Large-Diameter Banked Corners, ...Something bigger than the width of a sheet of MDF Options

[Wm_Brant]

All the track-building resources that I have seen that discuss banked corners, talk about building the corners the same way: Cut the corner and the straights entering and exiting at an angle slightly greater angle than the desired angle in the finished corner.

Pull the straights towards one another, which causes the curved area of the track to buckle downwards, creating the banking you want. You then install supports and attach the banked corner to the supports before releasing the tension on the straights.

See the BSCRA article on building banked corners, or Luf's video.

The above method well and good for banked corners with a diameter less than the width of a single sheet of MDF. My question is how do you create banked corners with a diameter greater than the width of a single sheet of MDF?

I know commercial 1/24 slot tracks have large diameter, steep, wide banked corners, so I am sure there is a way to build them, I just have not heard how to do it.

One thing that I have done, for a 10 degree banked corner, was to attach the supports to the track table FIRST, then attach the track to the supports, starting at the bottom of the curve. However, I do not know if this would work for a more steeply banked corner.

Anyone have any suggestions or pointers than might help?

-- Bill

[masmojo]

Well, Assuming that your curve could fit on TWO sheets or Pieces of MDF. You could quite simply, split the curve into 2 parts. Make sure to keep both pieces exactly the same size & have them meet exactly in the middle of the turn. Then you could simply adjust the angle/ steepness of the curve by adjusting the angle that the 2 pieces meet at, probably a piece that pivots to accomodate what ever angle you arrive at attached with a bolt as a pivot point. Once you get the desired angle, tighten the bolt at the desired anle & fasten your track to it. A cardboard mock up would probably be helpful to get establish approximately whats going to be necessary

[Brad Korando]

Hi Bill:

I don't have any routed track building experience, but...

Would it be possible to make the supports with the degree of banking you desire first and then screw the MDF pieces down to them? I would suppose that you'd double up the top of the support under the MDF seam and then work out from the center of the turn. You might then determine the point at which the banking degree starts to reduce to meet the straights. You could evenly space the supports between the last full banking degree support and the last support for the straight it attaches to. The angle could change in equal increments based upon the number of supports you have between the center of the turn and the straight. The MDF for the curve would have to be oversized a bit to account for the change in radius as it is screwed down to the supports. Once screwed down, you could mark the final radii of the inside and outside of the turn with a trammel or string and marker. Then you might take a sabre saw to cut the final shape of the turn.

Well, there's my guess. I hope there's something helpful there!

Brad

[Wm_Brant]

Brad --

I think we're thinking the same way, but since a banked corner is shaped like the section of a cone, the entire length of the banking must be curved. However, I don't know of a way to join two pieces of MDF that will not result in a flat section.

In my previous track that I mentioned above, I was able to cut my corner out of a 4x12' sheet of MDF, which allowed me to move the joints out to the entry/exit straights, where the flatness of the joint did not matter. However, that track was banked at only 10 degrees, and I'm wanting a significantly steeper banking than that.

However, if I carefully calculate the angles of the supports, I *should* be able to pre-cut and carefully position the supports and then screw the banking to them.

I'm just not sure if that's how the big track builders do it. I suspect that there might be a quicker way...

-- Bill

[Brad Korando]

Bill:

I'm not sure I understand what you mean by a flat section where the two pieces join. Do you mean a sort of crease or kink where the two pieces butt together? If so, have you considered not putting a support directly below the joint? You might be able to smooth out the crease by screwing a piece of MDF under the joint. The piece would be as wide as the track and lap past the joint 12" or so in each direction. The piece could be screwed into the track from below. It would be screwed tight along each side of the joint. If you're concerened about the track taking on the flatness of the piece below the joint, you could use tapered shims and screws to allow the track to continue it's curvature.

I'm probably not following you, becuase I would bet that this is how you fastended together your other track, with the possible exception of the shims. I'm interested in finding out what solution you come up with.

Brad

[taxi]

Just thinking out loud here...

If you enquire with your local timber merchants, you may find that you can get larger sheets than you would in a DIY store.

The other option may be to use thinner sheets of MDF or ply, and glue them together to create a laminated sheet of the correct size. By using thinner sheets you should be able to overlap the sheets at the joints to keep everything smooth and strong.

cheers,

Rob

[JEXY1]

Ribs!

When we build curved (in plan) walls for our exhibitions we make a numbver of ribs. Basically flat mdf strips cut to a radius and used as the formers to keep the wall curved.

Same principle for you banked curves. Roughly work out the radius (a true cone is a constant radius) and cut out several strips. Dont worry about getting the radius too spot on because you can just move it up or down the cone until you find the right spot. In total you want three ribs, top, middle and bottom.

To aid your construction, add some shallow slots to the rear of the sheet in stright lines from the centre point of your curve. This will make bending the sheet much easier.

If your still struggling to get the desired shape, wet the mdf sheet, front and back this gives it some flex, not too much mind or it will eventually return to its raw state - sawdust! fix it wet and leave it to dry out for a long time.

Temporary fix with some clamps the whole thing together, glue, screw and finish.

Hope this is of some help.

[Chris Frost]

Good question Bill !

When I wrote the BSCRA article on bankings, I didn't describe how to build larger radius banked turns - because I couldn't find anybody who had actually done it successfully to tell me how they did it.

Commercial builders certainly do build banked tracks with much larger radius turns than could be made from one sheet of MDF - and the best of them do it very well. I did ask some of the best commercial track builders how they did it, but (perhaps unsurprisingly) they didn't want to give away their "trade secrets". One of them told me he built a couple of track and threw them away before making one that worked properly!

