Advice on modifying car for districts please

[BADsDAD]

Hi All! First post here but we have followed advice that we read here as well as other online sources to build our first cars, one for each of my two boys. I wanted to say Thanks for all of the awesome info here!

We started by baking the blocks for 4hrs at 250deg. Weighed before and after, this dropped about 5% of the original weight, which surprised me a bit.

The pack rules allowed canted axles, so we drilled the rears with a purchased jig @-3deg and bent the DFW positive and adjusted for 3"@4ft.
RF is DFW and LF is raised about 3/16". We used dremel to slot RF axle for adjustments and then used loctite to secure once we were happy.

Cars are built with 5.25" WB and COG is about 1.25" forward of rear axle. I know this isn't terribly aggressive, but I worried about track quality as it was an old aluminum over wood framed track that had a reputation (per the scout leader) of cars jumping lanes, though none did this time.

Weight is 11/32" tool steel rod inserted into cars about 1/4 in front of rear axle, with the addition of a chamber in front of that which allows for minor adjustment with splitshot fishing weights or BBs. Cars weighed right around 4.97oz at race time.

We deburred and polished axles to 2000grit and then used the buffer and compound from my polishing kit for the finishing touch. We spent hours getting all eight of them to spin 20-25secs on PinewoodPro graphite. (I didn't read about Lemon Pledge until after)

Here is one of the 2 identical cars we built - (hope this link works)




We just took 1st and 2nd in the pack with ease. Next closest car was the scout leader's son's car which our slowest beat by at least 3-4 car lengths every time. Both boys now go to Districts, but the rules are requiring that we make a couple of modifications to their cars and here is where we need your help.
I asked my boys if they wanted to build new, but they really want to race their current "lucky" cars.

So, here is the issue -
New rules state:
-No canted/tilted axles
-3 wheels on track, 4 must exist on car
-Wheels must run flat on tread but "slight adjustments to axles allowed to make car run as needed"
-Wheels can only be "cleaned up" but not turned down or have tread sanded.
-Polishing axles is allowed

So now we have ordered new axles and wheels from Revell and are awaiting their arrival.
I will need to drill new axle holes in front of the existing rear since the holes are -3deg. I will use the jig and go forward about 1/8" to stay as far back as I can and still out of the weights (but man am I getting close here).

I will need to replace the DFW axle as it is bent and slotted, but with slight adjustments allowed per the rules, it sounds like I may be able to still rail ride? Thoughts on this?

Any advice you experts can give to make the rears track out to the heads but still ride flat on their treads??

Thank you!!!

[ironband]

A few things here:

1. Revell wheels are heavier than their BSA counterparts, and therefore slower on a normal sized track.
2. Revell axles, while having a better surface finish, are smaller than their BSA counterparts, and therefore slower than a properly prepped BSA axle.
3. Most BSA axles have a slight bend in them out of the box.

Crazy thing with BSA axle prep that I have recently learned is not to file the crimp marks off the shaft. Remove the burrs under the head, then sand the head with 1000-3000, and the shaft with 2000-3000 and then hit it with Brasso or some other metal polish. Spray pledge, then wipe off. If you spin them in a drill you can see which ones are the most bent.

For the rears, if you want you can fill the old holes with toothpicks and wood glue, then redrill in essentially the same spot to avoid your weights.

I've never done it, but I have heard tell of folks who have made rail riders with flat running wheels. At least you don't have to have the fourth touching, that makes it really tough! It'll take a lot of tuning, but I think you can leverage the natural bend of some of the axles to give you some movement to the outside on the rears and steer to the rail on the front. In a worst case scenario, you can use the old wax paper shim method to adjust your alignment (toe in will give steer even if the wheel runs flat!)

Good luck!

[BADsDAD]

Thanks for the info, ironband!

The Revell wheels should come this week and I'll definitely be weighing them to compare to the black ones from the kit. Ultimately though, it's up to the boys whether we use the colored ones or not. I'll just have to explain the disadvantages and let them decide.

I had wondered about the crimp marks when I was first starting to polish axles. After running the file on the head with the axle on the drill press and then polishing without completely filing off the crimp marks, I was wondering if they actually we an advantage once polished up given the reduction in surface area.

The toothpicks and woodglue is a great idea! I was planning to move them forward, but that sounds like the better plan since I can use the jig and be sure they are straight.

The rules state that I can adjust the axles to make the car track properly, just can't bend them to ride on the the edges of the wheels. Honestly, this makes me think I can adjust the axles and still use the -3deg holes for the rears, but then alignment may be really tough so maybe better to fill with the toothpicks and glue like you recommended. I'd hate to get there and have to explain that I have -3deg holes and have bent the axles to accommodate the changed rules from pack level. That might turn out badly for us.

I have found mention of a "screw method" of having the rears run out to the heads without needing to cant the axles, but only a mention of it, no details of what that entails. I'll keep searching...

Thank you again, ironband! Much appreciated!!

[Speedster]

I used the screw method a lot of years ago but it's a lot of work and not really worth doing. It tilted the rears ever so slightly and caused the rears to migrate to the axle heads. I think any bend in the axle is going to cause the wheels to go to the axle heads plus give the benefit to align the rears to be parallel to the block. I haven't tried it yet because I have no reason to do it but after things slow down I'll bend some nails 1/2 degree and see if the wheels go to the axle heads. My 1 1/2 degree bend will cause the rears to ride on the inside edge and that's what I want.

[ironband]

That's a good point - in my experience using a 1.5 degree bend will allow for migration and steer while the wheels still "ride flat". If the rule prohibition is against bent axles, then you might not go this way, but if the rules just say that the wheels have to "be flat" then that might be a way to go.

[whodathunkit]

Sporty, had his 4 wheel rail riding method here on DT some place ..
However here's a link to where to find it also!
https://www.derbyworx.com/sportys-4-on- ... ail-rider/

[Speedster]

Here's some weights on the wheels.
China - all #1. 2.72, 2.73, 2.75, 2.76.
USA. 8-2.55, 15-2.58, 11-2.55, 1-2.64 , 6-2.64, 5-2.53, 10-2.59

Pinewood Derby wheels are not flat, they are concave. If you set a wheel at the edge of a table, line your eye up with the very bottom of the wheel, use a strong magnifying glass, you will see daylight under the wheel. The wheels ride on the edges. The only good part of that is if there is a really bad spot in the middle of the wheel it shouldn't touch anything. It doesn't reduce friction. The bore on the China wheel seems to have the opposite taper as the USA wheel. I installed a set of China wheels and axles using the axle guide on a new wedge block that was purchased from Hodges Hobby house. Yep, I got 100 of them. The wheels migrated out to the axle heads both forward and backward. I suspect the taper might have something to do with it. Since you're using China wheels and axles you might give it a try before you do anything else with the rears. If you bend your front nail 1 degree or 1 1/2 degree and install it with Positive camber the lean doesn't show very much with Positive camber. I realize you don't want to get disqualified but I'm wondering just how much they are going to enforce the rules. This is the 1st time I have heard of anyone not letting you sand the wheels a bit. It probably does more damage then good but the kids like to do it.
Since the bores on the China wheel are smaller the the USA bore you might want to consider using a BSA nail for the front wheel. It won't take as much before you start moving the wheel.
Straight runners can be Fast too.