Mitigating negative cant problems for taller bodies

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
Online
User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1243
Joined: Sat Apr 20, 2013 7:26 pm
Location: Spotsylvania, VA

Mitigating negative cant problems for taller bodies

Post by Vitamin K »

So I'm planning an experimental build, which is an attempt to put together a competitive recipe (at the scout level) for a car that can be built by somebody with a minimum of time, resources and tools. I'm messing with the idea of using nickels for ballast, since tungsten is extremely costly, and lead has hassles of its own in regard to safe usage.

The problem is that this requires a taller body, due to the ballast's lack of density. I'm not overly concerned with the height of the weight, but I am concerned about the tops of the rear wheels brushing the side of the body when negative cant is applied.

Image

Any good ways to possibly mitigate this? Because of where the weight will sit, I don't have the option of carving into the body of the car. It seems like the options are:

- Restrict rear cant to a degree where the wheels won't touch (2 degrees?)
- Increase the gap for the rear axles

Or possibly a combination of both.

Any thoughts on this? Or other suggestions, perhaps?
User avatar
Stan Pope
Pine Head Legend
Pine Head Legend
Posts: 6856
Joined: Sat Jul 05, 2003 7:01 pm
Location: Morton, Illinois
Contact:

Re: Mitigating negative cant problems for taller bodies

Post by Stan Pope »

Pennies are effective ballast and fit into standard drill size holes! The are also convenient weight increments.

If you wish to limit play in axles, glue thin wood wafers to car body at rear axle holes.
Stan
"If it's not for the boys, it's for the birds!"
User avatar
davet
Master Pine Head
Master Pine Head
Posts: 539
Joined: Wed Jan 16, 2013 1:33 am
Location: MN

Re: Mitigating negative cant problems for taller bodies

Post by davet »

Can you place the weight at least 1/16" in from the sides then just sand into the side of the body a nice round indent matching the curvature of the wheel?
Online
User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1243
Joined: Sat Apr 20, 2013 7:26 pm
Location: Spotsylvania, VA

Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

Stan Pope wrote:Pennies are effective ballast and fit into standard drill size holes! The are also convenient weight increments.
The reason I went with nickels is because two nickels fit the width of a standard pinewood derby car width almost perfectly (and, thus, are more efficient in terms of space usage). They also fit directly into a 7/8" hole (you need a forstner or spade bit to make them, of course, but you also need the same with pennies (only, 3/4").
Stan Pope wrote: If you wish to limit play in axles, glue thin wood wafers to car body at rear axle holes.
Can you think of any possible speed penalties that you'd pay by widening the rear track by .125" or so?
davet wrote: Can you place the weight at least 1/16" in from the sides then just sand into the side of the body a nice round indent matching the curvature of the wheel?
Not with the nickels side-by-side, since they pretty much go right to the edge of the body.
ngyoung
Master Pine Head
Master Pine Head
Posts: 234
Joined: Thu Apr 03, 2014 6:36 pm
Location: Eyota, Minnesota

Re: Mitigating negative cant problems for taller bodies

Post by ngyoung »

I think you can get up to weight with a thinner body using 3/8" lead free rounds with the holes drilled across the body. Those are pretty cheap in the PWD supply section of most hobby stores. 1 behind and 2-3 in front of the axle hole. As for your design I think just putting an angle on the sides of the body should be enough to give you clearance.
User avatar
birddog
Master Pine Head
Master Pine Head
Posts: 307
Joined: Thu Apr 26, 2012 12:40 pm
Location: Denver, CO

Re: Mitigating negative cant problems for taller bodies

Post by birddog »

ngyoung wrote:As for your design I think just putting an angle on the sides of the body should be enough to give you clearance.
I agree. I don't think you want to mess with wider gaps as that can cause other problems.

birddog
Online
User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1243
Joined: Sat Apr 20, 2013 7:26 pm
Location: Spotsylvania, VA

Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

ngyoung wrote:I think you can get up to weight with a thinner body using 3/8" lead free rounds with the holes drilled across the body. Those are pretty cheap in the PWD supply section of most hobby stores. 1 behind and 2-3 in front of the axle hole. As for your design I think just putting an angle on the sides of the body should be enough to give you clearance.
I could do a lot better with lead, very true. I'm trying to avoid the use of lead in this build, though.

