Derby Talk

Elsewhere I described a suggested change for our Wotamalo Rules. In summary, the rule would limit the distance from each end of the car to the furthest axle to 6 inches. This change reduces the allowable wheelbase, leaving an additional 3/8" of car body possible behind the rear axle.

The purpose of this change is to reduce the advantage obtained by using expensive ballast.

However, I'd like to examine possible "unintended consequences", since competition with the rules as is has produced high participation rates. I'd certainly like to avoid suggesting something that would reduce participation!

So, please think through the implications and tell me how this might "go wrong."

Someone using tungsten is probably going to go all the way with purchased wheels and axles, expensive graphite, expensive oils, and anything else they can buy to give them a speed edge. I think the rule will not affect the outcome of the race. The same scout is going to win. Since you already allow an extended wheelbase, it might negatively affect the looks of a car that is being built for Best Design. I think it would not be worth it to put it in your rules.

Stan,

All's you have done is changed up the "extended wheel base measurement from 5-5/8 inch
to 5-1/8 now.. by shorting it by a 1/2 inch.

The purpose of this change is to reduce the advantage obtained by using expensive ballast.

whodathunkit wrote:Stan,

All's you have done is changed up the "extended wheel base measurement from 5-5/8 inch
to 5-1/8 now.. by shorting it by a 1/2 inch.

The purpose of this change is to reduce the advantage obtained by using expensive ballast.

While your wheelbase measurement may be correct, the important change is adding volume behind the rear axle. I think that the suggested change allows inexpensive ballast to accomplish the same CM locations in a 1/4" thick car as can be accomplished with tungsten in a fully extended wheelbase rule. If I had the number in hand, I'd suggest that the distances be comparable to the "rear slot" in an unaltered block.

Now, the "deep pockets" tungsten user might move to 3/16 cubes, etc, to further reduce the thickness of their cars, while maintaining the same CM location (relative to the rear axle), but this reduction is pretty small as compared to the frontal area of the wheels and as compared to the reduction in thickness when moving, for instance, from lead to tungsten in an extended wheelbase design.

Stan Pope wrote:Elsewhere I described a suggested change for our Wotamalo Rules. In summary, the rule would limit the distance from each end of the car to the furthest axle to 6 inches. This change reduces the allowable wheelbase, leaving an additional 3/8" of car body possible behind the rear axle.

The purpose of this change is to reduce the advantage obtained by using expensive ballast.

However, I'd like to examine possible "unintended consequences", since competition with the rules as is has produced high participation rates. I'd certainly like to avoid suggesting something that would reduce participation!

So, please think through the implications and tell me how this might "go wrong."

Well other then the fact that you couldn't use the stock axle locations. I understand your desire, but if you are going to change the rule I would go to 6 1/16" to allow the use of the stock axle locations.

A 3/16" thick car is not a good idea - too much flex - with truly negligible aero advantage - 1/4" is plenty thin, if not too thin.

Its funny - I really never thought about this Stan - but when we were revamping our rules there were many suggestions for not restricting to the stock wheel base - I guess I'm kinda glad we did. There is an advantage to moving the rear wheels back to something like 5/8" - but there is not a lot of speed advantage to moving the front wheels forward, though it does make tuning the car a bit easier.

I would have to agree with 5 J’s on the 6 1/16” from each end of the block. Otherwise they are moving the rear wheel (for those that use that end for the rear) forward by going to the extended wheelbase. Here is a picture:

Right! The "rear" groove-to-nose distance was my intended limit. Just couldn't get up to find and measure a block!

Stan - I cannot think of any unintended consequences, however, I am not sure that the use of high density mass such as tungsten provides a "significant" advantage. If you were going to put the maximum amount of tungsten behind an axle in a car that is 1/4" thick, you could get the same mass behind the axle using lead but you would have to have a 7/16" thick car. I would not think the 7/16" car would have a significant aero disadvantage.

Couple of other thoughts - at 1/4" there is the possibility of having detrimental flex. Also, at the extreme I outlined above using the maximum amount of tungsten behind the axle of a 1/4" car you would end up with a COM of less then 1/2". I would not recommend this.

I don't have your calculator up, but I would suspect that even if you allowed the rear axle to be set as far as 5/8" from the rear one could get enough lead behind the axle to give a perfect COM at a thickness with a negligible aero disadvantage.

