Rail Riding - relationship between COM and drift

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
Post Reply
ironband
Apprentice
Apprentice
Posts: 21
Joined: Mon Jan 30, 2017 10:58 am
Location: Livermore, CA

Rail Riding - relationship between COM and drift

Post by ironband » Fri Jan 12, 2018 12:40 am

Hi all,

'Tis the season! And my cub and I are hard at work on our cars for next weekend's pinewood derby.

Last year he wound up with a 1.5" COM on an extended wheelbase 3 wheel rail rider set to about 2" of drift over 4 feet. Racing on a wood track in pretty good condition. He took first in the pack, albeit in a fairly small field of 13.

This year, we switched to tungsten and a lower profile car, so we can set the COM between 5/8" and 1" for a little more speed.

One question that has been rattling around in my mind is whether a more aggressive COM has any bearing on the "right" amount of drift. I know that a more aggressive COM means less weight on the FDW, but I can't decide if that means we should use less drift (less weight more likely to jump the rail?), more drift (needs more steer to keep it on the rail since it has less weight riding on it?) or if it just doesn't really matter.

We won't have a chance to tune on the track, and so have to pick our drift and hope for the best.

What say you experts? Is there a relationship between COM and optimal drift on a 3 wheel rail rider?

Thanks in advance,

Andre



User avatar
MaxV
Merchant
Merchant
Posts: 489
Joined: Wed Jul 02, 2003 5:45 pm
Location: Phoenix, AZ
Contact:

Re: Rail Riding - relationship between COM and drift

Post by MaxV » Fri Jan 12, 2018 9:08 am

As you make the COG more aggressive, you need to increase the drift so that there is more pressure on the guide rail to help keep the car going straight. (since there is less pressure on the track itself)



Speedster
Pine Head Legend
Pine Head Legend
Posts: 1373
Joined: Sat Feb 18, 2012 2:48 pm
Location: Toledo, Ohio

Re: Rail Riding - relationship between COM and drift

Post by Speedster » Fri Jan 12, 2018 9:51 am

That is a fascinating question?
If someone has 3 identical scales, you might weigh the 3 wheels on the ground, perhaps using tungsten putty to shift the weight, and shift the weight from 1 1/4" COG to 3/4" COG leaving the drift the same. Then reset the drift and see if there is a gain, loss, or the same speed. Maximum Velocity might have already done this.



ironband
Apprentice
Apprentice
Posts: 21
Joined: Mon Jan 30, 2017 10:58 am
Location: Livermore, CA

Re: Rail Riding - relationship between COM and drift

Post by ironband » Fri Jan 12, 2018 8:43 pm

MaxV wrote:
Fri Jan 12, 2018 9:08 am
As you make the COG more aggressive, you need to increase the drift so that there is more pressure on the guide rail to help keep the car going straight. (since there is less pressure on the track itself)
OK, I can get behind that.

So if at 1.5" we had success with 2" of drift over 4 feet, do you have any guess for how much more drift would be appropriate at say 0.75"?

Or, would it still be 2" of drift, but to get that amount of drift more toe would be needed due to the reduced authority of the fdw?

Regards,

Andre



Speedster
Pine Head Legend
Pine Head Legend
Posts: 1373
Joined: Sat Feb 18, 2012 2:48 pm
Location: Toledo, Ohio

Re: Rail Riding - relationship between COM and drift

Post by Speedster » Sat Jan 13, 2018 7:06 am

Andre, here has been our experience for many years.
We always use the exact same model every year. It is a basic simple wedge cut to look a little bit fancy in order to remove as much wood as possible. The CG is always set at 3/4" using tungsten cylinders. 1 1/2 ounces are behind the rear axle slot. Dfw is set with 1 1/2 degrees of Positive cant and toe-in to get a drift of 2" in 4 feet on our tuning board. The tuning board is not moved after we have the drift. The car is then taken to my garage where the car is run down lane 1 of the Best track where it will travel exactly 30'. I have my track set for 30'. The track has a Micro Wizard timer. The best time is found and the car is then taken back to the test track. Always, and I mean Always, the car will drift to somewhere close to 36". Our Pack track and the 1st track at District are both Best tracks where the car will run 29'10". If we are in the top 8 fastest cars on the 1st track at District we then move to a Freedom track where the car travels about 40'. We have never won on the 1st track and lost on the 2nd track. From the records I have from previous District races I know how fast our car has to travel on my track to be competitive.
That's my story. I would concentrate on bore prep. I like the Walgreen's paper cotton swab shaft but that's just personal choice. I also use Liquid Glass after polishing the bores but again, I don't know if it helps. I cover the wood behind the Dfw with thick finger nail polish before applying graphite. We are limited to graphite. I'm not popular with what we do to the nail so I leave that up to you.
Best wishes to your team.



