Finalizing my car - 3 questions for the PWD masters

[Hyperfocus]

Hi folks,

I've been reading all of your conversations from the last 15+ years on here, and more or less have my car modifications figured out.

To set the stage: My son has a race this coming Saturday, the Awana Grand Prix. We randomly came across the Mark Rober video from near 10 years ago and since then have set out to produce the fastest PWD car we can. Here's the rules and our setup.

Running on a 35' Besttrack, unsure of what curve type it is, but I can tell you the exact track is the #435. Not sure if there was an older version of their track with a more aggressive curve or not.
Standard dimension rules, except clearance from base of car to track is 1/2". Overall width cannot exceed 2 3/4".
No rules noted regarding wheelbase modification, axle camber or 3 wheel rail riding.
No graphite allowed.

We're running just over a 5" wheelbase (I think 5.15"?), 1/4" thin body (flat wafer, no cut outs of side) with 3/4" COG in front of rear axel, which is properly maxed out at the rear of the car body.
Still deciding between a 3, 2.5 or 1.5 degree camber on the rear axles. 2.5 degree camber on FDW. 1/16th cut out for FDW. floating 3rd wheel etc.

Drilled the axle holes with a drill press and purchased alignment tool from Pinewood Pro; I did my best to hone in the angle of the drill bit to the table in my press using a 123 block, but I find theres still some movement in the alignment tool with the bit inserted, so its as good as its gonna get.

Using Max V's Krytox 100 oil.

Car body weight is VERY light, with majority of weight obviously around the rear axle, attaining the proper COG. More on this later.

Polished wheel bores using Tamiya swab and Novus 2. Cleaned with blue dawn soap, iso 91%, blown out with compressed air, then will apply a wax or silicone hydrophobic finish, depending on what I can get my hands on. Likely wax.

Awana axles polished, very lightly. They're already a pretty smooth nickel finish with no burrs, but improvement can always be found. Used 1500, 2000, 3000, and 4000 grit wet sandpaper for no more than 10 seconds each, then Brasso polish to finish off. Mirror chrome finish. Soaked in iso 91% for 30 minutes, then Jigaloo applied, left to air dry 4 hours, then applied a second time in the same fashion. Precision calipers noted little to no loss of axle diameter from pre to post polishing.

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Some interesting modifications I've done that may not make a huge difference but logically in my mind they should; before polishing, I used that DerbyWorx tool to hammer the axle head into a bevel to reduce contact surface area with wheel hub; also coned inner and out hubs for the same reason aiming to have as little surface area for friction to occur.

Much of everything I've done has been due to all of your knowledge and wisdom shared in this forum, so thank you for that. You've all been an invaluable resource.

I have three main questions though, to help me finalize my build this week.

1. I came across a product online by Turbo Derby called the Turbo Jig XL; they mentioned something in their promotion of that product regarding the clearance from the bottom of the car to the track being equal in the front and rear of the car. I've often found the front or rear of my cars to be higher/lower than the other, not equal, seemingly changed by how the front dominant wheels camber is adjusted for steer (lifting or lowering the body). Should I be drilling the axle hole in the front or rear to be higher/lower to make up that difference, so the front and rear clearance is equal? Does this matter much at all in the grand scheme of things?

2. I'm concerned that despite the 3/4" COG, my front dominant wheel doesn't have enough weight around it, and that might cause some wiggle or impact the rail ride. Should I be intentionally placing some small amount of weight right beside the wheel, embedded in the body, just to give it some stability? At this point the weight of the body+wheels+axles only rings in at around 0.7oz, the rest of the weight are tungsten cubes, tungsten putty and iron adhesive tape around the rear axle area. Should I place a small tungsten cylinder or two beside that front wheel or not worry about it? If I should, how much weight should I place based on the thickness of the body, it's 5.15" wheel base, and described current real weight distribution?

3. Given the body details I mentioned at the beginning, what camber would you recommend for the rear axles?

Cheers for any input! You have already been a blessing with this forum and it's archived knowledge.

Thanks

[Vitamin K]

I'm just gonna throw out my opinions here on how I approach things. I'm not claiming any of them is best:

- We usually set our wheelbase between 4.75 and 5, depending on how rough we expect the track to be (longer for rougher).

