Rail Riding - "How To Guide"

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.

Have you had success with a "rail rider"?

Yes
90
49%
No
8
4%
Somewhat
12
7%
Haven't tried yet
72
40%
 
Total votes: 182

quadad
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Re: Rail Riding - "How To Guide"

Post by quadad » Tue Sep 21, 2010 2:45 pm

pwrd by tungsten wrote:Running the rail:
...
3/16" height DFW
4/16" height raised
5/16" height rears
Wow ! I know you have mentioned it before, but assuming similar height wheels all around that puts the front of your car up on like a 1.6 degree angle. I know you are running fast, but I can't figure out how that doesn't make the bottom of your car a big wind catcher. What am I missing here ? :scratching:
pwrd by tungsten wrote: Most inportant thing is to have straight axles and well drilled holes.

Well drilled holes are the foundation of your car.
:bigups: Agree with you 100% there.



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pwrd by tungsten
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Re: Rail Riding - "How To Guide"

Post by pwrd by tungsten » Tue Sep 21, 2010 3:22 pm

It adds a little bit of toe-out. :-)

Think of a car with canted rears. As you lift the front you get some toe-out.


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FatSebastian
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Re: Rail Riding - "How To Guide"

Post by FatSebastian » Tue Sep 21, 2010 3:28 pm

quadad wrote:
pwrd by tungsten wrote:Running the rail:
...
3/16" height DFW
4/16" height raised
5/16" height rears
Wow ! I know you have mentioned it before, but assuming similar height wheels all around that puts the front of your car up on like a 1.6 degree angle. I know you are running fast, but I can't figure out how that doesn't make the bottom of your car a big wind catcher. What am I missing here ? :scratching:
I was also wondering if the divisors should be 32 rather than 16? Otherwise, wouldn't the clearance between a 1/4"-high guide rail and the rear of the car seem uncomfortably close to dragging? (Or is low clearance actually desired?)



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Re: Rail Riding - "How To Guide"

Post by asatxj » Tue Sep 21, 2010 4:08 pm

By raising the axles as high as possible you gain the ability to get more weight below the plane of the axles. This give some increase in stability and speed. I haven't tested it yet but many on other forums use this method exclusively. This season I may build a test car around this premise.


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Re: Rail Riding - "How To Guide"

Post by FinePine » Tue Sep 21, 2010 4:25 pm

To get a "level" car that has negative rear camber and positive front camber, the rear holes need to be a bit lower than the front to account for the angles of the axles. But all this "height of axle hole" talk is dependent on the shape of the bottom of the car. But even if it is flat, and it slopes down toward the back, this doesn't mean that the car necessarily has a larger frontal area. A wedge shape car is thicker in the back than in the front, so even with a "raised" front end, the top of the car could still be level, and frontal area would not be increased.

Still, need 3/8" bottom clearance by most rules.



quadad
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Re: Rail Riding - "How To Guide"

Post by quadad » Tue Sep 21, 2010 5:54 pm

pwrd by tungsten wrote:It adds a little bit of toe-out. :-)
I'm still scratching my head a bit about that. I drew up a car on the computer with axle holes placed as Pwrd By Tungsten described. It is possible to get the top (at least the front 4-5 inches) to be level or very slightly downward in this configuration. The bottom does drag (just under 1/4" clearance at the far back) unless you cut it - assuming you start with a 3/8" to 13/32" thick block. You have to work to make an acceptable shape AND provide enough room around all the axle holes (1/4" difference).
asatxj wrote:By raising the axles as high as possible you gain the ability to get more weight below the plane of the axles. This give some increase in stability and speed.
Yep, I have been a big proponent of weight below the axle holes, trying for very high percentages. I have been considering going up more like 15/64" though with the rear holes, which is greater than the 0.17" or so you get with the ProBody Jig, but not near the 5/16" Pwrd By Tungsten is suggesting. I can think through the 'How much clearance do I need ?' and "What weights will fit well under the axles ..." question, but my thick head is still mostly stuck on this toe out conversation. :pullhair:



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Re: Rail Riding - "How To Guide"

Post by pwrd by tungsten » Tue Sep 21, 2010 10:34 pm

FatSebastian wrote:
quadad wrote: Wow ! I know you have mentioned it before, but assuming similar height wheels all around that puts the front of your car up on like a 1.6 degree angle. I know you are running fast, but I can't figure out how that doesn't make the bottom of your car a big wind catcher. What am I missing here ? :scratching:
I was also wondering if the divisors should be 32 rather than 16? Otherwise, wouldn't the clearance between a 1/4"-high guide rail and the rear of the car seem uncomfortably close to dragging? (Or is low clearance actually desired?)

You are correct:

5/32" on rears
4/32" on raised wheel
3/32" on DFW

Sorry about that...

I have updated my origonal post. I got these numbers from one of the finest PWD tuners in the world. ;)


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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Wed Sep 22, 2010 9:40 am

FinePine - are you taking into consideration that 1.9 degrees of the canted axle will be taken up in the "bore slop" such that 2.5 degrees of axle cant will provide 0.6 degrees of wheel cant? (Somebody here can probably put this better into the Pinewood lexicon)

I am following this as this year we are going to pay attention to where on the block we start our holes. I may also modify a Pro-Body Jig to include provisions for drilling 1.5 degrees of negative axle cant, so I need to figure out the best spot for the bit to exit the Jig and enter the body. We can only run with a max of 1.9 degrees of axle cant as we need the wheel to stay flat on the track. I don't want to be on the edge - or worry about the "bind" having both the inside of the bore rolling on the lower edge of the axle and the outside of the bore rolling on the upper edge of the axle.

