DFW...I need explanation

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
Post Reply
SPEEDQUEST
Apprentice
Apprentice
Posts: 18
Joined: Wed Apr 20, 2005 8:57 pm
Location: Central NC

DFW...I need explanation

Post by SPEEDQUEST » Wed Jan 07, 2015 10:45 pm

I want to make sure I understand everything I have read on DFW. Most popular suggestions:
Drill hole straight.
Bend axle
Positive cant
Toe in
Set drift to 3"-4" over 4ft

With negative cant the the car is riding on the outer tread...correct?
That means the wheel is going to migrate in toward the body of the car...correct? And this will be compounded with toe-in..correct?
This goes against better judgement, wheel in contact with body has more friction than wheel against axle head.
NOW...
Once the car reaches the rail, the wheel is forced back to the axle head...correct?
So if the rear is aligned with no wiggle...the car "runs" the rail
Can someone explain this so a blue-collar guy like myself understands? Lol



User avatar
Noskills
Master Pine Head
Master Pine Head
Posts: 787
Joined: Tue Jan 31, 2012 1:37 pm
Location: Bellevue, WA

Re: DFW...I need explanation

Post by Noskills » Wed Jan 07, 2015 10:57 pm

Speed quest,
You forgot to mention to shave 1/16 off the DFW side so the same side rear wheel is off the rail. Some drill straight and used the bent axel to give some toe-in and some positive camber. Other drill with a little positive camber (like me). This will have the wheel move towards the body but it's offset (as you mention) by the rail pushing it to the head. I call this the ying and yang of the DFW. I am a simple country doctor so I hope this was simple enough. I don't get half the stuff some of the engineers say but we still do OK in the races.
Noskills
Also the amount of drift will depend on how aggressive your COM is and your wheel base. Longer wheelbase and less agressive
COM can get away with less drift. 3-4 in is pretty good in general. I like 2-3 with my extended wheelbase cars. Other with tracks can tune each car while I can only give my best guess.


"Nunchuk skills... bowhunting skills... pinewood derby skills... Girls only want boyfriends who have great skills!"
Napoleon Dynamite

SPEEDQUEST
Apprentice
Apprentice
Posts: 18
Joined: Wed Apr 20, 2005 8:57 pm
Location: Central NC

Re: DFW...I need explanation

Post by SPEEDQUEST » Wed Jan 07, 2015 11:43 pm

Thanks Noskills.
I have another question...
Per our rules... No canting.
So I am building car with extended wb. How light Can I go on DFW?
I am thinking around .75-.80 oz is about aggressive as I can go since I cannot cant rears.
Thoughts?



User avatar
LightninBoy
Master Pine Head
Master Pine Head
Posts: 203
Joined: Mon Feb 20, 2012 10:09 pm
Location: St. Paul, MN

Re: DFW...I need explanation

Post by LightninBoy » Thu Jan 08, 2015 12:05 am

You pretty much nailed it SQ ...
SPEEDQUEST wrote:I want to make sure I understand everything I have read on DFW. Most popular suggestions:
Drill hole straight.
Bend axle
Positive cant
Toe in
Set drift to 3"-4" over 4ft


Like NoSkills said, you'll want to shave off about 1/16" of the body around the DFW. This will more likely center the rears over the rail. This makes the car easier to stage consistently (most stagers will simply center the car on the rail by looking at the rear wheels).
With negative cant the the car is riding on the outer tread...correct?

I think you meant positive cant. If so, yes that is correct.
That means the wheel is going to migrate in toward the body of the car...correct? And this will be compounded with toe-in..correct?
Correct and correct.
This goes against better judgement, wheel in contact with body has more friction than wheel against axle head.
Positive cant on the DFW is unintuitive, I grant you. I had to take it on faith myself, but it is faster. Stan is the man when it comes to explaining this stuff ... I'll leave it to him (or you can search for posts by Stan Pope).

