Standard axle locations?

[Aron]

Hi, I'm new here. My name is Aron and my son and I just finished our 2nd year of races.I had a question about the standard axle locations. I would just like to get an opinion here. Just wondering where people have had more success, with the axle position. I have built a car the past 2 years and the first car had the larger spacing in the back which allowed for more weight to be put in the rear. My COG was about 7/8 of an inch in front of the rear axle. This year due to the design of the car I had the smaller spacing at the rear so I used tungsten weights to help out. both cars weighed 4.9ozs. My COG on the new car was worse at about 1 1/4 inch in front of rear axle. To my suprise the new car beat last years car by 2 car lengths. I do think I did a little better job on this years axles but I was still a bit suprised at the outcome. Just wanted to see if anyone else had any input or experience to share.

[FatSebastian]

Aron! Do your rules require you to use the original axle slots in the block? If so, then it is almost universally recommended that the slot closest to the edge should go to the rear. Some people who are required to use the original slots even cut the back of block off and fasten that piece to the front to move the rear wheels as far back as physically possible!

(If you are not limited to using the original slots, then it is usually best to extend the wheelbase as far as your rules allow, with the rear wheels are far back as possible.)

Based on your description (and if I did my math right), the slower car has its COM 4.375" from the starting pin, and the faster car has its COM 5"from the starting pin, so the faster car has a higher COM overall when placed on the track. Generally speaking, the further the COM is from the starting, the higher the speed when the car reaches the flat of the track.

[Stan Pope]

I can supply some input, FS. Aron lives a couple of towns over and (I'm pretty sure) attends the local pack. If so, he is in my district.

We do not require use of slots and many, many folks choose to drill axle holes regardless of where they place the axles.

If Aron studies the stuff here on DT and if he and his son choose to rebuild his car (or build another), my grandsons could have some more serious competition at the district races. That is not to imply that there is not enough competiition already ... There are some really good builders floating around our district.

My grandsons got there new builds underway during the past 2 days ... I'll post some sketches and measurements of what they are doing so far. I think that needs to go in the "construction" forum.

[Aron]

Thank you very much for the input gentlemen! As I said I am still quite new at this stuff but I am learnig quickly. It's quite a fun little hobby for the boys and I. FatSebastian~ Those are some very helpful suggestions....Thank you!

Stan~ We didn't go to districts last year but I do hope to attend this year. Do you have any info on the rules for that race? The local pack my son is in is not the most organized! I have heard many people say that they get to go test run their cars the week before. Our local pack dosen't do this. Oh and the reason the cars are at 4.9 is because their scale is off by 1/10th. It's the same for everyone but seems a little odd.

Thanks again

Also~ Have you guys had any issue with front end shake before? I had this happen on one run and the car actually came off track on the flat. Luckily we won the other 3 runs.

[Stan Pope]

Thank you very much for the input gentlemen! As I said I am still quite new at this stuff but I am learnig quickly. It's quite a fun little hobby for the boys and I. FatSebastian~ Those are some very helpful suggestions....Thank you!

Stan~ We didn't go to districts last year but I do hope to attend this year. Do you have any info on the rules for that race? The local pack my son is in is not the most organized! I have heard many people say that they get to go test run their cars the week before. Our local pack dosen't do this. Oh and the reason the cars are at 4.9 is because their scale is off by 1/10th. It's the same for everyone but seems a little odd.

Thanks again

Also~ Have you guys had any issue with front end shake before? I had this happen on one run and the car actually came off track on the flat. Luckily we won the other 3 runs.

Aron, the district rules are posted at http://www.wotamalo.org/. Well, it takes time and effort to set up the track, but I bet if enough of your district reps wanted it done, it could get done. It is a good learning experience, and it helps your pack's members to get the best out of their trip to the district races.

You will want to put that 0.1 OZ of weight back in the car before the district races ... The extra weight usually puts your car at the finish line a bit earlier than it would otherwise.

On the wiggle situation, first decide if it was caused by the track or by the car. If a big bunch of other cars showed the same behavior, then don't do anything drastic to the car before digging into the question a lot more.

Most likely, though, only a few if any others did that. Then you need to diagnose the cause. The diagnosis is helped if you can describe the motion more thoroughly. Describe the motions of the front and rear and whether they move in sync or in opposite directions, etc.

Sometimes you can tell a lot about a car by watching it roll in slow motion across a table top, especially if the table is tilted a bit, maybe 1/2" higher on one side. Then you can let gravity move the car. Look at what the wheel do and how the front and rear seem to affect the steering. Multiple rolls with the wheels starting in different locations on the axles can show up "rough spots" that need to be addressed. Finger spin of the wheels can show if the wheels have about the same friction losses or if there is a grave imbalance. Spinning while tilting the car can show if there are special friction issues with the nail head and hubs.

[Aron]

Thank you once again Stan for the info!

Any place around here that you know of that has a test track?

[Stan Pope]

Thank you once again Stan for the info!

Any place around here that you know of that has a test track?

My usual practice has been to check around at Cub Roundtable and ask for someone with a derby or practice session coming up, then wheedle an invite. CRT is 2nd Thursdays in Wotamalo.

[Aron]

I got another question for you Stan. My car weighs in at 5.0 acording to the scale but the scale only goes in 1/10s. So it either weighs 4.9 or 5.0. Do I neeed to be under 5.0 at the district races?

[Stan Pope]

I got another question for you Stan. My car weighs in at 5.0 acording to the scale but the scale only goes in 1/10s. So it either weighs 4.9 or 5.0. Do I neeed to be under 5.0 at the district races?

Wotamalo District races weigh to 0.01 oz ... 5.00 good, 5.01 bad. With the wide availability of very inexpensive 0.01 scales (e.g. Harbor Freight pocket scales) this seems reasonable. Plus, if there is a problem with calibration differences, the amount of weight reduction to bring the car into conformance is small enough to handle easily with a craft knife! I like to sneak up on the weight and leave about 0.05 oz in the form of a bead of hot glue that can be trimmed easily ... even with a thumbnail!

A reading of 5.0 oz on a well calibrated scale probably means that the car weighs between 4.95 and 5.05 ounces. The 0.1 oz variation is measureable on the track. A common ploy is to tune the car as heavy as possible without tripping the scale to 5.1 oz. The choice to weigh in on 0.01 oz scales pushes the speed difference well "into the noise" and avoids the question of whether or not it is "cheating" to deliberately be on the heavy end of the 5.0 oz (e.g. 5.04 instead of 4.96 oz) range.

It is interesting how many of the cars weigh in at district inspection at either 4.99 or 5.00 oz!