Why are rear wheels so far forward?

[1reason]

Next week is my son's fourth derby race and despite countless hours and study on my part and lots of effort on his part, his best is 3rd place. Unfortunately I only now found out I've "over engineered" his car, including grinding down the axles too far and placing the weight too far back etc... (I think I made every mistake possible to boost performance.

Anyway, back to my question. I'e noticed almost all cars' rear wheels are located within the wood area with wood behind. If the goal is to move as much weight as far back as possible in relation to the axles, why not move the axles to the very far back and leave the wheels "hanging" rear of the rear bumper?

To stay within the length limits, the body must be shortened a relative amount. I haven't found a local rule that says where the wheels must be other than wheelbase distance and width distance.

I get that the amount the front is cut off reduces the height of the weight, but moving the rear wheels back allows for all the weight to be concentrated closer together without leaving space for the axles to be in between.

Ok, with that asked, at a minimum I don't understand why the rear wheels wouldn't always be located so the rear edge is as far back as the rear edge of the body?

Thanks in advance

 

[5KidsRacing]

In general, you want the weight as far back as possible to get the most weight "drop" or "time on the hill" as possible. So if you put all of the weight in the front of your car it would not drop as far as if you put it in the back. So let's look at what you said.... If I took a 1-3/4" wide x 7" long x 1/4" thick car body and I put tungsten cubes 1 layer thick starting at the back I would have something like 4 rows of 6 cubes each row. Lets call this the fastest weight placement (with still being stable). The weight is as far up the hill as I can place them (with still being stable). Now lets add the wheels/axles..... If I glued the rear axles to the back of the car, the car would now be something like 7-5/8" long.... which is too long..... so I would have to cut 5/8" off the front of the car to get it legal. when I cut the 5/8" off the front I move the weight 5/8" down the hill losing speed.

 

[1reason]

Thanks for your reply and I understand what is gained by moving the rear wheels backwards is lost by cutting off the front, but some cars (winning cars on youtube for example) have wood behind the rear wheels.

Also, even if you cut the front off to compensate for moving the rear wheels further back, the ability to concentrate the ballast to lower the rotational energy cost when the car enters the horizontal from the arc appears advantageous.

Maybe a better question is why allow ANY wood behind the rear wheels and not move than as far back as possible (even if only to the edge of the wood).

In this video http://youtu.be/MDrNwe0nPgA?t=5m17s (it should start at 5:17 where it displays three fast cars at the starting gate) all three cars have the ability to move the rear wheels further back.

What am I missing?

Thanks again

 

[5KidsRacing]

Well, first that is an old video and things have changed and moved forward and faster. Those cars are running in the Pure Stock class and had to adhere to a 4-3/8" wheelbase and could not cut off the back and move it to the front.

I don't know what cars you are seeing on YouTube, but the fastest cars in the nation have tungsten cubes stuffed in the wood behind the rear axles.

Concentrating ballast and using a disc isn't faster than using tungsten cubes and moving more weight up the hill and being able to adjust it during tuning.

By the way.... that car in the video? That LA1 car? Man that guy was fast!

 

[DerbyDad4Hire]

The rules of that race required the stock wheelbase. Wow thanks for bringing that oldie back up! Man I dominated the track!!!! That is when DerbyDad4Hire was born. I bet a lot of the guys here don't know my race name was LA Sound.

You have to follow your race rules and it is never better to shorten the length of the wood in any way. Every bit you take off is less time on the hill.

 

[1reason]

Well, first that is an old video and things have changed and moved forward and faster. Those cars are running in the Pure Stock class and had to adhere to a 4-3/8" wheelbase and could not cut off the back and move it to the front.

I don't know what cars you are seeing on YouTube, but the fastest cars in the nation have tungsten cubes stuffed in the wood behind the rear axles.

Concentrating ballast and using a disc isn't faster than using tungsten cubes and moving more weight up the hill and being able to adjust it during tuning.

By the way.... that car in the video? That LA1 car? Man that guy was fast!

Thanks, that explains it (I didn't know the wheels "had" to be placed there). Glad I'm not losing my mind

 

[1reason]

The rules of that race required the stock wheelbase. Wow thanks for bringing that oldie back up! Man I dominated the track!!!! That is when DerbyDad4Hire was born. I bet a lot of the guys here don't know my race name was LA Sound.

You have to follow your race rules and it is never better to shorten the length of the wood in any way. Every bit you take off is less time on the hill.

Ha, cool. I STUDIED your car last night for maybe an hour or more. The fenders and length, height, etc.. and the thickness and especially the rear wheels being backwards.

I was going to post a question about fenders in it's own thread, but maybe this will be just as good.

