COM with Stock Wheels

[renner]

Been reading the boards for months and the info on this site is extremely helpful. My son is a new Tiger Cub. Our rules require stock BSA wheels that cannot be lathed, sanded, or modified. Based on the only slight imperfections in the new BSA wheels, do you have a recommendation for appropriate COM? I was thinking of going with 1", but that might be too aggressive if you can't turn down the wheels.

Note, based on what I have been reading, I plan to do a rail rider with the recommended 2.5 up camber on the rears and 1.5 down on the FDW. Am I going too far overboard for the first time out? The body is reasonably well configured based on the Star Wars theme requirement of my son and his helpful little hands.

[Murph]

Hi Renner,
Thank you kindly for your inquiry. A 1" COM is just fine and is a very comfortable number for the start of your journey building BSA PWD Cars for Cub Scouts!
If you have alignment problems, let us know and we can help!
Murph

[renner]

Thanks Murph. I'm going to try some more aggressive things in the "Dad" class, but I'd rather have my son finish respectible than not finish going for the win. I have no idea what the competition will be doing.

I'll post pics of the cars afterwards, but right now they are under wraps until the event. They really turned out nice.

[tkp]

I would only agree- newer builder, stock car, stock base, wheels etc- 1 inch would be perfect.

TKP

[renner]

We got a fleeting opportunity today for my son to try out a couple runs with his car on an older 32' wood track. No timer or anything fancy. We ran it against my cars from 30 years ago. He consistently got second to the car I built my last year in scouts, which was pretty fast but not a winner in its day. Looks like we have work to do. The good news is it went straight as an arrow along the rail with no sign of wobble. The overall alignment I believe is good except that we had like 6" drift in 4' on the kitchen floor. We are going to shift the weight back from 1" to 3/4" COM and adjust the steer less to maybe 2" or 3" in 4'. He also wants to trim about a 1/4" off the back of the car and add a bumper on the front, but that's a lot of work for dad with the scroll saw to still make it look right and it will ruin his beautiful paint job. I'll try to talk him out of that one. I suspect we have issues with the graphite and/or wheels as well because they don't spin as smooth or long as they used to no matter how much we try to work in the graphite (Come to think of it, maybe he dropped it or something when I wasn't around and never 'fessed up). But the rules state we have to use the stock wheels with no modifications so I'm not sure how to go about fixing that besides replacing them and starting over.

I wish we had a track available for more testing because its amazing what we learned with just those 4 runs down that old track. I believe the track he will be racing on is a 42' aluminum one but I've never seen it to know for sure.

Any idea roughly how much time relates to 1 car length of distance? I figure we have to find maybe another 2-3 car lengths worth of speed and I don't have any idea how difficult that will be.

[tkp]

1 car length is typically around .03

IE- 2.41 to 2.44 is about exactly 1 car length victory.

Sounds like your on right track-no pun intended, for adjusting steer, COM....but check those wheels and axles...could be losing speed there.

If you finger spin the wheels how long do they spin for?
What type of graphite are you using?
When you roll the car back and forth, does the wheel go to the axle head in both directions- front and back?
What is spacing between wheels and body?
How does wheels and axles look from any dings etc?
Are you running 3 wheels touching?
Curious- how much weight do you have in your current car?
How much weight did you have in old car?
What is old car's COM?

TKP

[renner]

- When we first prepped the wheels they would finger spin for around 25 seconds. Now we only get about 15 - 20 seconds depending on which wheel. (Note my old car wheels spin for 30 seconds silky smooth) At slow RPM's two of them tend to wobble side to side slightly which they didn't used to do. And they have a tiny tick when they spin, like it got a scratch or dirt in the bore. Seems like I noticed all this after the last time I tried to pack the wheels with graphite and work it in.

- We are using the Hob-E-Lube dry graphite with molybdenum. The stuff I used is kind of old. Can the stuff go bad over time? Would a fresh tube help?

- The rears go to the axle heads going both directions. The FDW goes to the body, which I thought it was supposed to do with the steer and positive camber built in. Maybe I misunderstood that part.

- The spacing on the FDW (left front) and right rear is .030 set with a gauge. The left rear is slightly more, maybe .045 or so but I figured that was okay because I didn't narrow the front like I should have when we first built it. I discovered that oops after we glued the axle. With the left rear wheel on the axle head, it shouldn't rub the rail.

- There is a small scratch across the tread on one wheel which I don't know where it came from. The rest of the wheels look fine. I did not pull the axles off because we finally got the alignment we wanted, but I realize I'm probably going to have to do that. I want to mark the axle TDC with a different colored marker so I can put them back in without major readjustments if I reuse the axles.

- We are running the right front wheel up, three wheels in contact.

- The new car weight is 5.023 oz right now, I figure we can sand that much off at weigh-in if we are over on their scale. The old car was 4.998 oz.

- The old car had an extended wheelbase but the COM was 1.5". If you compare apples to apples, the new car COM is back about 1/4" closer to the rear than the old car.

Now that you've laid it out for me so clearly, I know what we have to do. Tear out the wheels and axles, clean them out, and see if the tick and wobble is still there. If so I'll get a new set and start over with the axle polishing. I know we can get them to spin better. My son is getting tired of all the work and rework, so maybe I'll just do a late night session and fix it. He's done pretty good for a Tiger on his first car. I offered to help him build a new car with what we've learned but he has too much invested in this one and doesn't want to start over. And it really does look nice.