I could do an angle on the sides if I wasn't using nickels, but because the nickels will pretty much run right up to the edge of the body, I don't have much free wood to work with. Also, angling the body is a bit of a trickier cut, and I'm trying to come up with something that a busy parent can help their kid do with a few borrowed hand tools.
birddog wrote:I don't think you want to mess with wider gaps as that can cause other problems.
What's the major risk with wider gaps? If well-aligned rears migrate to the heads of the axles anyhow, what other issues do you run into? Debris fouling the wheel/axle gap? I'm not challenging the statement, just curious about the actual effects.
ngyoung
Master Pine Head
Master Pine Head
Posts: 234
Joined: Thu Apr 03, 2014 6:36 pm
Location: Eyota, Minnesota

Re: Mitigating negative cant problems for taller bodies

Post by ngyoung »

I said lead free. So basically the cheap zinc or whatever rounds that are at the hobby shops.
Online
User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1243
Joined: Sat Apr 20, 2013 7:26 pm
Location: Spotsylvania, VA

Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

ngyoung wrote:I said lead free. So basically the cheap zinc or whatever rounds that are at the hobby shops.
Ah, my mistake. Reading comprehension fail.

If I went with a smaller round size, I'd probably just drop back to pennies though, since they're zinc, and cheaper than weights you'd actually buy.
User avatar
Stan Pope
Pine Head Legend
Pine Head Legend
Posts: 6856
Joined: Sat Jul 05, 2003 7:01 pm
Location: Morton, Illinois
Contact:

Re: Mitigating negative cant problems for taller bodies

Post by Stan Pope »

I just measured the angle between the bore axis and the hub-rim tangent: 85.5 degrees. That means that if the camber is less than 4.5 degrees and if the bore rests on the axle (it better), then the rim will not rub the body.

BTW, my "bent by hand" axles average 4.5 degrees! :)
Stan
"If it's not for the boys, it's for the birds!"
User avatar
Stan Pope
Pine Head Legend
Pine Head Legend
Posts: 6856
Joined: Sat Jul 05, 2003 7:01 pm
Location: Morton, Illinois
Contact:

Re: Mitigating negative cant problems for taller bodies

Post by Stan Pope »

Here are some stats for folks considering similar weighting methods:

Image

Some quick computations would show that:
--- the density of nickels is slightly more than for pennies.
--- a stack of pennies across the back of the car would force the axle hole to be shifted about 1/16" forward of the nominal best 5/8" location. The stack would result in a 3/4" tall car with a rounded rear profile and about 2 oz behind the rear axle.
Stan
"If it's not for the boys, it's for the birds!"
Online
User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1243
Joined: Sat Apr 20, 2013 7:26 pm
Location: Spotsylvania, VA

Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

Hmm...I wonder if I've been over-canting our axles in the past, because I've actually run into this as a problem before, when we made balsa toppers for the kids' cars.

Does this take into account the 'slop' from the axle being narrower than the bore?
Stan Pope wrote:I just measured the angle between the bore axis and the hub-rim tangent: 85.5 degrees. That means that if the camber is less than 4.5 degrees and if the bore rests on the axle (it better), then the rim will not rub the body.

BTW, my "bent by hand" axles average 4.5 degrees! :)
User avatar
pwrd by tungsten
Master Pine Head
Master Pine Head
Posts: 723
Joined: Wed May 24, 2006 1:51 am
Location: Seattle, WA

Re: Mitigating negative cant problems for taller bodies

Post by pwrd by tungsten »

This is a major issue and why I advocate thin cars. (Or at least one reason :-)

You will need to taper the sides or create little axels on the side. Much easier to cut the wood at 3/8" and add exposed weight to the top....
Last edited by pwrd by tungsten on Wed Feb 11, 2015 12:05 am, edited 1 time in total.
W Racing!!!!
User avatar
Stan Pope
Pine Head Legend
Pine Head Legend
Posts: 6856
Joined: Sat Jul 05, 2003 7:01 pm
Location: Morton, Illinois
Contact:

Re: Mitigating negative cant problems for taller bodies

Post by Stan Pope »

Stan Pope wrote:BTW, my "bent by hand" axles average 4.5 degrees! :)
Measured on the angles chart that VitaminK helps make! :)

My measured limit does not accommodate slop... When running, the wheel bores must nestle firmly against the axle, so slop should not be an issue. OTOH, if you bump and grind down the track OR if the back end gets some side-to-side rotation, then they could touch! The limits would depend on how much the axle diameter was reduced!
Stan
"If it's not for the boys, it's for the birds!"
Online
User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1243
Joined: Sat Apr 20, 2013 7:26 pm
Location: Spotsylvania, VA

Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

Stan Pope wrote:My measured limit does not accommodate slop... When running, the wheel bores must nestle firmly against the axle, so slop should not be an issue. OTOH, if you bump and grind down the track OR if the back end gets some side-to-side rotation, then they could touch! The limits would depend on how much the axle diameter was reduced!
I guess I tend to err on the paranoid side, as far as clearances go. I'm going to do some testing with a plank car and some nickels on my scales and see if I can lower the stack of nickels (and, accordingly, the body) and still hit a reasonable CoM
Post Reply