- I won't try to drill angled holes without a precision drill jig. I've found attempting to do so on my drill press table is too janky. Something like the original Block from Jewkes would probably let you do this, but I don't trust the squareness of a typical block on its own. If I didn't have a precision jig, I would just use bent rear axles and go with Stan Pope's bias-weight method for alignment of rears.

- Already been over the whole oil process in the other thread, so won't bore you there again.

- I don't really use any Derby Worx tools on axles. If I have a lot of time, I will put a slight bevel on the underside of the axle head during the polishing process.

- I often notice that my cars end up with the nose slightly up. This probably isn't ideal, since with bent rear axles, you can get a bit of toe-out if the nose is up. Ideally you'd adjust for this in the drill of the front axles, but that would require a little trial-and-error and I have been too lazy.

- I usually go with 3 degrees on my rears if I'm using my precision jig, since that's what it's set at. If I do bent rears, I usually end up with about 4.5 degrees on the rears due to how much they end up getting bent. For the front axles, I like 7 degrees or more of positive camber on the DFW axle, because I really want to get the edge of the wheel against the rail, as opposed to the flat part. Having a lot of bend in the front axle can make tuning hard, so I usually drill with about 5 degrees of positive camber and then use a 3-4 degree bent axle to make up the rest and allow for drift tuning.

[FatSebastian]

You've done your homework, and gotten an excellent response from VK (as usual), and good info over in the wheel wax thread. I'll only add a couple of thoughts...

Still deciding between a 3, 2.5 or 1.5 degree camber on the rear axles.

One of the under-advertised benefits of camber is that it lifts the tread face off the track; a piece of paper should slide in and out under the outside wheel without much (if any) resistance (like a feeler gauge). IIRC, I am not sure that 1.5 degrees is quite enough for that, because the tolerances between the bore and axle may allow the wheel to rest flat across the thread. (Maybe this value sufficiently accommodates those whose rules require "no angled axles"?)

Honestly, I wonder if 1/2 degree specifications are at the limit of installation accuracies, and maybe even the manufacturing tolerances of some commercial jigs (we have a top-of-the-line jig where the difference in camber between the rears is more than 1/2 degrees, which bugs me a little, yet the toe remains zero which is most important).

Should I be drilling the axle hole in the front or rear to be higher/lower to make up that difference, so the front and rear clearance is equal?

I'd agree with VK that it won't be debilitating, and since it sounds like you might already have yours drilled then don't worry about it. But if not, and you have the means to drill the DFW just a tag higher or rears a tad lower to level the body, then why not? The positive camber of the DFW tends to lift the front relative to the rears which have negative camber, which (in addition to potentially affecting toe very slightly as noted) funnels extra air between the body and the racing surface; in theory it is probably better to force any inequity in airflow over the top instead of underneath if not absolutely parallel with the track.

Running on a 35' Besttrack... We're running just over a 5" wheelbase (I think 5.15"?)... with 3/4" COG in front of rear axle... I'm concerned that despite the 3/4" COG, my front dominant wheel doesn't have enough weight around it

Based on these numbers using your CoM and wheelbase estimates, I calculate your DFW weight to be about 0.73 oz; for a very well tuned car on a nicely assembled track, 3/4 ounce on the DFW seems like it should be sufficient, but I must defer to those who have specific experience with Best Tracks as far as the optimal DFW weight.

Car body weight is VERY light... the weight of the body+wheels+axles only rings in at around 0.7oz

Yes, that seems lightweight. I would recommend weighing the down-force of the DFW directly with a scale, and if you want to play it safe, you can add trim weight near the nose.

[Hyperfocus]

- I won't try to drill angled holes without a precision drill jig. I've found attempting to do so on my drill press table is too janky. Something like the original Block from Jewkes would probably let you do this, but I don't trust the squareness of a typical block on its own. If I didn't have a precision jig, I would just use bent rear axles and go with Stan Pope's bias-weight method for alignment of rears.

I use a drill jig as well as a drill press table, because of the little bit of play I can feel when using a handheld drill, I try to mitigate that with a honed in vertical drill press.

- I don't really use any Derby Worx tools on axles. If I have a lot of time, I will put a slight bevel on the underside of the axle head during the polishing process.

Yeah I'd have to do a whole bunch of testing with a track of my own to really know if there is a benefit. I imagine it's minute.