I have no particular reason for 1.5 other than it should provide enough to keep the wheel on the axle head but keep me safely away from the 1.9 degree critical angle.



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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Wed Sep 22, 2010 11:36 am

FinePine wrote:To get a "level" car that has negative rear camber and positive front camber, the rear holes need to be a bit lower than the front to account for the angles of the axles.
A car with straight axles and axle holes will sit at a height of approximately 0.5435" minus the depth above the bottom that you drill the axle hole (using a nominal wheel of diameter 1.183" with a nominal bore of 0.096").

If the rear axle has a 3 degree negative cant it will only lower the rear 0.0016" (using 1/32" wheel to body gap).

If the DFW axle has a 3 degree positive cant it will only raise the front 0.0192" (using 1/32" wheel to body gap), and that is assuming all 3 degrees of the bent axle is used to provide positive cant, when in reality some will be used to provide toe-in for steering to the rail.

So if drilled all your axles at the same height and had 3 degrees of negative cant on the rears and 3 degrees of positive cant on the DFW the front would only be 0.0208" (less than 1/32") above the rear as measured at the axles. Or another way to look at it is the body would tilt up approximately 1/4 of a degree.

If you wanted to level the car - you would want to drill the rears 0.0208" below the front. However, PBT drills the rears 1/16" above the DFW which will further lower the rear. Using the quoted three degrees of cant I calculate the body will tilt up 1.12 degrees if a standard wheelbase is used. This will give a bit of toe-out on the rears as PBT states. Note sure how this equates to speed. I would think trying to set up level would be best - but PBT has much more experience than I, and experience beats theory anyday.

Somebody may want to verify my math.



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Re: Rail Riding - "How To Guide"

Post by FinePine » Wed Sep 22, 2010 4:26 pm

*5 J's* wrote:FinePine - are you taking into consideration that 1.9 degrees of the canted axle will be taken up in the "bore slop" such that 2.5 degrees of axle cant will provide 0.6 degrees of wheel cant?
I haven't observed a loss of camber. If the contact point of the wheel to the track is vertically between the ends of the bore (and if there is enough weight on that axle), the angle of the wheel should match the angle of the axle, no? The front, with positive camber and a deep dish wheel, and light axle weight, may be different. My car is in the display case now so I can't pull it out and have a look for a few weeks. Anyway, things will change when the car is moving, which is the part that actually matters.



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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Thu Sep 23, 2010 5:27 am

Yes there is a loss in camber as the wheel attempts to lay flat to the track. This causes the wheel to roll on two points once you pass the critical angle of 1.9 degrees - the inner/bottom of the bore and the outer/upper of the bore as the axle sits diagonally across the bore. This loss of camber also allows us to run 1.5 degrees of camber - but keep the tread flat to the track as required per our council rules.

Note that this 1.9 degrees is calculated on nominal values of an axle diameter (0.087") and hub bore (0.096") and bore length (0.273"). This gives 0.009" of bore slop to be taken up over a bore length of 0.273", which gives an axle angle of 1.89°. Any differences is these values would change the critical angle. With the older style wheels without the 0.02" step - the bore length was 0.293" which gave a critical angle of 1.76 degrees.

This is illustrated in this post.

Note that this is not true in the other plane, or toe. With toe, the axle will remain in the bore valley such that wheel toe angle = axle angle.



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Re: Rail Riding - "How To Guide"

Post by FinePine » Thu Sep 23, 2010 9:12 am

Thanks for the link. Later in the same thread, Stan's casual observations match my own, which is that the wheel tilts up to match the angle of the axle.

There are three moments acting on the wheel that determine whether it lies "tread flat" or is angled (car at rest on a level surface). If you look at these, it seems that they will cause a rear wheel with negative camber to tilt. One moment is from the weight of the wheel, one from the weight of the car, and one from the nail head preventing the wheel from traveling up the axle (if you roll the car a bit to allow it to get out there). I think the latter two are stronger than the first. It could be calculated easily enough, but I'll leave that to someone else. At low angles, friction may be high enough to prevent the system from reaching its lowest potential energy state.



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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Thu Sep 23, 2010 9:48 am

yea, I have never gone with more than 1.7 degrees of negative camber due to our rules requiring the wheels to sit flat on the track. With 1.7 and less the wheel do sit flat - perhaps at greater angles above 1.9 - wheel angle = axle angle.



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Re: Rail Riding - "How To Guide"

Post by FinePine » Thu Sep 23, 2010 10:04 am

I'd be interested to know: if you push the top of the wheel in to match the angle of the axle, does it stay there? And if you then roll the car a bit, what happens?



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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Thu Sep 23, 2010 10:32 am

I have a bit of my daughters car running during the alignment process
Imagehere

The cars can be seen here and here

I will have to see what happens when I push the wheel to match the axle angle.



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