ironband
Apprentice
Apprentice
Posts: 21
Joined: Mon Jan 30, 2017 10:58 am
Location: Livermore, CA

Re: Rail Riding - relationship between COM and drift

Post by ironband » Sat Jan 13, 2018 7:16 pm

Speedster wrote:
Sat Jan 13, 2018 7:06 am
Always, and I mean Always, the car will drift to somewhere close to 36".
Thanks for the info Speedster - but do you mean 36", as in 3 feet of drift over 4 feet of running? My tuning board isn't that wide...Is this a typo?

Regards,

Andre



Speedster
Pine Head Legend
Pine Head Legend
Posts: 1373
Joined: Sat Feb 18, 2012 2:48 pm
Location: Toledo, Ohio

Re: Rail Riding - relationship between COM and drift

Post by Speedster » Sat Jan 13, 2018 8:55 pm

Sorry for the confusion. My tuning board is 6" wide. The car is originally set to drift 2" in 4'. After tuning on the track the car then drifts 2" in 3'. The axle is then glued in and we go racing.
We have been very fortunate to win many 1st place finishes and never worse then 3rd. This year my special scout happens to be my neighbor. Her name is Emma. Yep, the Cub Scouts race February 10th and the Girl Scouts race February 11th. It's going to be a Fun weekend.



ironband
Apprentice
Apprentice
Posts: 21
Joined: Mon Jan 30, 2017 10:58 am
Location: Livermore, CA

Re: Rail Riding - relationship between COM and drift

Post by ironband » Sat Jan 13, 2018 9:59 pm

Ah, that makes sense - thanks for the clarification! We're on a wood track (20 year old Piantadosi, but in pretty good shape).

My daughter's name is Emma - so we'll be rooting for you!

Andre



Speedster
Pine Head Legend
Pine Head Legend
Posts: 1373
Joined: Sat Feb 18, 2012 2:48 pm
Location: Toledo, Ohio

Re: Rail Riding - relationship between COM and drift

Post by Speedster » Sun Jan 14, 2018 4:45 am

The Piantedosi is a beautiful track. If you have an opportunity to help set it up you will be able to check the joints. Hopefully someone will run some cars down the track before the actual race. Your rail rider would make an excellent Test car if you're allowed to do it.



ironband
Apprentice
Apprentice
Posts: 21
Joined: Mon Jan 30, 2017 10:58 am
Location: Livermore, CA

Re: Rail Riding - relationship between COM and drift

Post by ironband » Sun Jan 14, 2018 12:28 pm

It is a nice track. I spent the better part of yesterday with our cubmaster extending the stopping area - over the years the original foam rubber compacted in the box, and previous years' attempts at adding grip tape, sandpaper, and who-knows-what else on top have been ineffective. We built an extension area with just railing (and some rubber drawer liner attached) so that the cars will high center and stop. We used last year's cars to test it out.

We agreed it wouldn't be fair for us to test this year's cars since the rest of the pack doesn't have access. Anyway, this year's cars aren't ready yet :)

Fortunately, I am on the set-up committee, so I can make sure the joints are aligned as well as they can be. What we do in our pack is we have the adults/den chief race as the preliminary event, and use that to test the track and make sure everything is good to go prior to the main cub event. That's worked well for the last couple years, and I don't think we've had a single car leave the track in cub racing since we started that practice. Our adult races have no award beyond bragging rights, so if something goes horribly wrong we all just laugh about it and carry on :)

Do you have an opinion on whether wood tracks really need a more aggressive drift than aluminum? Unless the track is really rough, I don't really see this as being an issue, but I have read elsewhere that wood tracks might need a touch more.

Andre



Speedster
Pine Head Legend
Pine Head Legend
Posts: 1373
Joined: Sat Feb 18, 2012 2:48 pm
Location: Toledo, Ohio

Re: Rail Riding - relationship between COM and drift

Post by Speedster » Mon Jan 15, 2018 5:01 am

Andre, I'm not sure what is meant by a "rough" wood track. If the Guide Strip is really rough then perhaps a more aggressive drift might help to keep it on the rail. We raced on a Piantedosi track up until two years ago when we acquired a Best track. The way that track was designed and the type wood used I would guess you are very fortunate. Even though I'm able to increase the speed of the car on my track it has never changed the outcome of the race. When we win, we win. and when we lose, we lose.



Post Reply