COM Balance Point

Hermanator

Hammering Axles
Oct 20, 2017
5
6
3
50
I’m building a 1/4” ladder car for my son running a 5 3/4” wheelbase which is weighing 10 grams right now. We’ll likely run 4” in 4’ of steer. We will run 2 ounces of tungsten (12 cubes) in the far rear. Right now the balance point is right at 1/2” in front of the rear axle with 2 ounces on each side of the axle. My question is will the car be stable (& not wheelie) if I put 2 more ounces right in front of the rear axle or should I slide some cube in front of rear axle forward to increase the balance point? If so how far in front of the rear axle should the balance point be for a Cub Scout race on a 42’ aluminum track? We ran 3.04-3.06 (on average) on a non ladder car with graphite & no fenders last year with a balance point of 7/8” & 2” of steer in 4’. Just trying to get faster.
 
Hi Hermanator,
I've only been doing this since March so please take my opinion with a grain of salt but I think you'll find most people on this forum run two rows of 6 cubes directly in front of the rear axle. If the other aspects of your build are good you should be fine.
 
  • Like
Reactions: Craven
I’m building a 1/4” ladder car for my son running a 5 3/4” wheelbase which is weighing 10 grams right now. We’ll likely run 4” in 4’ of steer. We will run 2 ounces of tungsten (12 cubes) in the far rear. Right now the balance point is right at 1/2” in front of the rear axle with 2 ounces on each side of the axle. My question is will the car be stable (& not wheelie) if I put 2 more ounces right in front of the rear axle or should I slide some cube in front of rear axle forward to increase the balance point? If so how far in front of the rear axle should the balance point be for a Cub Scout race on a 42’ aluminum track? We ran 3.04-3.06 (on average) on a non ladder car with graphite & no fenders last year with a balance point of 7/8” & 2” of steer in 4’. Just trying to get faster.

Take this with a grain of salt as I am not a pro. However, I will share what I have learned over the last 8 months.

When trying to compare one car to another and asking oneself if it will be faster, a number of items need to be considered.

How stable did last years car run? What axles & wheels were on it. What wheel/bore prep was done? What "improvements" am I trying to make?

To answer your question ...it will not wheelie, but will it run straight? It depends. How good is your foundation (drill)? Is your alignment near perfect? What wheels and prep will be used? Remember that wheel bore to axle slop along with excess wheel gap can contribute to a car that wiggles.

What you have going for you is the long WB is normally quite stable and will track straighter. A true .500" COM is quite aggressive for a scout car. What I have read here is that something around .625" - .700" is a good COM for a scout car. That along with a WB around 5" seems to be the consensus that I have read.

While having the COM as far back as possible has the most potential to have a fast car, I believe that giving up a bit on the COM is not a bad thing. paying CLOSE attention to ALL of the other details (specifically the wheel/bore prep & Drill) will yield the best results.

Again, this is just my humble opinion.

You have come to the correct place to learn and get answers ...Good Luck.
 
  • Like
Reactions: Craven and Reecedad
Thanks TBR.
Last year we ran a 1” COM with bent axles all the way around. Had 1 gram Maximum Velocity wheels with .092 axles tuned with approximately 2” of steer over 6’. Polished the axles up to 3000 grit & used a plastic polish on the wheel bores. Wheels were at roughly 2.5 degrees & the 4th wheel didn’t touch. It ran very stabile with no wobble. However it wasn’t as aerodynamic as a 1/4” car. This year we’re drilling the rears at 2.5 & measuring with 123 blocks, pins & feeler gauges. I’m 98% confident in the set up. He’s also running DD4H .093 axles & 2.0 gram wheels as the 1.0 gram wheels last year were to flimsy. We’ll polish to 3000 on the axles & do the wheel bores again & also run Pledge on the axles this year. My sons hope is to get to a sub 3 second graphite car in 42’’. Should I be polishing the axles all the way to 12000? Does it help a graphite car?
Thanks again,
 
  • Like
Reactions: Reecedad and Craven
EE0BC80B-5F58-4223-B4E0-F9CACA2BD2D7.jpeg

The green car was last years car & he won by 2-3 car lengths each race.
 
  • Like
Reactions: Reecedad and Craven
Thanks TBR.
Last year we ran a 1” COM with bent axles all the way around. Had 1 gram Maximum Velocity wheels with .092 axles tuned with approximately 2” of steer over 6’. Polished the axles up to 3000 grit & used a plastic polish on the wheel bores. Wheels were at roughly 2.5 degrees & the 4th wheel didn’t touch. It ran very stabile with no wobble. However it wasn’t as aerodynamic as a 1/4” car. This year we’re drilling the rears at 2.5 & measuring with 123 blocks, pins & feeler gauges. I’m 98% confident in the set up. He’s also running DD4H .093 axles & 2.0 gram wheels as the 1.0 gram wheels last year were to flimsy. We’ll polish to 3000 on the axles & do the wheel bores again & also run Pledge on the axles this year. My sons hope is to get to a sub 3 second graphite car in 42’’. Should I be polishing the axles all the way to 12000? Does it help a graphite car?
Thanks again,

I was waiting for someone more experienced to chime in here because I certainly don't want to monopolize the conversation with my limited experience.