- I usually go with 3 degrees on my rears if I'm using my precision jig, since that's what it's set at. If I do bent rears, I usually end up with about 4.5 degrees on the rears due to how much they end up getting bent. For the front axles, I like 7 degrees or more of positive camber on the DFW axle, because I really want to get the edge of the wheel against the rail, as opposed to the flat part. Having a lot of bend in the front axle can make tuning hard, so I usually drill with about 5 degrees of positive camber and then use a 3-4 degree bent axle to make up the rest and allow for drift tuning.

I will aim for 3 degrees then, as long as it fits within the rules limits for total width. 7 degrees of positive camber is pretty intense haha. Do you feel that such a degree makes tuning the wheel alignment for steering easier? I've shaved the inside of my wheels so I wonder if a high degree of camber would cause the wheel tread to bend if running on the edge like that.

I am curious about the details of how you personally precisely align your rear wheels if using bent axles (not using angled axle holes)? Is there a specific method you find most effective?

[Hyperfocus]

Running on a 35' Besttrack... We're running just over a 5" wheelbase (I think 5.15"?)... with 3/4" COG in front of rear axle... I'm concerned that despite the 3/4" COG, my front dominant wheel doesn't have enough weight around it

Based on these numbers using your CoM and wheelbase estimates, I calculate your DFW weight to be about 0.73 oz; for a very well tuned car on a nicely assembled track, 3/4 ounce on the DFW seems like it should be sufficient, but I must defer to those who have specific experience with Best Tracks as far as the optimal DFW weight.

Car body weight is VERY light... the weight of the body+wheels+axles only rings in at around 0.7oz

Yes, that seems lightweight. I would recommend weighing the down-force of the DFW directly with a scale, and if you want to play it safe, you can add trim weight near the nose.
[/quote]

Thanks I will try the 3/4oz. Would it matter if the weight is embedded center of the car, or closer to the side of the DFW?

And a question I've asked VK and will ask everyone here, how do you personally like to precision tune your rear wheels (given the axles are bent with 3 degrees camber, not drilled into the block at 3 degrees)?

[Vitamin K]

- I won't try to drill angled holes without a precision drill jig. I've found attempting to do so on my drill press table is too janky. Something like the original Block from Jewkes would probably let you do this, but I don't trust the squareness of a typical block on its own. If I didn't have a precision jig, I would just use bent rear axles and go with Stan Pope's bias-weight method for alignment of rears.

I use a drill jig as well as a drill press table, because of the little bit of play I can feel when using a handheld drill, I try to mitigate that with a honed in vertical drill press.

You shouldn't feel any play if your bit is sized properly to the jig. What I did when my drill bit for my jig broke was to buy a 12-pack of #43 bits off of Amazon and used the one with the snuggest fit.

- I usually go with 3 degrees on my rears if I'm using my precision jig, since that's what it's set at. If I do bent rears, I usually end up with about 4.5 degrees on the rears due to how much they end up getting bent. For the front axles, I like 7 degrees or more of positive camber on the DFW axle, because I really want to get the edge of the wheel against the rail, as opposed to the flat part. Having a lot of bend in the front axle can make tuning hard, so I usually drill with about 5 degrees of positive camber and then use a 3-4 degree bent axle to make up the rest and allow for drift tuning.

I will aim for 3 degrees then, as long as it fits within the rules limits for total width. 7 degrees of positive camber is pretty intense haha. Do you feel that such a degree makes tuning the wheel alignment for steering easier? I've shaved the inside of my wheels so I wonder if a high degree of camber would cause the wheel tread to bend if running on the edge like that.

I wouldn't be worried about the tread bending. I don't think there's enough weight for that unless you're running super-thin razor wafers.

If you've bent the axle to say, 7 degrees, tuning gets a little bit tricky, though it's still doable. Having less actual bending on the axle gives you a little more leeway when twisting for adjustment.

I am curious about the details of how you personally precisely align your rear wheels if using bent axles (not using angled axle holes)? Is there a specific method you find most effective?

I use Stan Pope's bias weight method.

[FatSebastian]

Would it matter if the weight is embedded center of the car, or closer to the side of the DFW?

Considering that the car is supported by three wheels, putting trim weight close to the DFW also puts a wee bit more on both the DFW and the rear wheel behind the DFW, and a wee bit less on the opposite rear wheel. Practically, it probably doesn't matter too much, and since it isn't our normal practice I would just consider putting trim weight wherever is most convenient to get the desired outcome.