In your fenders, did you place weight at the bottom? I imagined in my head that a longer continuous row of ballast could be placed through the body and fenders to concentrate the weight location.

Also, it would appear that fenders offer a means to place ballast lower than the body, thereby aiding in lowering the center of gravity (I have zero clue how important that is though). So I guess I have a few questions in this regard (hopefully I don't over do it as a newbe here). will a lower center of gravity via weight in the lowest part of the fenders allow for the COM to move rearward, because I'm guessing it's all about moving the COM back?? Any other benefits to placing weight low in the fenders that I'm not thinking of, or is the center of gravity not that big a deal?

Thanks so much, and congrats again on winning the race!

 

[5KidsRacing]

I think for a scout race, and maybe even a NPWDRL race, you are overthinking the weight placement aspect of your build. Your speed is going to come from first alignment/rail running and then axle/wheel prep..... weight placement like you are discussing is way down the list.

If you can make an extended wheelbase car, make the rear axles 5/8" from the rear and try to put 2 ounces behind the rear axles and the rest to make it 5 ounces in front of the rear axles.

 

[5KidsRacing]

Oooh sorry, forgot the other question...

I believe all of the fenders of the fastest racers are either, pine, balsa or plastic (GEE fenders rock!)..... I don't know of any league racer that adds weights to their fenders with any success or added speed.

 

[1reason]

I think for a scout race, and maybe even a NPWDRL race, you are overthinking the weight placement aspect of your build. Your speed is going to come from first alignment/rail running and then axle/wheel prep..... weight placement like you are discussing is way down the list.

If you can make an extended wheelbase car, make the rear axles 5/8" from the rear and try to put 2 ounces behind the rear axles and the rest to make it 5 ounces in front of the rear axles.

I don't mind "over thinking" the car, it's half (or more) of the fun for me.

With that said, You're totally right, and I appreciate the guidance. That's what I really needed, was to be told what to focus my energies on because each year I run out of time and have to rush through what appears to be wayyyyy more important stuff.

I have to use the stock wheelbase length, but I will search for best practices with axle/wheel prep and alignment/rail running. I can't begin to express how glad I am to have finally found this site. I wish I did three years ago.

If you happen to know the link where I should first go to learn about rail ridding and alignment and or wheel/axle prep off the top of your head that would be great, otherwise I search for it.

Thanks again

 

[1reason]

Oooh sorry, forgot the other question...

I believe all of the fenders of the fastest racers are either, pine, balsa or plastic (GEE fenders rock!)..... I don't know of any league racer that adds weights to their fenders with any success or added speed.

Thanks

 

[TRE]

All the info is right here at this site just take a day or night and read thru this site you will learn alot..i have read thru it a few times

 

[1reason]

All the info is right here at this site just take a day or night and read thru this site you will learn alot..i have read thru it a few times

Yeah, lots of great info and I started reading this site a few days ago. I report back how my son does next week

 

[Kinser Racing]

LA1 was ahead of it's time. Did you see those massive fenders on Bones" car? How 4 years will change things.

 

[mxquad]

Funny I raced an Outlaw event in Chicago last Saturday and John's old LA1 car was there racing. I recognized it first time I saw it race.

 

[Guest]

Funny I raced an Outlaw event in Chicago last Saturday and John's old LA1 car was there racing. I recognized it first time I saw it race.

Fascinating!

 

[birddog]

If you happen to know the link where I should first go to learn about rail ridding and alignment and or wheel/axle prep off the top of your head that would be great, otherwise I search for it.

For a quick jump start and wonderful step by step instructions for how to build a fast car, my favorite place is right here:

http://5kidsracing.webs.com/

Of course the advanced training can be found on this forum, but for an easy place to start without having to search through a bunch of posts, that site is the best. Personally, I think it is better than any book I've purchased on building PWD cars. He should really think about turning that site into a book.

In some sense, I don't want my Scout competitors to find it as it took me months of reading through stuff to find out what he has collected in one easy to read place!

There. The secret is out.

5kids must just be too humble to point you to his own site.

birddog

 

[Guest]

If you happen to know the link where I should first go to learn about rail ridding and alignment and or wheel/axle prep off the top of your head that would be great, otherwise I search for it.

For a quick jump start and wonderful step by step instructions for how to build a fast car, my favorite place is right here:

http://5kidsracing.webs.com/

Of course the advanced training can be found on this forum, but for an easy place to start without having to search through a bunch of posts, that site is the best. Personally, I think it is better than any book I've purchased on building PWD cars. He should really think about turning that site into a book.

In some sense, I don't want my Scout competitors to find it as it took me months of reading through stuff to find out what he has collected in one easy to read place!

There. The secret is out.

5kids must just be too humble to point you to his own site.

birddog

+1
The abridged version of things.