You'll want to treat the body area where the wheel hub meets the body. I cover the area with CA glue and sand it super smooth. Others prefer to use nail polish. League racers typically glue delrin or Teflon washers to the body for the hub to contact, but this is not normally legal in scout racing.
Once the car reaches the rail, the wheel is forced back to the axle head...correct?
Not typically. Usually it stays against the body and only goes to the axle head if it contacts some roughness in the track (for example - an uneven track seam).
So if the rear is aligned with no wiggle...the car "runs" the rail
Can someone explain this so a blue-collar guy like myself understands? Lol
I took a shot. Hopefully that helps.



User avatar
Stan Pope
Pine Head Legend
Pine Head Legend
Posts: 6888
Joined: Sat Jul 05, 2003 7:01 pm
Location: Morton, Illinois
Contact:

Re: DFW...I need explanation

Post by Stan Pope » Thu Jan 08, 2015 4:32 am

First a question: How is the "no camber" rule inspected? Possibilities include
1. Can the wheels be placed to rest flat on level surface? If so, pass! Or
2. Place the car normally on a flat surface and roll it for a few inches. If paper can be slid under the wheels from either side, the axle is cambered, fail!

If #1, then as mentioned elsewhere, apply a very small amount of camber, around 1/2 to 1 degree. After rolling, the wheel will settle on an edge.

Positive vs negative camber on DFW:
Positive camber: the wheel slides on the rail a small distance. Force times distance = work = energy lost!
Negative camber: the wheel slides on the rail a longer distance ... more work = more energy lost!

Rubbing rim vs hub:
Distance from the wheel center factors into the friction loss ... larger distance = more work = more energy lost.

Help?


Stan
"If it's not for the boys, it's for the birds!"

User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1087
Joined: Sat Apr 20, 2013 7:26 pm
Location: Derwood, MD
Contact:

Re: DFW...I need explanation

Post by Vitamin K » Thu Jan 08, 2015 6:14 am

It took me a little while to get my head around the whole concept, too.

One 'epiphany' for me was that the major benefit of "rail riding" was not so much that the car traveled a straight path (though, obviously, this is a good thing!), but that the rear wheels stayed off of the rail. Since the rear of a properly weighted car carries the lion's share of the weight, the speed losses when the rear wheels hit are much greater than when the front wheel(s) do. Thus, the value of having a properly angled front wheel that skims and steers into the rail (while blocking the rears from doing so) becomes apparent.



User avatar
whodathunkit
Pine Head Legend
Pine Head Legend
Posts: 2046
Joined: Sat Apr 25, 2009 5:56 pm
Location: Forgan, OK

Re: DFW...I need explanation

Post by whodathunkit » Thu Jan 08, 2015 11:02 pm

LightninBoy wrote: (most stagers will simply center the car on the rail by looking at the rear wheels).
Would it help the stagers out..
If there were 2 black sharpie marks down the back side of the car body
that help line the back end of the car body up.. by lineing it up with the rails on the track..
So that when the car is staged on the track by someone else..
it is staged as close to the center line of the track with the back wheels pulled out.

Thought's... anyone! ;)


What type of automobile can be spelled the same forwards & backwards?

User avatar
Vitamin K
Pine Head Legend
Pine Head Legend
Posts: 1087
Joined: Sat Apr 20, 2013 7:26 pm
Location: Derwood, MD
Contact:

Re: DFW...I need explanation

Post by Vitamin K » Fri Jan 09, 2015 7:36 am

I wanna say that one of the popular books (Pinewood Derby Speed Secrets?) mentions something about a paint job with stripes to act as a visual aid for staging.

Also, I wanna say that the NPWDRL has a system for where you can put a mark on the back of your car to indicate how you want the stager to place it on the track.
whodathunkit wrote:
LightninBoy wrote: (most stagers will simply center the car on the rail by looking at the rear wheels).
Would it help the stagers out..
If there were 2 black sharpie marks down the back side of the car body
that help line the back end of the car body up.. by lineing it up with the rails on the track..
So that when the car is staged on the track by someone else..
it is staged as close to the center line of the track with the back wheels pulled out.

Thought's... anyone! ;)



Post Reply