[FatSebastian]

...how do you personally like to precision tune your rear wheels (given the axles are bent with 3 degrees camber, not drilled into the block at 3 degrees)?

We first ensure that the rear wheels migrate out while the car is rolling frontward and backward, after which we like an adaption of Sporty's rather straightforward "no front wheels" alignment method that allows the front of the car to skid down an incline. Sporty used a piece of tape on the car's nose but we further adapted it by using a thumbtack* to elevate the front of the body into its racing configuration. The car should roll fairly straight if the rear-wheels are correctly aligned. We use axle pliers to rotate the axle, which are the only specialized tool required with this method.

(*We say "thumbtack" but technically we use something larger that is thumbtack-shaped: a furniture tack or upholstery nail with a smooth, rounded head. We sometimes rubbed a little graphite on the tack head also to facilitate the sliding action.)

[Loud2ns]

Answer #1- The nose elevation matters more when drilling the rears on an angle. If using bent rears, make sure to keep the nose at the ride height of the dfw when tuning them to keep from inducing toe in or out.

Answer #2- What is the weight on your dfw? With that cog, I wouldn't worry about it being too light.

Answer #3- I'd go with 3 degrees on the rears.

Sent from my SM-G991U using Tapatalk

[Hyperfocus]

Answer #2- What is the weight on your dfw? With that cog, I wouldn't worry about it being too light.
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I weighed it last night, looked to be about 0.9 oz at the DFW, with the 3/4" COG.

[FatSebastian]

looked to be about 0.9 oz at the DFW, with the 3/4" COG.

[Loud2ns]

You definitely don't need to worry about being too light.

Sent from my SM-G991U using Tapatalk

[FatSebastian]

...how do you personally like to precision tune your rear wheels ...?

...we like an adaption of Sporty's rather straightforward "no front wheels" alignment method ... using a thumbtack to elevate the front of the body into its racing configuration.

I see that during our extended derby hiatus that LightninBoy's method refined the "no front wheels" approach a bit further. Worth a try!

[Loud2ns]

...we like an adaption of Sporty's rather straightforward "no front wheels" alignment method ... using a thumbtack to elevate the front of the body into its racing configuration.

I see that during our extended derby hiatus that LightninBoy's method refined the "no front wheels" approach a bit further. Worth a try!

This is how we were tuning our bent rears at the same time with the Stan Pope's method of cantilevered weight.

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[Hyperfocus]

Result from Saturday was we took 1st in the races. My son was very excited! It was close though, which admittedly surprised me. I knew a good amount of other racers had seen the Mark Rober video and were focusing on their weight placement, canted axles and rail raiding. I was expecting to be ahead of everyone by at least a car length given my use of Krytox 100 (instead of WD40, which everyone else used), my low profile aerodynamics, extensive wheel bore and axel prep of polishing and 3M silicone dry type lubricant (wheel bore) and Jigaloo (axles), 3 degree camber on all wheels, somewhat tuned in DFW etc. etc.

I did my best with rear wheel tuning, struggled to have wheels move towards axle head when moving in both reverse and forward, so just set the wheels with as little toe in or out as I could visually, ensuring they moved out to the axle head when moving forward. Definitely some bouncing around the track on different runs. Was adjusting the DFW between each race trying to dial it in based on what I was seeing.

On top of this I had lathed my wheels, to be super light and trued, coned inner and outer hub.

The person who came in 2nd was right on my tail, and they didn't do many of the extra modifications to my car that I had done. So back to the drawing board wondering where to even start haha.

The best time I saw from my own car was about 2.97 on the 35 ft track, otherwise I'd see around 3.03 to 3.04.

The worst part is that our Awana group has a kind of funny rule, where your weight limit is 5oz or 150g. But 150g is about 5.29 oz. So the time I've gotten is with 5.29oz, not 5. Yikes.

The best speeds I've seen are around 2.45 to 2.5 seconds for the 35 footer. Not sure if I'll be able to figure out how to build a car to get anywhere near those speeds with 5oz weight max, without having access to track or buying my own to practice and test car builds.

Any advice or thoughts on where I should focus my efforts are appreciated!

YouTube short of the final race (with 2nd place riding my back) is linked below. My son's car was the gold/black car, placing 1st.

https://youtube.com/shorts/qbBFsOTZ0cs?feature=share