How far back on the COG?

[hwsjr]

A newbie question, but here it goes: How far back can the COG go before you have to worry about the rear wobbling with the wheels in the standard slots? I've read that you shouldn't get it too far back or that is a problem, but how far is too far? And for this question, does it matter if the car is a rail rider or not?

[Warthog]

Around 1 inch in front of the rear axles is a safe COG for rail rider or straight aligned car.

[PWD]

We have built cars with very aggressive rear COG and never,ever had a car leave the track. I have read this concern for years on the forum but I have never seen it.

What we have had is cars with wheel chatter, cars that fishtail all over the place and cars that look to slow down on the flat straightaway section.

COG really depends on length of track. On a 32 foot track there is little worry. We have had cars that have a COG 1/2 " in front of rear axles. THey would fishtail but were still very fast.

When you get on longer tracks it is a bigger issue. You don't mention the length of track you will be running on. Really need this information to give an opinion.

I think rail riding will allow you to have a more aggressive COG but I have not seen data to prove this. But rail riding only really helps with one of the negative characteristics of a very rear weighted car. It helps with fishtailing. It might help a little with wheel chatter. I am really curious if it will help with the phenomena where the car looks to slow way down on the flat section and the other cars catch up.

A weight distribution of 4:1 or 1 ounce on the front wheels is very safe for pretty much any length of track. I don't believe you loose much on a 42 foot plus track. BUt you would probably loose too much on a 32 foot track.

[hwsjr]

Thank you for the replies.

The track for the Pack is 32" and the District is 42". We were lucky enough to win the Pack and place 4th in the District last year. Last year's car had a COG around 1.25" in front of the back axle and 1 oz. behind the axle. The version we have been working on for this year has the COG around .8" in front of the back axle and has 1.25 oz behind the back axle. Based on what I was reading here I was worried about stability.

[hwsjr]

PWD, when you had a COG only .5" in front of the back axle, was that with a standard wheel base? Also, how much of the weight was behind the back axle?

[RLLD]

I can conceptualize how having some weight (say 1oz) behind the rear wheel and then some in front might be a nice combo.

I am wondering if perhaps I should try .5 oz behind the rear wheels and 1.25 oz in front.

[2kids10horses]

RLLD,

The amount of weight you add, and where you put it, depends upon the body design and the weight of the other components i.e. wheels and axles and paint.

Generally speaking, you want to concentrate the weight as far back as you can without sacrificing stability. As a rule of thumb, for newbie builders, that means that the center of mass should be about 1 1/4 inch in front of the rear axle.

This means that if you were to place your car body (with wheels on) on a pencil, with the bottom of the car on the pencil 1 1/4 inch in front of the rear wheels, the weight you add should be placed so that the car will balance on the pencil.

Of course, you first need to know how much weight you need to add to bring the completed car up to 5 oz.

Step 1: Weigh the car body and wheels and axles.

Step 2: Determine how much weight you need to add to bring it up to 5 oz. I take my cars to the Post Office with a whole bunch of fishing weights. Set the car on the scale, and add weights until it reaches 5 oz. Put the added weights in a ziplock bag.

Step 3: figure out where you can put the weights so that the car stays balanced.

Step 4; Either hollow out wood or glue the weights on. (Or combination of both.)

Step 5: fill holes with putty.

Step 6: Reweigh.

Step 7: Adjust as necessary.

Step 8: Rebalance.

Step 9: repeat steps 2 thru 8 until satisfied.

I usually like to go in a little light to the official weigh in. It's easier to add weight at the last second than it is to remove it. And, you don't have to have every single last 1/100th of an oz. Anything around 4.95 oz or above is ok.

Going in at 3.5 oz final weight is not enough, however. You will be slow.

The more skillful you become determines how far back you can set your Center of Mass. I repeat, the father back, it more difficult to align, and it could be slower than a less aggressive CM. That has to be balanced against the fact that the farther back the CM is, the more potential energy the car has. Therein lies the art.

2K10H

[Smell of pine]

We have built cars with very aggressive rear COG and never,ever had a car leave the track. I have read this concern for years on the forum but I have never seen it.

Last year at our district race, the car in the next lane to my son's, lifted as it hit the flat section of the track, and jumped onto my son's car and damaged my son's car.

We added permanent weights in strategic places, then we had a hollow cavity built into the car with a hole in the back of the car closed off with a small screw where we could add or subtract some tungsten powder to dial it it perfect. It worked well for us.

[BigSilver]

Rear COG can be a problem but it can also be a lot of fun for the crowd. At our PWD race last year we had a Tiger car with very aggressive CG that would slowly raise the front wheels off the track at about half way down the flat section of the track and pop a wheelie all the way through the finish line. He came in fourth overall for the pack and first in his den. His car was a real crowd pleaser and how he kept it strait with only two wheels contacting is just plain good luck. His car never created a problem for any of the others as it never jumped the track. Again this was just plane luck.

[2kids10horses]

Smell,

Two years ago, another car lifted right out of it's lane, knocked my son's car right off the track, and then it bounced back into it's own lane and finished the race!

None of the officials 'saw' it, and since it finished in the proper lane, it counted as legal run!

All the spectators saw what happened.

Oh well. He still took 3rd in the Pack.

Later he went on to 3rd in the District!

2K10H

[Go Bubba Go]

Rear COG can be a problem but it can also be a lot of fun for the crowd. At our PWD race last year we had a Tiger car with very aggressive CG that would slowly raise the front wheels off the track at about half way down the flat section of the track and pop a wheelie all the way through the finish line. He came in fourth overall for the pack and first in his den. His car was a real crowd pleaser and how he kept it strait with only two wheels contacting is just plain good luck. His car never created a problem for any of the others as it never jumped the track. Again this was just plane luck.

Too cool!

If I had a test track I think I'd try to build one just like that!!!

[RacerRusty]

Rear COG can be a problem but it can also be a lot of fun for the crowd. At our PWD race last year we had a Tiger car with very aggressive CG that would slowly raise the front wheels off the track at about half way down the flat section of the track and pop a wheelie all the way through the finish line. He came in fourth overall for the pack and first in his den. His car was a real crowd pleaser and how he kept it strait with only two wheels contacting is just plain good luck. His car never created a problem for any of the others as it never jumped the track. Again this was just plane luck.

Oh I would really love to see a film of that!!! So cool!!!

[Go Bubba Go]

Rear COG can be a problem but it can also be a lot of fun for the crowd. At our PWD race last year we had a Tiger car with very aggressive CG that would slowly raise the front wheels off the track at about half way down the flat section of the track and pop a wheelie all the way through the finish line. He came in fourth overall for the pack and first in his den. His car was a real crowd pleaser and how he kept it strait with only two wheels contacting is just plain good luck. His car never created a problem for any of the others as it never jumped the track. Again this was just plane luck.

Afterthought on your comment about "good luck".

If one were to shape the bottom of the car correctly in the rear, I think you could intentionally achieve the straight running wheelie car.

I think it would only require a slight downward slope from each side toward the middle. That way, the friction from the bottom of the car rubbing against the center rail would have the tendency to self-correct the direction of the car. If the car starts to veer left, the contact point in the rear would be have moved slightly rightward, inducing a clockwise force that would move the rear to the left, correcting the front of the car back to the right (and vice-versa). The greater the distance between the rear wheels and the back of the car which rides on top of the rail during the "wheelie", the more efficient the correction.

Just a thought (I smell another Open Class car idea!!!).