What I do know about commercial banked tracks is that they come apart in sections no bigger than about 8 by 4 feet and the sections are the right conical shape (so whatever bending was needed was permanently built in during construction).

In this thread people have put forward some very plausible ideas as to how to build these banked bends - but they haven't actually done it. Getting an approximately conical shape is easy enough, a major problem is getting a smooth joint from one sheet to the next. So has one of the earlier replies hit on a way the commercial builders use to produce successful banked turns, or have they hit on a method used on a track that had to be thrown away? Until it's tried we don't know.

So if anybody does KNOW how to do it, we'd like to hear from you - better still with a few photos!

Chris

[shaunbmx]

There are a number of ways you can do this and be sucsessful. Its pretty straight forward.

Most of the above suggestions will work.
Thin sheets and pre cut ribbs work the best.
Play around with it till your happy.
You will be able to avoid flat spots on joins.

Sheet materials MDF & Ply avalible in sizes up to 5' x 10' and jumbo boards can be special ordered from some suppliers up at sizes of 6' x 12'.

Sorry i don't go more into the technic but the guys have the basics covered above.

Shaunbmx

[Slotracing Al]

Great shaunbmx
So you've actually done all this
Please tell us more.

[shaunbmx]

No not exactly that but many things very close to it.

My company builds contest & skatepark equipment, its all CNC machined and during long all night sessions at the workshop throwing boards on and off the machine have come up with some pretty odd stuff. All of which has been possible to make given enough thought. See the pics, sure there is some headaches but there is a science behind everything you want to make, i mean it can be worked out exactly. We draw it out in CAD, the CNC will obviously just cut what you tell it to, and if its worked out right then it generally goes together well.
The bowl section of the ramps has an 18mm surface made from layers comprising of sheets from 3mm to 9mm depending on the bowl radius but all bend well.

The bowl is more complex than a bank turn as it has a transition/radius going in both directions where as the bank turn has only one radius, which obviously decreases toward the inside of the turn but like Scott said if you are hand cutting ribs you can slide them up or down the turn to get the right fit so the accuracy of CNC is not essential.

As for joining boards/surfaces on a bank or anywhere for that matter, slot them either with biscuit joint or using the layers of ply your track surface is made from. Have a surface over lap that way you will can eliminate the chance of a flat spot on your join.

We did made some 1/12 scale ramps for marketing models all down to the beams and slots.

I also tried to make the barrel roll jump from the james bond film with a single lane routed corner into short straight into a twisted launch, landing was just some ply no slot but little success was had (scaley porsche was kinda close) due to lack of time spent getting it to work before getting fed up with it.

The pics below the bowl has an 11' rad at the top and 4.5'rad at the bottom.

How big / what radius was this bank turn gonna be.?

Shaunbmx

[Chris Frost]

Hi shaunbmx
Thanks for that very interesting explanation
This must be about as near as we'll get to practical experience in building slot car bankings short of a commercial slot track builder telling us how he does it. So, armed with this knowledge, perhaps somebody will try building a slot car track with a big banked turn. Let's put in a request - please tell us how you get on.

As you say, the two plane curves in these bowls must be more difficult than the single plane conical curves needed in a slot car banking.

To answer the question how big are the largest banked slot car turns (I've not measured one myself) - I'll refer to the photos and small plans of tracks on Steve Ogilvie's web site
http://www.bmts.com/~ogilvie/trackdesigns.htm
(Steve Ogilvie Custom Tracks are one of the leading builders of raceway tracks.) The outside radius appears to be something like 8 feet (2.4m) for the largest banked turns. The track surface is usually around 40 inches (1m) wide and is normally made from 1/2 inch (12mm) MDF.

The King track photo (the bottom right photo on the web page) shows how lightly supported the turn is compared with your bowls. Obviously the weight of a slot cars is trivial compared with the riders on a skate park. However the extra support must help with holding the sheet in the right place. It sounds like the inside radii are a bit tighter than on a skate park so it sounds like several layers of thin MDF rather than one sheet of 12mm is the way to go.

Perhaps I should also mention that the fastest cars will lap a King track at over 60mph (average), this combined with their small size does make slot cars rather sensitive to irregularities in the track which would be so small that riders on a skate park wouldn't notice them.

I do suspect shaunbmx is being a little modest about the amount of skill and experience involved in building your structures. It's surprising how good people get at doing their day job compared with somebody doing it for the first time. Achieving a smooth transition between 1 sheet and the next is something quite a few people have trouble with flat slot car tracks, no doubt banked turns are be more difficult.

Chris

[Wm_Brant]

I think I found a usable reference on large-diameter banked turns. It's in a looooong thread over at Old Weird Herald's 'Slot Car Talk' forum.

It's a thread with 439 posts (it exceeded the maximum number of responses to a thread on their software) about the building of a commercial 'Blue King' slot car track. There are lots of pictures, and some decent links to other references. A lot of chaff, too.

Now, not everybody needs or wants a 'Blue King' track in their basement. However, the Blue King has all banked corners and the banked corners are big, which is what we're interested in here. A 'standard' Blue King is 155' (47.25m) long with the largest bank having a diameter of 14' (4.3m). That's bigger than a single sheet of MDF, and so it addresses my question.

I have to admit that while I have read through all the posts, I have not 'digested' them yet. However, I did want to get this out here so other people who are interested in this subject can also read through the posts.

It's in a thread entitled 'Where's Waldo, I Mean Swiss', and it follows the construction of the track from early stages to the finish. Enjoy!

-- Bill