LMP Chassis

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Ned
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LMP Chassis

Post by Ned » Wed 19. Oct 2016 21:56

I just tested my first chassis for 1/32 LMP bodies. It runs much better than I thought it would, given that the battery is located between the front and rear wheels on the right side. The battery is loaded from the top which requires removal of the body. Brass spring strip is used for electrical contacts instead of magnets. The wheel base is adjustable from 3.20" to 3.55".

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The chassis is ultra low in front to accommodate the low cowling and NASA ducts in the front of many LMP race cars.

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I need to get the body mounted and do more testing. I've already identified over 20 very minor changes to make to the design. Then I'll make it available for purchase on Shapeways.

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Keld
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Re: LMP Chassis

Post by Keld » Thu 20. Oct 2016 21:40

I think it is a mistake to handle the battery from above, and also using the clip instead of magnets. When the car crash the spring effect in the copper clips can be damaged, and then no power ;)
/Keld

Ned
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Re: LMP Chassis

Post by Ned » Fri 21. Oct 2016 02:04

Thanks for the feedback Keld. The only drawback I see to loading the battery from above, is that the body has to be removed and that takes a little extra time, perhaps 6 seconds. My bodies are securely held on to my chassis with magnets, bumpers, and stops which prevent the body from shifting on the chassis during a crash. Unless your're running a race that lasts longer than 20 minutes (the run-time with non-stop racing on a fully charged battery), I don't think the extra time is relevant. I've never been in a race that lasted more than 20 minutes. If long distance racing is your primary interest, then the extra time would be a drawback of a top loading battery compartment. Is there something else I'm missing?

Battery terminal connectors made from spring strip brass or copper, like pictured below, have several advantages over magnets.
1. No need to remove insulation from the battery.
2. No need for magnetic positive (+) terminal on the battery. These batteries are difficult to find.
3. Less space required for connection to battery which allows battery to be fitted inside narrower and lower bodies.
4. Less interference with the car interior. Walls of the battery box need be only .32" high when loaded from above, instead of .40" high. That difference of 2mm can be very significant when trying to minimize the amount of cockpit and air ducts that have to be removed to fit a body to a chassis.
5. Works with protected and unprotected batteries.
6. No magnets required to make electrical connections from battery to PCB, which are unreliable in dusty environments with iron oxide in the soil, like around my outdoor track.
7. No magnets required to keep the battery in place. Gravity and friction do the job.

I think I can design the chassis and connectors so that the chance of damage is very small.

My main concern is whether or not spring strip brass or copper about .006" thick and .20" wide is readily available at a reasonable cost around the world. I have some samples I obtained from Wes but I can't find a place to buy more. If anyone knows where I can buy this please let me know.
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I could make 2 versions of each chassis available on Shapeways, one with a bottom loading battery box like the original magracer and one with a top loading battery box. What do you guys think? All feedback is welcome.

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Keld
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Re: LMP Chassis

Post by Keld » Fri 21. Oct 2016 07:57

you really try to think about everything :yes

but one problem could be that the batteries are not the same length, I do have some batteries that not fitting the controller (to short) so i use them in cars, but again, there will always be a issue.
/Keld

kiklo
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Re: LMP Chassis

Post by kiklo » Fri 21. Oct 2016 09:40

Hi Ned.

Please keep us updated on performance on the LMP. Curious about weight distribution influence on the sideloaded battery.
Racing Regards

Kim K.

Ned
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Re: LMP Chassis

Post by Ned » Sat 22. Oct 2016 04:11

First a correction. I misspoke when I started this topic. This is not the first chassis I've built for an LMP body. The first one was a hybrid chassis for my 2000 Cadillac Northstar LMP. This is the first chassis I've built using CK4 which has a top loading battery located on the side of the chassis. This was largely an experiment in an attempt to minimize the height (thickness) of the front half of the chassis. Both chassis are shown below.

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Yesterday I mounted an LMP body to this new chassis. The rear axle, crown gear, wheels, tires, and body are from an SCX PRO 2003 Audi R8 slot car. The body weighs 18 grams. Today I spent some time testing it out against some other cars. I am extremely pleased with how this car performs, chassis and body combined. I don’t have an automatic lap counter and timer built into my layout, so I can’t be precise about lap times. My wife clocked several laps of mine at 10 seconds using her kitchen timer. ;) It measures in full seconds only. The fastest time ever recorded is 9.5 seconds 1 ½ years ago with a different timer. That was a stock magracer with enhanced tires on a newer and smoother outdoor track. I suspect this car is just as fast on my track, probably faster.

