Advice on modifying car for districts please

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BADsDAD
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Advice on modifying car for districts please

Post by BADsDAD » Sat Feb 09, 2019 5:17 pm

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! :bigups:

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) :mrgreen:

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

Image


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. :roll:

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!!! :D



ironband
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Re: Advice on modifying car for districts please

Post by ironband » Mon Feb 11, 2019 9:57 am

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
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Re: Advice on modifying car for districts please

Post by BADsDAD » Mon Feb 11, 2019 6:28 pm

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!!



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Re: Advice on modifying car for districts please

Post by Speedster » Mon Feb 11, 2019 7:21 pm

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
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Re: Advice on modifying car for districts please

Post by ironband » Mon Feb 11, 2019 7:48 pm

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.



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Re: Advice on modifying car for districts please

Post by whodathunkit » Mon Feb 11, 2019 8:43 pm

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/


What type of automobile can be spelled the same forwards & backwards?

Speedster
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Re: Advice on modifying car for districts please

Post by Speedster » Tue Feb 12, 2019 2:34 pm

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.



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Re: Advice on modifying car for districts please

Post by BADsDAD » Sat Mar 02, 2019 11:07 am

In between pack races now, waiting for my oldest Son's race to start. Trying to decide whether to remove some graphite as we have 5mins of pit time before the race starts in about an hour.

My youngest Son's car just got 4th at districts and we are pretty happy, but what was weird is that it got faster after each heat of about 11 heats. It picked up about 2mph after each heat up to the point that it ran faster on its last heat than the first place finisher's average. Problem is, the first few heats really drug it down to 4th overall.

Any ideas on why this might be? Too much graphite? I would suspect lane choice, but it never ran slower than the previous one and always picked up ~2mph.



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Re: Advice on modifying car for districts please

Post by Speedster » Sat Mar 02, 2019 4:00 pm

That's common. I think it's been said it takes about 6 runs before the car will do its best. I think this might be more common to those who get to the race and start loading the wheels up with graphite. That is not a wise thing to do if racing in our District and maybe yours. The cars are scored "Elapsed Time" and each car will run once in each lane on a 4 lane Best track, 29' 10". The 8 fastest cars move to a 40 foot Freedom track and each car races 8 more times. You might have the fastest car there but get eliminated on the 1st track.



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Re: Advice on modifying car for districts please

Post by Speedster » Sun Mar 03, 2019 6:27 am

Sporty's 4 wheels on the ground rail riding was fairly basic. Set the car up to rail ride with Positive cant and toe in on the dfw and set the drift. Set the non dfw with Negative cant and slight tow out. The big mistake I made was to raise one front wheel as though I was going to run 3 wheels. I only ever did this once. I'm thinking if the front holes or slots are set so the front wheels, with straight axles, are both on the ground to start, this should be a lot easier. One of the front wheels is still going to be dominant because of the slop in the bores. Choose the one with the leastt weight and make that the dominant wheel. That's going to raise the front of the car slightly if the axle is bent 1 1/2 degrees and set with Positive cant. The non dfw should now take a very slight adjustment to adjust it down so it provides a Negative cant with toe out.

Does this make any sense?



BADsDAD
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Re: Advice on modifying car for districts please

Post by BADsDAD » Sun Mar 03, 2019 8:50 am

That does make sense and thank you for the insight. As it turns out, after modifying our existing cars for the districts, we learned that others did not and were still running canted 3-wheel rail riders. The boys were ok with how we did given the circumstances, but we got clobbered pretty hard where we had totally run away with the pack race. They have their pack trophies and there is always next year! :-)
I feel it was one of those situations where I know they were trying to make the district rules to allow for a more fairness, but in not enforcing them, it punished those of us who changed our cars up to comply. It was a lesson in sportsmanship anyway for them, they win too much as it is. LOL..



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