Which axle location to use for front and back wheels? (2024)

Hi, I am a newbie. Our pack rules state that we have to use the stock axle locations on the wood block in the BSA PWD kit.

My question is which axle location do I use for the front and which one for the back? Does it really matter and if so can you explain why?

Thanks, John

Hello John,

Welcome to Derby Talk!

Use the slot closest to the end as the rear axle slot. This allows you to concentrate the weight in the rear of the car (more potential energy) with less of a possibility to pop a wheelie.

Randy Lisano
Romans 5:8

Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

Randy's answer is correct for most tracks that are in use now. These tracks have a starting pin about 4 feet higher than the finish line and slope at around 30 degrees, gradually becoming less steep until after 8 to 12 feet the track flattens out at the same level as the finish line. On that style of track it does matter.

Some old (very, very old) tracks have a constant slope from start to finish. On these, the energy available to convert to speed is independent of where the Center of Mass (CM) is located. Car stability on that track is reportedly best with a centralized CM.

There there are some unusual tracks such as the "S" shaped track in which the starting line slope is very gentle. Then after a few inches (or feet) the slope increases and until the slope reaches about 30 degrees and the remainder of the track is similar to the commonly used current tracks. Best CM location depends on the details of the track, but boil down to how fast the cars are going as the slope starts to increase. Slow speeds (short, gentle starting line slope) favor forward CM, faster speeds (longer or steeper starting line slope) favor rearward CM!

Another track that provides a "home field advantage" has "whoop-de-doos" down track. These are short hills that the cars must climb over. Reportedly, these tracks favor more centrally located CM than the more commonly used current tracks. Again, "the devil is in the details!"

Shop around Derby Talk to get more good info on "weight distribution" and "CM location." Those two concepts are intricately intertwined!

I know there are some S-Shaped and incline track out there somewhere, but as Stan states, the vast majority are tracks that have an incline transitioning to a the horizontal.

Regardless of the track type, I would still use the slot closest to the end as the rear of the car. Then you can position the COG where needed based on the track type it will run on.

Randy Lisano
Romans 5:8

Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

gpraceman wrote:Regardless of the track type, I would still use the slot closest to the end as the rear of the car. Then you can position the COG where needed based on the track type it will run on.

Randy is "spot on!" If you don't know the details of the track, it is the safest choice, and will be "best" more than 95% of the time. It is also safe to run about 4/5 of the weight on the rear wheels. From there, you can get more aggressive or more conservative if you know more about the track.

Thank you all for your replies and explanations, it is very helpful to understand why.

I have been reading this forum and all the information is very helpful.

John:

The other thing to consider when you have to use the stock axle slots is whether your local rules allow you to "move" the slots. This is usually accomplished in one of 2 ways, the first more common and the second less so.

1) "Pre-install" the wheels in the back axle slot and measure off the amount of "extra" wood behind the rear wheels. Cut off a vertical slice of the "extra" wood and reattach it to the front of the car. This allows you to push the CG even further back (i.e. higher potential energy) at the starting gate while maintaining the stock wheelbase and stock axle slots.

Depending on your local rules, this may or may not be allowed. Often the local rules aren't clear on this, so check with your local race chairman for an advance ruling. Some folks will consider this "creative", others will cry "foul". When in doubt, clarify ahead of time.

2) Perform a similar exercise on both the front and back axle slots, relocating the "extra" wood to the middle of the car. This allows you to push the CG even further back (i.e. higher potential energy) at the starting gate and extend the wheelbase (if done correctly can yield more stability and better alignment).

I would strongly not recommend #2 to a Newby for 2 reasons:

a) If you don't have 3 consistent perpendicular vertical cuts you can end up with a fairly disjointed looking car and some horrendous alignment problems.

b) This is a little too "creative" for a lot of observers, and unless clearly allowed (and confirmed as acceptable in advance) can turn into a heated discussion very quickly. This is especially true if the car performs very well, which it likely will if you keep reading this forum and taking the advice folks like Stan and Randy give you.

Good luck.

One final note: Since you must use the stock axle slots, make sure you check them for squareness and "test fit" the axles first before spending a lot of time cutting and shaping the block. Occasionally you will get a block where an axle slot will split when you insert the axle. Better to find this out right away so you can get a new block than to discover this after hours have been spent cutting, sanding, shaping, painting, etc...

For ease of explanation, let's assume that the existing axles slots are 1" from one end of the 7" block for one slot and 2" from the other end for the other slot. For our purposes, I will refer to the 1" from the end slot as the "1 inch slot" and the other slot as the "2 inch slot".