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This car has a 26 tooth crown gear on the rear axle, compared to a 30 tooth gear on a stock magracer. The Audi has .779” diameter rear wheels compared to .839” on a stock magracer. I estimate that the top speed of the Audi R8 should be 7% faster than a stock magracer. That appears to be the case going up a 23 ft long straight with 5% grade. I compared the Audi speed against 2 other cars with stock gearing by running them on the same lane, one right behind the other, using the same controller.

The car handles very well, especially around tight right-hand corners. When running clockwise, my track has 3 tight right-hand corners. It has only one tight left-hand corner which can be avoided by taking a different lane. Only 2 of my cars can negotiate that left-hand corner consistently without some steering input. This car is not one of those two. (That corner is the later part of an S curve where the bank changes from right to left. I'm trying to modify the S curve slightly to make it easier to negotiate.) The car handles most left had corners with ease. I have not been able to test the lane changing ability of this car because something is wrong with the steering. It’s not the controller. It’s probably the coil or the control module on the PCB or related wiring. The car does not react at all when the steering wheel on the controller is turned. This does not concern me. I’m certain that the lane changing will work properly with a working coil and receiver board.

On a smooth flat level surface, this car tends to drift to the right. I’m not sure if it’s because of the greater weight on the right side of the chassis or if it’s due to slightly more negative camber on the right front wheel. (This was not deliberate. A hole for a kingpin in the upper suspension beam, located a few thousandths of an inch inboard or outboard, can affect the amount of camber. The geometry of the wheel and tire can also affect camber.) The “wear” on the front tires clearly indicates negative camber on both tires. On the right tire, the wear pattern is a consistently narrow band along the inside edge of the tire. On the left tire, the “wear” shows up as a variably wide band on the inside edge. The wear pattern on the rear tires extends all across the face of the tread which indicates that the entire tread is making contact with the track surface.

The good handling is probably due in part to the high quality aluminum wheels and good rubber tires. I sanded all 4 tires just a little before mounting them on the axles. They were almost perfect to start.

Concerning the spring metal strip for electrical connector, thanks to Wes, I found a vendor called Greenweld in the UK:
http://www.greenweld.co.uk/cgi-bin/sh00 ... 1#aCDT0091

It’s called bronze spring strip, 5mm wide by 0.2mm thick by 4 metre long. That should be enough to build at least 75 cars, a lifetime for me. The cost is about 5 British Pounds, which is less than $6.25 USD today. I hope to find a vendor in the U.S. also, so that folks living on this side of the pond can buy it closer to home.

I intend to make a few minor modifications to Chassis Kit CK4 and to design it to use the bronze spring strip mentioned above. Then I will offer it for sale on Shapeways. Hope to finish this within a few days since I’m leaving town next week for a couple of weeks. Eventually I will probably design 2 versions of most chassis and make them available on Shapeways. I suspect that most guys will prefer the chassis with the top loading battery and bronze spring strip connectors. With my collection of chassis kits on Shapeways, you will be able to convert almost any 1/32 slot car to a magracer.

kiklo
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Re: LMP Chassis

Post by kiklo » Sat 22. Oct 2016 12:39

I guess you could move the motor and gear arrangement ca 5 mm to the left to compensate for the battery weight to the right ?
Racing Regards

Kim K.

Ned
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Location: Sedona, AZ USA

Re: LMP Chassis

Post by Ned » Sat 22. Oct 2016 17:43

That's a good idea Kim. Thanks. I'll definitely consider it. The battery and motor weigh the same. Shifting the motor to the left would improve balance somewhat.

I've heard that the torque of inline motors tends to lift one of the rear wheels of slot cars off the track on some corners. That's why some slot racers don't like them. I suspect Keld, an experienced slot racer, could speak to this. In the case of my chassis, would the torque tend to lift the left rear wheel off the ground? If so, shifting the motor to the left would help to offset it. Is the effect of torque a relevant consideration in these magracing cars? Any slot racers out there that know the answer?

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Keld
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Re: LMP Chassis

Post by Keld » Sun 23. Oct 2016 22:49

I really don't think torque matters here in magracing, but correct, I will newer use inline in my cars, and that is not a problem I have since there are space enough in my size 1/24
if it is the left wheel or the right depend on what way you fit the gear :rolf

but it is easy to find out just put a motor on the table and power it up, if there is enough power in it it will rotate, if not it will only jump a little.
/Keld

Ned
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Re: LMP Chassis

Post by Ned » Tue 25. Oct 2016 03:10

kiklo wrote:I guess you could move the motor and gear arrangement ca 5 mm to the left to compensate for the battery weight to the right ?
I modified the chassis so the motor can be mounted further to the left side. Also made several other small changes. Included a hole in the left side of the battery compartment to insert a magnet just in case one is needed to hold the battery in place. This chassis in now available to buy on Shapeways at
https://www.shapeways.com/product/227CC ... d=60846517

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