Basics: Stock wheelbase, lead powered car. CG ~5/8" in front of rear axle. (discussion and prototype photo on this thread http://www.derbytalk.com/viewtopic.php?f=11&t=8230 ) Stock weight wheels, prepped with LG and graphite.
Track is a 43' BestTrack in decent shape but with some small flaws were someone filed down the areas where the track sections come together to "make it smoother" Our rail riders go down it without issue, but I typically have had a bunch of drift dialed in.
So our plans:
2.5 degree negative camber on the rear wheels using bent nails
5 degrees positive camber on the DFW using bent nail.
Wheel off the track slightly toed out.
I have never used more than about 2 degrees on the DFW but read that 5 or more was better. And I trust everything I read on the internet...
So does that sound about right?
Also, we have a 20ish foot wood track that I made several years ago. I have used it a ton for testing and dialing in cars in the past. But I was never able to just set 1" drift over 6 feet. But this year we plan on just aligning it without the front wheels on a test board, Putting on the front wheels and setting the drift at 1-2" over 6', then doing some slower coasting trials down the bottom sections of our track to fine tune it to run as free as possible. Does this sound like a good approach?
As always, thanks!
- Vitamin K
- Master Pine Head
- Posts: 980
- Joined: Sat Apr 20, 2013 7:26 pm
- Location: Montgomery County, MD
With bent-axle rears, having them in good alignment is critical. Consider building Lightninboy's speed square tester, or using Stan Pope's bias weight method, or both.
Your drift numbers sound rather low to me. I would suggest starting with something like 3-4" over 4 feet. Personal opinion, there.
Agree with Vitamin...you will want more drift than that I use 8-10" at 7 feet. Which is close to Vitamin's numbers at 4'. So ball park guess at 6' would be 7-8". Stan built a calculator to figure it out a bit better than ball park. Ultimately it depends on a lot of factors but 3-4" in 4' is the normal starting point.
If you have drilled straight holes on the rear and have good, straight axles, you might want to install them and see what the rear wheels do. Bending rear axles might hurt you more than they're going to help.
I didn't realize you were that far into the build. as long as the rear wheels migrate out to nail head rolling forward and backward, you'll be fine, but I fought with bent axles for a long time when I was first learning and could never get them right. Took me a while to understand the geometry and physics of everything, then straight just seemed to make sense to me and it just seemed to click and be easier for me to get the car setup right.
Do not be afraid of bent axles as long as you use the Lightninboy rear alignment process linked earlier in this thread. It is fast, easy and foolproof. If you mark with Sharpie at 12:00 and bend towards the mark, install axles with the mark in the 12:00 position. Minor adjustments from there one side at a time and the rears will be aligned in less than 5 minutes.soccerdad wrote:Thanks, Great information. Glad to know about the drift. I am now not sure on the bent nails... I already drilled the holes straight and cut the cars. But I may be able to fill and redrill without too much pain. Question- if you use straight axles on the DFW, then how to you adjust the drift?
It is so easy and accurate that you can adjust so that both wheels migrate to the head forward and backwards at the same rate, perfect alignment.
Once it's aligned make a new 12:00 mark as it sits in the car with a etching tool. Then whenever you take the axles out just keep them marked for LR or RR and you'll always be able to reinstall exactly as they came out without having to realign each time.
You don't even need a speed square. We made a tool by gluing 3 paint stir sticks (free Home Depot) together side by side. Then at a 90 degree angle we glued a single stick flat on the top so it sticks out to one side about 6". We slide the 3 sticks along a edge table with the one at the 90 degrees used to rest the car on. We used to use a tack with double stick wrapped thick enough to get the front same height as with the DFW installed. We have also just rolled up duct tape until it was thickness we needed to get the correct height.
We are doing one car with bent axles since I am used to doing it that way. And one with straight. So today we will set up our test track and see what we got. Should be fun. We already did the initial drift testing. Seems like we need to have a fair amount of toe in on the DFW to get a 3" drift. I used a 1.5 degree bent nail and the bend is set almost full forward to get the drift. The pictures of the Lightninboy alignment process for the rears are not up anymore. But I did some front wheel off tests with it on a small center runner and it goes pretty straight with the axles migrating out in both directions. Since these are only 5/8" CG cars, maybe the rear is just really dominating the setup for my drift trials. We will test some more and have lots of tuning stuff ready for tech day this Tuesday when we get to hit the real track.
So thanks to all that helped. I learned a bunch. Unfortunatly, this is our last race ever. So it is nice to finish on a high. But still a little sad. I have been an advocate for our program for so long they actually named the spirit trophy after me. A humbling honor. So thanks again. Here are some pix and proof that lead powered cars can still be competitive!
Thanks! I could not do the above listed alignment techniques because the photos are not there anymore and I didn't understand the working. So I just used a board without my front wheels on and put a small skid under the center of the car to bring it up to normal height. I then just adjusted them to have the car go straight, while having it as free as possible. I figure the faster it rolls straight, the toe of the rears must be as neutral as possible. Once that was done I put the fronts back on. But with the 5/8 CG, I had to crank in a lot of toe in on the FDW to get a 3-4" drift on a 4' table. With that setup, it was pretty fast on the real track. But once we found that the other car we had was a little faster, I started adjusting this one again. We still had a little wiggle in the car going down the track. So we rolled one rear axle back a little and it did not help much. So we rolled it forward a tiny amount and it got better. Then we turned the other one a tiny bit in the opposite direction and it got even faster. We then adjusted them again a little more and it slowed back down. So we reset to the fast position and left it. This was the fastest car we have ever put on the track. I think the LG prepped wheels helped, but the CG and alignment were keys as well.Vitamin K wrote: Any observations/ideas in regards to the tuning of bent rear axles?
BTW, you cant tell from the pix but the bottom of the car is also skinned with the super thin plywood.
Other notes on the race. This year we had way more fast cars than ever. The internet and forums like this are giving everyone more information and you can really tell. Also we are seeing more factory built chassis and parts, which is a little disappointing. But our turnout improved so good and bad.
Finally, the fastest car of the night, (not in our class) was just a basic wedge (pretty thick) with cheese holes cut in it. So aero it was not. But it just smoked down the track. Just a dad and his son throwing together a car that hit the perfect happy spot. I am sure they did their homework, but they did not get hung up on being trick. Not sure what other prep they did but it was great seeing that car beat all the kit cars.