EDIT: 1/4" Tungsten cubes have a mass of approx 1.68oz each. The most you can get behind an axle is 10 blocks (2 rows of 5) which gives you 1.68 oz behind the axle. If you put the balance of the required mass directly in front of the rear axle, you will have a real aggressive COM, but you could match this COM by using a 1/2" x 1 1/4" x 7/16" chuck of lead behind the rear axle (and the balance of the required mass directly in front of the rear axle.)

I'm holding a car I did not build. It has 12-1/4" cubes behind the rear axle and 12 cubes in front of the rear axle. The last 2 1/8" of the car is 1/4" thick and the rest of the car is less than 1/4". I do not know how much a single cube weighs but I think about .168 oz because the gentleman likes to add 4 oz. of weight to his cars. If Stan's rule drifts Eastward I will no longer be able to cut off the back of the car and move it to the front. That is one way this could go wrong. Another way might be the negative appearance of a car that was not built for speed.

*5 J's* wrote:The most you can get behind an axle is 10 blocks (2 rows of 5)...

Although we don't recommend it, we have fit 14 cubes (2 rows of 7) behind the rear axle.

*5 J's* wrote:A 3/16" thick car is not a good idea - too much flex - with truly negligible aero advantage

Plus, IIRC, the smaller cubes are more irregular and take up slightly more volume.

FatSebastian wrote:

*5 J's* wrote:The most you can get behind an axle is 10 blocks (2 rows of 5)...

Although we don't recommend it, we have fit 14 cubes (2 rows of 7) behind the rear axle.

You can, but this leaves nothing for a "frame" for the car (for stiffness) with no advantage because that much weight behind the rear axle creates a car with such an agressive COM that it would be unstable even with tight tolerance machined axles and wheels. Now, having said that one could create the same COM with a block of lead 7/16" thick in lieu of the 1/4" thick tungsten - so if one was so inclined it could be done with lower cost lead.

*5 J's* wrote:You can, but...

That's why we don't recommend it. However, we sometimes build with 2 rows of six cubes.

FatSebastian wrote:IIRC, the smaller cubes are more irregular and take up slightly more volume.

Also, 3/16" cubes have a reputation for being less dense.

Stan Pope wrote:

whodathunkit wrote:Stan,

All's you have done is changed up the "extended wheel base measurement from 5-5/8 inch
to 5-1/8 now.. by shorting it by a 1/2 inch.

The purpose of this change is to reduce the advantage obtained by using expensive ballast.

While your wheelbase measurement may be correct, the important change is adding volume behind the rear axle.

Stan Pope wrote:Elsewhere I described a suggested change for our Wotamalo Rules. In summary, the rule would limit the distance from each end of the car to the furthest axle to 6 inches. This change reduces the allowable wheelbase, leaving an additional 3/8" of car body possible behind the rear axle.

The purpose of this change is to reduce the advantage obtained by using expensive ballast.

However, I'd like to examine possible "unintended consequences", since competition with the rules as is has produced high participation rates. I'd certainly like to avoid suggesting something that would reduce participation!

So, please think through the implications and tell me how this might "go wrong."

Drats.. I missed it by 1/16".. o'h well you can't win them all.

I thought I recalled Grandson building with 4 rows of 6 1/4" cubes last year ... but my memory is not as sharp as it used to be ... in fact, I think that it never was that sharp! CM was aggressive! And car was stable!

First thing he did was to build 2X6 weights with superglue while forming the weights against a jig lined with waxed paper. The weight assemblies then went into through-holes in the car body, nestled close to the rear axle and fixed with epoxy.

Thank you all for contributing to the analysis. I am fearful suggesting changes to what has been a pretty good set of rules ... at least we get very good district race participation. Sure don't want to introduce changes that would reduce participation!

I'm holding a car I did not build. It has 12-1/4" cubes behind the rear axle and 12 cubes in front of the rear axle. The last 2 1/8" of the car is 1/4" thick and the rest of the car is less than 1/4". I do not know how much a single cube weighs but I think about .168 oz because the gentleman likes to add 4 oz. of weight to his cars. If Stan's rule drifts Eastward I will no longer be able to cut off the back of the car and move it to the front. That is one way this could go wrong. Another way might be the negative appearance of a car that was not built for speed.

I think the idea behind the rule was to stop cutting and moving...