I appreciate all the help!

Joe

P.S. I got a chance to do two more runs today before they tore down the wood track. I made the weight and steer adjustments I mentioned previously. Still lost by 1 1/2 car lengths. The car tended to bounce around just a tad but that was likely because of the track.

[tkp]

Hate to cause any additional work for your young tiger, so this information is as you see fit.

How much actual tungsten did you add to your son's car?
Do you have a pic?

But yep- if you have a spin of 15-20- you definitely need to be able to be 25 or 30 seconds. And the fact that you notice a tick or wobble will halter performance without question. Sometimes you can break in wheels to much if you use dremmel etc- it can increase bore size and cause some slight wobbles in your wheel/s. Sounds like this may be the case. If it's only 1 wheel that doesn't spin about 25 secs, then perhaps you can replace only one- but don't know the exact situation.

Chances are your lube is just fine.

Regarding your front wheel- you should be fine as your rail riding.

The small scratch on the wheel you may be able to slightly sand away manually- per your rules- I have had to do this prior and lightly sanded by hand just enough to help but not too much to reduce overall tread. Perhaps use grit of 800-1200- something pretty fine. If you have to take the wheel off, check the bore to ensure they are clean, no scratches, no big increase in bore size and perhaps use a plastic polish on the wheel bore if needed. While your at it, polish up that axle again.

Additionally you made some adjustment on steer and it appears it helped on overall speed but you noticed some wobble. What you may have done is went a Touch away from where steer needs to be for this track. However the smoother aluminum track may mean less aggressiveness on steer- net - keep the steer you have now would be my recommendation as I believe you are very close and again, a smoother track will not need to be as aggressive.

I do believe however, that the general question that started the thread-- COG with stock wheels-- you are good with now- seems you went from 1 inch to 3/4. That should be fine if your Rail Rider set up is solid. Let us know how your progress and again, don't want to overload your son on this so if you make changes, see if you can hone in the 1 or two bad wheel/axles or whatever the key couple of items are that will assist in increasing speed without a complete overhaul.

TKP

[46u]

Our 2013 car is going to be a rail rider since we did so well this year with the straight runner. I am going to take off 1/16 off the body on the FDW.

[renner]

Feb 26, 2012 20:45:51 GMT -6 tkp said:

How much actual tungsten did you add to your son's car?
Do you have a pic?

The weight of tungsten and screws is 2.97 oz. I could cut out a bit of the wood in the center of the car but I would have to put tungsten back in virtually the same place. There is not much original wood left in the back of the car and as you can see the front of the car has all the excess wood removed without changing the car theme. The COM is now at 3/4".

Sorry I can't show you pics from the top until after the race on the 17th because I've been sworn to secrecy.

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[renner]

And the results are in...

Our Tiger Scout won his rank and got 6th overall in the Scout class.
Our youngest son not only won the "Dad" class, he also set the track record.
And Mom's "Hot Stuff" did pretty well too.

Thanks for all the advice and ideas. While we didn't try all the recommended things, we couldn't be happier with with our first derby. The ideas are already percolating for next year.

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[46u]

Congratulations. My grandson is in tiger scouts and this was our first year. He won 1st in pack and just like your son at district he won tiger scouts and came in 6th over all. I wish they had a dads race and a state race.

[RacerX]

AWESOME JOB GUYS FROM DERBY WORX!!!

[renner]

Today was the District Derby Race. Our District/Council races seem really late in the year. We built a new car for district to account for the rules variations and to incorporate what we learned from the first car.

The district race was held outdoors on a windy day! There was dirt and debris all over the track even though they tried to clean it fairly regularly. Cars were bouncing off the track, wheels flew off, one of the kids in our pack ended up with a bent axle of about 20°. I guess it was still fair because everyone was having problems.

But after all the carnage and the results were tallied, my son got SECOND PLACE! We qualified for the Council race tomorrow and the cars were impounded, so I never got to see if our car had any significant damage. I have to imagine the alignments on all the cars are shot.

Now we know for next year, use a very conservative COM for the district race.

[renner]

For the Council Race we shifted as much weight forward as we could, which was really only the tungsten putty used for trim. The non-dominant front axle got bent somehow from the District Race (see above post), so we straightened that out more or less. The Council Race was the same track as Districts, but it was cleaner and the car ran much smoother with no wipeouts. We ended up taking 4th in the Council Derby (1st of the Tigers) and my son was extremely pleased.

Biggest thing we learned from this is not to assume that the District/Council tracks would be nice and smooth tracks like our pack track is. We will allow the ability to move weight around the day of the race and use a 3/4" COM if its a good, clean track but adjust it to 1" or more if its like it was this year. We have a few more years to try to dial it in, but those top three cars that beat us were out of our league. We lost to the winner by about 5 hundredths of a second, 4.344 to 4.297, which in my book is quite a bit to make up.

[Murph]

Nice going guys!

You never know what type of track you will be racing on when you go to a Cub Scout PWD race! There are still some nice wooden tracks out there that are taken care of.

If you know you are running on a wooden track and the joints look rough, you will need to add more"drift" or turn into the rail if you are running a rail rider.

Murph