I understand the goal of wanting to run a sub 3 second pass and it is admirable. However, I have learned that trying to compare one track to another is almost a waste of time. This is especially true if the track is set up and taken down. It may not be as level or the hill may not be the same angle. So remember that while your times may be faster/ slower on your home track, they may well not be directly comparable to last years runs. Having said all of that, from what you are writing, you are headed in the right direction.

My only experience so far is with graphite cars. We are hoping to eventually dip our toes in the pro leagues. Your question about how far to polish an axle for graphite ...I think you would see mixed responses here as I have read both theories. I don't believe there is a huge difference, but we are all looking for any advantage that we can. I don't have the means to perform a meaningful test at this time. I will tell you that we did polish to 12,000, then followed that with DD4H's polishing step with leather. Several of our cars (after break-in/burnishing) have in excess of 90 runs on them. I have found that we are not seeing any significant drop in times after 16-18 runs. However I prefer to lightly re-graphite and clean the wheels after 10 passes. The number of runs is largely due to the fact that I am new at this and discovering how to approach tuning. The first three cars all had significantly more passes than the later 3. This is probably because I figured out what worked with our wheels, axles, and prep. A general pattern developed on our package. I believe this pattern will be a bit different for each combination or package from each builder. This is because of WB, weight, prep process and other variables unique to each car and builder.

Good luck and welcome to the addiction. ;)
 
  • Like
Reactions: Reecedad
If you're running the same track this year as the one in the pic, it looks like you're golden at the 1/2" COM. On a track with gaps or rough edges, you'd want a bit bigger a COM number, to keep the car planted.

If you have the ability to tune on the track, make use of it. Run the same lane every time, and adjust the steering to give the best combo of fast and stable run.
 
  • Like
Reactions: Craven
Crash Enburn.....Thanks, yes it’s the same track. We’ll be able to make a run or two the night before setting up. Hopefully all goes well if not we’ll be tearing into it all night to adjust it.
 
  • Like
Reactions: Craven
I’m building a 1/4” ladder car for my son running a 5 3/4” wheelbase which is weighing 10 grams right now. We’ll likely run 4” in 4’ of steer. We will run 2 ounces of tungsten (12 cubes) in the far rear. Right now the balance point is right at 1/2” in front of the rear axle with 2 ounces on each side of the axle. My question is will the car be stable (& not wheelie) if I put 2 more ounces right in front of the rear axle or should I slide some cube in front of rear axle forward to increase the balance point? If so how far in front of the rear axle should the balance point be for a Cub Scout race on a 42’ aluminum track? We ran 3.04-3.06 (on average) on a non ladder car with graphite & no fenders last year with a balance point of 7/8” & 2” of steer in 4’. Just trying to get faster.
I build all my cars with 2 oz behind and the rest right in front of the rear axle. It will work like a charm if you steer it enough.
 
  • Like
Reactions: ljo
Thanks all. We raced yesterday with 2 oz behind & 2 oz in front with 4” of steer in 48”. 6 lane track & have to run each lane twice. The car was untouchable in 5 of the 6 lanes. There was one lane with a bad/rough spot that upset any car that was truly tuned. Thankfully we slid sideways across the finish line both times on the bad lane. Qualified for the finals & took the steer out of the car for the bad lane. His car slowed down .04 to .05 sec per race but completed each lane in the lead and took 1st place. Now it’s on to districts in April.
 
  • Like
Reactions: Crash Enburn
Thanks all. We raced yesterday with 2 oz behind & 2 oz in front with 4” of steer in 48”. 6 lane track & have to run each lane twice. The car was untouchable in 5 of the 6 lanes. There was one lane with a bad/rough spot that upset any car that was truly tuned. Thankfully we slid sideways across the finish line both times on the bad lane. Qualified for the finals & took the steer out of the car for the bad lane. His car slowed down .04 to .05 sec per race but completed each lane in the lead and took 1st place. Now it’s on to districts in April.
Nice! Congrats! sounds like there was some excitement there with the sideways sliding - always gotta be something at these races that keeps it interesting!
 
Thanks all. We raced yesterday with 2 oz behind & 2 oz in front with 4” of steer in 48”. 6 lane track & have to run each lane twice. The car was untouchable in 5 of the 6 lanes. There was one lane with a bad/rough spot that upset any car that was truly tuned. Thankfully we slid sideways across the finish line both times on the bad lane. Qualified for the finals & took the steer out of the car for the bad lane. His car slowed down .04 to .05 sec per race but completed each lane in the lead and took 1st place. Now it’s on to districts in April.


First - Congratulations!

Second, a couple of questions.

Was it a bad joint or a dip/bump in the track that caused the issue? You also stated that you took steer "out of the car". So you put less steer in the car and it was not as stable and slowed down but didn't derail? Or did you put a ton , of steer in it causing it to slow down?

Just trying to figure out why it was derailing and what/how you compensated to keep it in the lane?

Wondering if it was a dip/bump, if you were on the edge for COM allowing the front to get light and derail?

Sorry about all the questions, but I am always "thinking" about these things and trying to figure out the cause & effect relationship.

BTW! Good luck on District next spring!
 
Last edited: