest stock car in your collection. It'll be your King and used as a control when you're building and testing your modded car. Your goal is to be faster than that car.
For your own build, you should start with a fast car too. Scout out castings that you know are already fast to begin with and source them for parts. Race cars from your stash until you find the good ones. You can use the Redline Derby Fantasy League archive of car ratings to help identify some older castings.
When scouting cars, look for wheels that don't wobble from side-to-side. Spin the wheels and watch the surface that will contact the track. A car with a longer, wider wheelbase is also a good place to start. Cars with narrow wheelbases will have a lot of wobble.
Choosing a chassis
Try choosing your chassis by deciding where the weight can and will fit. If you have a metal chassis, always deburr and polish anywhere the wheel may rub against the chassis. With a plastic chassis, smooth the chassis best you can along with a wash in warm soapy water then dry.
You should always test fit placement before you affix anything to the chassis...axles, weight, body. It's hard to undo anything, so like they say: Measure twice, cut once. Try using clay or putty to temporarily hold things in place if you need, but don't add too much because that weight might not be there when you use the JB or glue.
Wheels and axles
Even though they seem smooth, car wheels have lots of imperfections and they need to be removed. Sand the wheels while they're still attached to the donor chassis, or before you remove them from the donor car. Start with 400 grit sandpaper and sand until the car rolls smooth and sounds smooth on a countertop. Silence is golden! Then sand the wheels with 1200 grit paper until the wheels are smooth and shiney.
Next, try polishing the axle ends where the wheels will ride. Using automotive polishing compound and a Dremel with a cloth disk works well. Polish one end at a time being careful not to let the wheel slide into the compound. You don't always need to have FTE axles! If you can find them, great, but don't underestimate a non-FTE axle that is well polished.
Before attaching your axles, swap them from front to rear and flip them to determine the best and fastest arrangement. Do some dry runs against your fastest stock cars.
When you're ready to glue down your axles, make sure each wheel has the same amount of play when you're affixing it to your chassis. You don't want one wheel with more wiggle than the other. Use cardboard as spacers between the wheel and chassis when setting.
And don't forget a basic rule: Fast cars go straight. They do not hug the wall or bounce side-to-side. In many cases, avoiding it entirely is impossible but do your best to get things aligned. When you have just the chassis and axles in place, find a countertop or your track and adjust the alignment. Using an alignment jig to keep axles straight will help a lot. Making an alignment jig is a good side project that is quick and easy.
Make slight adjustments here and there but not too much as it will bend an axle and ruin your work. When you're happy, test it again with the car all put together. Wash, rinse, repeat...
Last but not least, put graphite powder on the axles/wheels. Do one wheel at a time and add it directly on the end of the axle and work it into the wheel opening with a small paint brush. Pull up on wheel and give it a spin, grinding the graphite in. Once all wheels are done, roll on countertop with slight pressure down to seat in the graphite. Do some more testing and before your put the car in the box, give it some more graphite...never hurts.
She blinded me with science
Each of those is something you can do to try and make your car a little bit faster, but let's not forget some of the science before what makes a fast car. I won't go into much detail here...that's what Google is for...but search and learn about gravity racing physics. Look up Pinewood Derby science and check out YouTube for some ideas. Learn about weight placement, wheel size, friction...the tips, tricks and theories used in Pinewood Derby racing applies to diecast racing too, it's just a lot smaller.
In the end, you might have straight wheels, shiny axles and all the science you can find, but remember that in many ways your car is only as good as the track it's racing on. In most cases, you won't have access to the tracks, so you'll need to do your best using photos of the track. You're only in control of your car, so focus on that and leave the rest to lady luck.
It's a learning process
Making a fast car is a learning process and an iterative process. Your first modified car probably won't be much faster than before. Don't get discouraged. Just keep entering races and you'll get all the practice you need.
And remember, this is supposed to be a fun hobby. Half of the fun is the journey to the starting line, let alone watching your car roll to victory. Don't lose sleep over your cars or your racing. Have fun, take your time and enjoy the hobby.