Assuming you stick with the basic block described above with no modifications to the location of the axle slots, you can build the car in one of two orientations depending on which slot (1 inch slot or 2 inch slot) you choose to utilize for the front slot.

1 inch slot in front (Low potential energy) method: Use the 1 inch slot as the front slot. After you have done the appropriate cutting of wood and placement of added weights to achieve the desired 1 oz. weight on the front wheels and 4 oz. weight on the rear wheels, stop and consider the position of your "effective weights" with respect to the starting gate. You have effectively placed 1 oz. of weight 1" uphill from the starting gate and 4 oz. of weight 5" uphill from the starting gate.

2 inch slot in front (Higher potential energy) method: Use the 2 inch slot as the front slot. After you have done the appropriate cutting of wood and placement of added weights to achieve the desired 1 oz. weight on the front wheels and 4 oz. weight on the rear wheels, again stop and consider the position of your "effective weights" with respect to the starting gate. You have effectively placed 1 oz. of weight 2" (rather than 1") uphill from the starting gate and 4 oz. of weight 6" (rather than 5") uphill from the starting gate.

In the 2 inch slot in front orientation, you have effectively moved the entire 5 oz. weight of the car 1" further up the hill from the starting gate as compared to the 1 inch slot in front orientation. For the typical inclined ramp followed by long flat section track, this extra (free!) potential energy will pay big dividends!!!

EDIT: To address letsgo's question succinctly, in the 2 inch slot in front orientation, you are effectively using the 2 inch "overhang" as you call it to hold the weight further uphill from the starting gate. It gives you no benefit to "overhang" in the rear, and actually contributes to some potential instability by placing more weight behind the rear axle.

I will spare you the math regarding exactly how much extra potential energy you gain with the 2 inch slot in front orientation (i.e. f * d sin theta, where f = 5 oz, d = 1 inch and theta = angle of incline of the ramp (typically 30 degrees, but depends on the particular model and set up)... oh, yuk ).

Last year I managed to get 2 oz of tungsten of weight BEHIND the rear axle, even though I cut off the back of the block and put it on the front.

My COG was .75 inches in front of the rear axle. But, to acheive this, I had to move some weight farther forward. This spread the weight out. I'd prefer to have it more concentrated.

That said, I just read MaxV's article in his newsletter where he states he did some tests and found that distributing the weight out was better than concentrating it in one spot.

I'm not too sure about that, maybe he's right, but I'm going to concentrate my weight as much as possible.

I've held in my hands the two fastest pinewood derby cars of 2005 (Snuffleupagus and HammerHead) and they both had very concentrated masses. Both were primarily in front of the rear axle.

I don't pretend that I can make a car that comes close to those, but they are good role models for us mere mortals!

2kids10horses... and the goat is at the breeders! (Again!)

p.s. I know I said "yuk" above, but if you are interested in some mathematical analysis showing the conversion from Potential Energy of a typical Pinewood Car at rest behind a starting gate to ideal speed (i.e. no friction, other losses) and elapsed race time, see the linked post below:

viewtopic.php?p=15075

I just stumbled across it as I was doing some "casual browsing". Pictures and everything (thanks, Teeeman!)

If you don't care for the math, just stick the short (1 inch) slot in the back and leave the cypherin' to those brave souls who seem to like that stuff.

Our Pack requires keeping the "stock" wheel base. Over the past couple of years we've had very good success with shorter cars. Usually the front axle is about 1/4 inch from the front and the rear axle about 1/2 from the rear. The weight is concentrated 3/4 inch in front of the rear axle.

Our cars resemble a VW Beetle, results are a short compact car. My sons car has yet to loose a "heat" in two years.

IMHO, the important issues are, axle alignment, weight placement, axle prep and wheel prep. The last step my son does, is what I call "orienting" the axle to the wheel. With the axle in the wheel and car body, he installs graphite into the inner wheel hub, then with the Dremel tool and a felt fob, he turns the wheel with the dremel tool.

This process is repeated 4 times on each wheel. I firmly believe this polishes the inner bore of the wheel and works in the graphite.

Be careful not to apply too much pressure to the wheel, or spin the wheel with the dremel tool for more than 30 seconds. It can lead to friction/heat which can distort the hub bore and you wind up with a wobbly axle to wheel fit.

Just my 2 cents worth.

Jeff

Rainman wrote:My sons car has yet to loose a "heat" in two years.

It is reasonable to continue doing what works for you ... until you run against someone who does as well the things that you do well, and does a bit better the things you don't. Then more learning happens!

Good racing, Jeff!