derby car set up advice

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FatSebastian
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Re: derby car set up advice

Post by FatSebastian » Fri Mar 12, 2010 12:23 pm

doct1010 wrote:Please remind me why positive camber on the DFW?
Notionally the wheel rides against the body rather than the nail head in this configuration. If the wheel hits a defect in the rail, it will slide a bit outward on the lubricated nail before it hits the nail head and jars the front of the car body. So in principle it acts like a shock absorber?



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pwrd by tungsten
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Re: derby car set up advice

Post by pwrd by tungsten » Fri Mar 12, 2010 12:32 pm

Doc

For negative DFW the contact point of the wheel is the top of the rail and the distance of contact is the full radius of the wheel. For positive the contact is a portion of the radius of the wheel and the contact is at the bottom edge of the wheel which is better.


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Re: derby car set up advice

Post by *5 J's* » Fri Mar 12, 2010 12:35 pm

FatSebastian wrote:
doct1010 wrote:Please remind me why positive camber on the DFW?
Notionally the wheel rides against the body rather than the nail head in this configuration. If the wheel hits a defect in the rail, it will slide a bit outward on the lubricated nail before it hits the nail head and jars the front of the car body. So in principle it acts like a shock absorber?
:thumbup: Bingo! Nice explanation FS.



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Re: derby car set up advice

Post by 62vetteefp » Fri Mar 12, 2010 12:44 pm

huhh? Why is rubbing at the bottom of the wheel (at radius R) different than hitting it at the top of the rail which is also at radius R on the wheel?

Perhaps some sense might be made if we add in that the area at the bottom of the wheel (w/ positive camber) is against a wheel contact point that is at the point of starting rearward and then moves away from the rail and therfore has no rubbing friction. It just touches the rail and moves away.

While w/ negative camber and toe in the contact point is at the front of the wheel when that wheel contact point still has some downward vector direction and therefore does have some relative movement to the rail and therefore friction is taking away speed?

???? Sounds good but is this what really happens?



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pwrd by tungsten
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Re: derby car set up advice

Post by pwrd by tungsten » Fri Mar 12, 2010 12:49 pm

THe radius would be the same if no camber.... With positive camber the inside edge lifts up a bit. So the radius is reduced.

Yes on the bottom realtive movement is minimized...


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Re: derby car set up advice

Post by 62vetteefp » Fri Mar 12, 2010 12:54 pm

I have not bent a axle yet but at 1.5 degrees does the wheel sit flat on the track? How about at 2.5 degrees?



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Re: derby car set up advice

Post by *5 J's* » Fri Mar 12, 2010 12:58 pm

Depends on bore "slop". Generally at 1.5 tread sits flat, 2.5 it is on edge.



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Re: derby car set up advice

Post by pwrd by tungsten » Fri Mar 12, 2010 1:11 pm

Do your wheels need to sit flat to meet race requirments?

With large axles 0.091-0.093 the wheels will tilt for sure. Awana axles and 91SGAs, 93SGAs fall in this category.

For BSA axles at 2.5 you can almost make the wheels look like they are flat if you try hard for checkin but it would depend on rules. Must be flat at all times -vs- must be flat at some point... :D


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Re: derby car set up advice

Post by doct1010 » Fri Mar 12, 2010 1:11 pm

pwrd by tungsten wrote:Doc

For negative DFW the contact point of the wheel is the top of the rail and the distance of contact is the full radius of the wheel. For positive the contact is a portion of the radius of the wheel and the contact is at the bottom edge of the wheel which is better.
I'm going out on a limb here. It appears to me the contact patch is greater with [junk] camber. If the wheel is cambered to the point of lifting isn't the entire bottom edge of wheel in contact with lower 1/3 of rail? Negative cambered DFW the contact patch of the wheel is at the top 1/3 or less of the rail and I would think smaller, possibly only width of rail edge. One would also have to factor in loss from wheel/body contact vs. axle head.

I may be out in left field on this, won't be first time. ;)



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Re: derby car set up advice

Post by pwrd by tungsten » Fri Mar 12, 2010 1:16 pm

:D

For negative a small patch touches against a 90 degree angle that is not entirely smooth...

For positive a small patch touches against a flat surface with little relative movment.

It is still a small patch because the wheel being round will only contact at one point...


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Re: derby car set up advice

Post by FatSebastian » Fri Mar 12, 2010 4:45 pm

:thinking: I'm not sure that the exact size of the contact patch with the rail is a significant issue. Frictional force tends to be independent of the contact area and is instead a function of the coefficient of friction and the normal force applied. For a spinning object, the radius of contact is also a factor, but in this case the contact radius (the inside tread wall) seems to be the same in each case?



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Re: derby car set up advice

Post by Stan Pope » Fri Mar 12, 2010 5:03 pm

doct1010 wrote:
pwrd by tungsten wrote: Bend the front (DFW) axle either 2.5 or more. (I use 3-4 degrees) Rotate the axle so that the bend is going down... (Positive camber) .....
Please remind me why positive camber on the DFW? I know it works and the fast boys are doin it, and that in itself may be enough for some. I need to understand the advantage.
When rail guiding, DFW "slides" against the rail. More slide, more friction loss. Less slide, less loss!

Now look at the direction of movement of tread edge, the part of the wheel that slides.

With negative camber, the direction is toward the rear, but at about 45 degrees from horizontal.
That puts the amount of slide at about 70% of the distance traveled.

With positive camber, the direction is toward the rear, but at a very small angle from horizontal, which puts the amount of slide at less than 10% of the distance traveled.


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Re: derby car set up advice

Post by doct1010 » Fri Mar 12, 2010 6:00 pm

62vetteefp wrote:Perhaps some sense might be made if we add in that the area at the bottom of the wheel (w/ positive camber) is against a wheel contact point that is at the point of starting rearward and then moves away from the rail and therfore has no rubbing friction. It just touches the rail and moves away.

While w/ negative camber and toe in the contact point is at the front of the wheel when that wheel contact point still has some downward vector direction and therefore does have some relative movement to the rail and therefore friction is taking away speed?
OK, Just had an AH-HA moment, a gestalt. The relationship to rail is rotational. The higher up on the wheels rotation more energy consumed. Close?



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Re: derby car set up advice

Post by 62vetteefp » Sun Mar 14, 2010 9:28 pm

Yes. Perhaps vectors is confusing (mechanical dynamics term). Vector is direction of motion. If an object is going in a horizontal direction the vector is 100% horizontal. If vertical, then vector is 100% vertical.

At the bottom of the wheel (with positive camber) the contact patch vector is completely horizontal in direction and the vehicle direction is also 100% horizontal so there is no relative movement between the wheel patch and the rail.

However with negative camber and toe in, the wheel patch is touching the higher up corner of the rail and at that point the wheel patch has both a horizontal and a vertical vector (contact patch moving down and rearward). But the vehicle vector is still 100% horizontal.

So there is a little difference that causes some rubbing.

I wonder if what I just said is right. been a long time since Dynamics.



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Re: derby car set up advice

Post by 62vetteefp » Sun Mar 14, 2010 9:51 pm

Also in looking at the variation in run times. Last year our 3rd place car had a range of .039 seconds while first had 012. This year we had a range of .038 while the person who beat us had a range of .011 (same person :thinking: (but he had very light wheels)).

Can I assume the issue is that our car was too rear heavy which caused the front to skitter and therefore put a lot of random inputs which gave us the random results? So doing the above recommendations should help.

thanks for everything!!



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