Question of which wheelbase

[561 racing]

Guys, something has been picking at my brain now for a little bit. I saw a quote earlier that said "the longer you wheels stay on the hill the faster you are" I always thought that means the extended wheel base would be better, but as I have been thinking about it, it would seem that by doing that I put the front wheels off the hill sooner and thus the standard wheelbase might be better? Am I in the ballpark or am I overthinking it, I know I lose a little bit of stability but is it worth it?
Thanks!
Brian

 

[Crash Enburn]

I think the quote is wrong. It's not a matter of the wheelbase, but of your center of gravity (CoG). By having the CoG as far back and low as possible, it will maximize the falling distance of your weight, maximizing your speed.

The longer wheelbase can provide more stability. That is not to say it is necessarily faster. Then again, the Unlimiteds all seem to be running a long wheelbase... YMMV.

 

[Guest]

I've often wondered if the reason behind an extended wheelbase in the Unlimited class is that by extending the lever arm, it puts more down force on the front bearing, essentially enhancing that bearings performance.

 

[bracketracer]

I was under the (perhaps false) impression that by placing the wheels farther forward it allowed you to put more weight behind the rear axle for a given COM. So your weight is farther up the hill.

 

[quadad]

My 1.5 cents:

Shorter WB cars stay on the hill longer because they are not tranitioning to the flat until the (further back) front wheels get there. The longer WB cars are more stable for a given COM, but from my perspective they tend to require a lower COM (more weight in the back) in order to compete with the shorter (but not necessarily shortest WB cars).

Regarding the bearing cars, I think there have been plenty close to the 5" WB territory. They are not all max or close to max WB. Some of the reason you may notice a lot of close to max WB bearing cars is just the personal preference of the builders, especially those that have been succesful of late. Remember that one particular, long term very successful and prolific builder of bearing cars likes the longer WB cars and he has sold a fair quantity of his cars to other racers. Nothing says a shorter WB bearing car can't compete - there is a challenge for you aspiring bearing car builders !

 

[HurriCrane Racing]

I agree with Quad Dad. The shorter wheel base will give you a little more time on the hill but will be more difficult to keep stable.
I think the current typical street stock base is a good combination of the two extremes. Some of my fastest cars are short wheel based. But they were a little more difficult to tune. I'm planning on sending in a short wheel base care for the May race.

 

[Kinser Racing]

The shorter the wheelbase the longer the weight stays on the hill but there is a trade off with tuning and stability. I have a short wheelbase Unlimited that was the top of the game at one time. It also has canted rear independent axles. I don't know of another Unlim that has had that. We'll see this month what my new Unlim does.

 

[561 racing]

Kinser, I would love to see a picture of that unlimited if you have a picture still. Ps thanks you all for your help, lots of good information.

 

[Kinser Racing]

I'll see what I can find. I've changed computers and I think I only have one on Photobucket.

 

[Guest]

I was under the (perhaps false) impression that by placing the wheels farther forward it allowed you to put more weight behind the rear axle for a given COM. So your weight it farther up the hill.

Now I am totally confused!

When I was speaking with ODD and he made this statement I interpreted it like Eric and figured he meant to say something slightly different.

The mis cue kind of stuck in my head and got me wondering if he meant the other though.

After thinking about it further I had somewhat concluded that the original statement was correct. The fact that BR thinks otherwise gives me serious pause.

Edit: I'm sorry, what were we talking about?

 

[bracketracer]

Doesn't the COM only provide a boost when the car is in transition anyway?* If so, then why would it matter where the front wheels are? The body would be in transition until the rear wheels reach the flat regardless of where the front wheels are.

*I read that on the pinewood physics site so I do take that with a grain of salt. There are so many interactions of car balance and COM that it would be difficult to attribute a boost to any one thing IMHO.

 

[Crash Enburn]

Speaking "physicsally"... The CoM is the Center of Mass. The point in which the weight is centered in the car -- the density of the weight just varies the size of the mass volume, but the CoM is the balance point. Moving your wheels forward doesn't really let you put more weight higher up (further back) because you are countering by putting the wheel weight further forward. That is just spreading your weight out, but the CoM is still essentially in the same place.

