Long or short rear fenders?

[derbydentist]

So i am making my first set of fenders and as has been expressed many times here on the forums it is kind of a pain - especially cuz Im working with balsa. My question is are short or long rear fenders better? I am not making trailing fenders. I am at a 5" wheelbase.
Thanks in advance for any advice

 

[B_Regal Racing]

I do not think it matters all that much, if it matters at all. Make them aesthetically pleasing. Don't make them higher or wider than the wheel itself and make sure they do not drag on the track or rail.

 

[derbydentist]

Thank you BRR! I wondered because I've seen both styles in pictures on this site. Here is where I'm at with the fenders it's our first pinewood derby. Is there a measurement someone can give me as to how far in the inset under the fenders need to be to avoid the rail- looking at the gee fenders it seems that it is a few millimeters more than it needs to be because the wheel will hit the rail before the fenders would. Sorry one more question- I've never seen or tested aero with a wind tunnel or anything like that so based on just sheer dumb thoughts in my head it seems like the trailing fenders wouldn't really be much of a help- can someone help me understand the logic behind that one?

 

[Crash Enburn]

Looks like you're off to a great start!

The rail is 1/4" tall. As long as your fenders wouldn't contact the rail before your wheels would, you'll be fine. It is impossible for the rear fenders to hit the rail before the rear wheels as long as they are not wider (internally) than the wheels.

Trailing fenders help to keep the airflow clean. If you see a wind tunnel test of a wheel, or even a wheel with a spoiler (fender) in front of it, there is a lot of turbulence behind the wheel. The trailing fender helps eliminate that.

 

[Skywalker Racing]

Those are some nice looking fenders!

There is turbulence that occurs behind your front wheel and behind your car. If you can mitigate that, you're better off. This is where trailing fenders can help.

Here is a helpful video to demonstrate:

 

[BulldogRacing]

Longer the better!

 

[derbydentist]

Thank you- crash, skywalker and bulldog- I love all the info I can get! Sounds like I need to go to work making some trailing fenders too. Skywalker I couldn't see the video - maybe it's just my phone not allowing me to play it - I'm very interested in seeing it- do you have a link I can go to?

Bulldog we are still talking about fenders

 

[Skywalker Racing]

Skywalker I couldn't see the video - maybe it's just my phone not allowing me to play it - I'm very interested in seeing it- do you have a link I can go to?

Happy to help.

I actually posted the URL, and the site automatically converted. I'd try to view from a desktop or search YouTube for "Why Trailing Fenders Work in Pinewood Derby"

 

[derbydentist]

I will definitely do that thanks!

 

[LightninBoy]

What's kinda ironic about trailing fenders is that for years the argument was that fenders don't help in PWD because the cars aren't going fast enough for drag to matter. But its actually because of these relatively low speeds that the turbulence behind the wheel matters; the slower you go, the more this rear turbulence contributes to the overall drag.

 

[Thinkin'Bout Racin]

What's kinda ironic about trailing fenders is that for years the argument was that fenders don't help in PWD because the cars aren't going fast enough for drag to matter. But its actually because of these relatively low speeds that the turbulence behind the wheel matters; the slower you go, the more this rear turbulence contributes to the overall drag.

That means I should REALLY benefit from a set of trailing fenders!

Thanks for the information, it is always nice to understand the why!

 

[derbydentist]

Yea I was reading some older posts like 2010 that were saying that same thing that it doesn't really benefit anything and the speeds are too low. But I agree and I think the trailing fenders are definitely more important than they used to I think. Sooooo that leaves me with a dilemma I have two short fenders and two long fenders and not enough time to make another set before our race- should I use the long fenders as trailing fenders and the short ones in front or the short ones in back and no fenders on the front? Which combo would be best?

 

[Crash Enburn]

I believe there is more benefit to have fenders on the leading side of the wheels than on the trailing.

(Although it would be an interesting experiment to see if using the long fenders as a trailing fender to the fronts [with no rear fenders, then] would perform better or worse than the classic leading fenders on both axles)

******************

Every time I see your name, "DerbyDentist", I can't help but think of the movie "Down and Derby".

 

[derbydentist]

Haha classic movie- I need to watch that again it's been a while! I think I just might have enough room in my house for a full track haha.Well I will probably just keep it how I planned it then with the fenders in the front of front and back wheels. It sounds like my area doesn't have any district Races I can go to so it will just be a pack race I need to worry about. I was kicking around the idea of sending my car in to race with you guys though - not sure what class I would qualify for. I will finish the build and see how fast I am and make some decisions. Anyone know relative times i should be at on a best track 35'. And how that compares or translates onto the longer track you guys race on? A good time on our track has been around 2.48

 

[davet]

I see you're running a 5" wheelbase with front, leading fenders. I'm trying a 5 1/8" wheelbase right now which is shorter than the 5 1/2" I'm used to. I'm concerned about the bottom edges of the front, leading fenders contacting the track surface at the transition. I see yours are much closer to the track than mine. Have you run that car yet? Did your fenders contact the track surface?

 

[derbydentist]

Yea It was our first car and first fenders but we have a best track that we put it on to make sure that no matter what angle the car went down the track that it didn't touch the rail. The fenders didn't touch no matter how crooked I put it on the track and as I slow rolled it at the transition it was good - not sure if at 5 1/2" though if they would still be good I've never tried longer than 5"

 

[davet]

Thanks.

 

[Jimmy & his 2 Kids]

Hey Derby Dentist your fenders look awesome. I went thru the exact same thing. Made short for front and long for the rear then saw Bulldog's fender vid making X3 fenders and then I saw that all the pros are running those as well. Look at the cars in my avatar. I did mine last minute and not enough time to make more. They worked out fine. I did the usual speed tricks and my 2 kids cars crushed it at our churches Awana GP. I love working with balsa since it is so soft. The first fender took me an hour to make but theN got the hang of it and pumped the rest out in less that time. It was actually therapeutic I think!
Will try to link Bulldogs fender vid. Highly recommended you watch it. Worth every minute and entertaining to boot!

 

[derbydentist]

Hey Derby Dentist your fenders look awesome. I went thru the exact same thing. Made short for front and long for the rear then saw Bulldog's fender vid making X3 fenders and then I saw that all the pros are running those as well. Look at the cars in my avatar. I did mine last minute and not enough time to make more. They worked out fine. I did the usual speed tricks and my 2 kids cars crushed it at our churches Awana GP. I love working with balsa since it is so soft. The first fender took me an hour to make but theN got the hang of it and pumped the rest out in less that time. It was actually therapeutic I think!
Will try to link Bulldogs fender vid. Highly recommended you watch it. Worth every minute and entertaining to boot!

Thanks Jimmy - I did see part of Bulldogs video and the design that everyone seems to be running and I will have to try that on my next build. What exactly is x3 mean? is it just the concept of having front leading and trailing fenders and leading fenders on the rear wheels and then connecting the portions between or is it something else I'm missing? Maybe I need to watch the rest of that video to get my answer haha

 

[Jimmy & his 2 Kids]

The one thing I do know (suspect)... think I heard Bulldog say it is the X represents the cross center section. Draw a line from each point diagonally to the opposite point (top front to bottom rear) and you will get an X. Not sure about the 3. Those lines will also guide you in getting your shape. Definitely watch the whole vid. I know its almost an hour long but it will go fast and its TOTALLY worth it. After linking it to that post I found myself watching it AGAIN. LOL
Hope that helps.