Which then brings up the idea of a polar moment. It is easier to rotate an object with the mass closer to the turning point then further out. If you have a car with a short wheelbase and a COM 3/4" in front of the rear axles, and a long wheelbase car with a CoM in the same place, it will take more energy to rotate the LWB car than the SWB one. Does that translate to speed loss in the transition? I would hazard a guess of "yes", but likely a very, very small amount.

The "boost" in the transition comes from when your car still has its CoM on the slope when the other car's CoM has already hit the flat.

Long story short, you want to have your CoM as far back and as low as you can get it for maximum benefit.

 

[Bullet]

hmmm...for highest potential energy (mass x height X gravity), you would be trying to maximize the height. This would say place the weight as high as possible on the car, the top back of the car. Maybe this could cause stability issues though? The COM will still be dropping as long as it is above horizontal. Pushing the rear axle back as far as possible keeps the weight higher for longer I assume? Pushing the front wheels forward would cause the car to rotate sooner, dropping the back of the car faster when its in the turn. As mentioned earlier, it seems it comes down to a stability vs maximum potential issue.

 

[Crash Enburn]

I saw a graphic a while back that showed that (surprisingly) the weight travels a further distance when low in the car vs. up high.

Having the wheels all the way back just facilitates having the weight in back w/o making a car into a wheelie machine.

 

[561 racing]

Ok, so whose totally confused here but me. All I know for sure is I'm going to have to buy another body from John and do more testing.

 

[Guest]

Doesn't the COM only provide a boost when the car is in transition anyway?* If so, then why would it matter where the front wheels are? The body would be in transition until the rear wheels reach the flat regardless of where the front wheels are.

*I read that on the pinewood physics site so I do take that with a grain of salt. There are so many interactions of car balance and COM that it would be difficult to attribute a boost to any one thing IMHO.

Hi BR,

I keep thinking about this post of yours and I have to disagree with your source.

On a purely "boost" viewpoint the shorter wheelbase will have a quicker MOI at the transition than the extended.

It would be a much more difficult stallion to tame, but she has the most power.

It is just like the wheel with a heavy tread versus a heavy hub/ light tread.

In theory there is more energy left over.

 

[GravityX]

Doesn't the COM only provide a boost when the car is in transition anyway?* If so, then why would it matter where the front wheels are? The body would be in transition until the rear wheels reach the flat regardless of where the front wheels are.

*I read that on the pinewood physics site so I do take that with a grain of salt. There are so many interactions of car balance and COM that it would be difficult to attribute a boost to any one thing IMHO.

Hi BR,

I keep thinking about this post of yours and I have to disagree with your source.

On a purely "boost" viewpoint the shorter wheelbase will have a quicker MOI at the transition than the extended.

It would be a much more difficult stallion to tame, but she has the most power.

It is just like the wheel with a heavy tread versus a heavy hub/ light tread.

In theory there is more energy left over.

This all sounds to me like some testing is in order! Short vs Long WB, now can we possible maintain the same COM to bring more validity to the test? Yet another test to run in the very distant future. Stay tuned people, stay tuned. Who am I kidding, I barely give myself enough time to do what I've got going on now. LOL

 

[bracketracer]

Doesn't the COM only provide a boost when the car is in transition anyway?* If so, then why would it matter where the front wheels are? The body would be in transition until the rear wheels reach the flat regardless of where the front wheels are.

*I read that on the pinewood physics site so I do take that with a grain of salt. There are so many interactions of car balance and COM that it would be difficult to attribute a boost to any one thing IMHO.

Hi BR,

I keep thinking about this post of yours and I have to disagree with your source.

On a purely "boost" viewpoint the shorter wheelbase will have a quicker MOI at the transition than the extended.

It would be a much more difficult stallion to tame, but she has the most power.

It is just like the wheel with a heavy tread versus a heavy hub/ light tread.

In theory there is more energy left over.

That's why I quoted the source of the info Joe. I don't think I believe it myself. At least I won't believe it until I can see it for myself! lol! Paul, get to testing!

 

[Guest]

Yeah. After going back thru the thread I saw that Eric laid it out pretty clearly.
C'mon Paul!
I am already doing the wind tunnel.
It is your turn to take a hit!
JK
I recognize BR's dry wit enough to know that Eric was on the money and BR was speaking with tongue in cheek.

 

[Kinser Racing]

A lot of times for me to see things I have to greatly exaggerate the example.