Insane speed car

[RC10th]

I've been thinking of building a speed car on and off for the past couple of years. I knew/know I could build something pretty serious but due to lack of places to use it, lack of grip and not wanting to invest the type of money it would require basically resigned it to just an idea.

I've got the urge again to build something and thought about scaling back to something using basic reasonably cheap off the shelf stuff. After seeing a belt driven touring car hold up to big SCT power years ago on 4s LiPo I knew belt drive was the way to go. Also after seeing another belt driven touring car with a sedan body of all things go 158mph on only 4s in Australia confirmed that a touring car based vehicle would be enough. I'm not chasing any records or goals but just out for a bit of fun without spending crazy money.

I've done some basic maths and while debating which power plant to start with came up with the following. Also where the tipping point is between RPM vs horsepower remains a thing of trial and error in my opinion (without fancy math)

The following is rotor shattering although just theoretical RPM and doesn't take voltage drop into consideration. If the car leaves the ground and/or spins the wheels the rotor will likely explode though. The 1/8th motors are just over 4 horsepower and I cant find any power specs on the SCT motors.

4000kv SCT on 14.8v = 59,200rpm
Fully charged 16.8v = 67,200rpm

2200kv 1/8 on 22.2v = 48,840rpm
Fully charged 25.2v = 55,440rpm

2600kv 1/8 on 22.2v = 57,720rpm
Fully charged 25.2v = 65,520rpm

The speed control is 150amp, will support 2s - 6s and also has the option to dial in 15 points of boost and 15 points of turbo timing. This should be enough to send something into meltdown fairly easily if you get it wrong.

I chose the Sakura Xi mainly because I already had one, plus the parts are dirt cheap. I really wanted to use the new Awesomatics but I'm not spending $800 on a new chassis alone. This will be a slow rolling build as I'm expecting a fair bit of tinkering and parts are slow to arrive in Australia, ie need a different spur gear, gonna take at least 2 weeks to get here.

[NomadRacer]

Hey mate. There's a thread on another forum I visit about speed runs. I will search for it first chance and post the link.

[RC10th]

Thanks, the only one I really know about is ROSSA.

http://www.rossa.org.uk/


Finally started to put some pieces together. I didn't think the stock plastic bulkheads would be up to the task so I replaced them with some aluminum pieces for a bit more precision.

I also swapped out the pulleys to change the transmission ratio. Instead of being 2.0 (38/19) it is now 1.5 (33/22). This should hopefully allow for a more reasonable gear ratio range without needing crazy large pinions. I also had to swap out the rear belt for a shorter one and thankfully it fit perfectly, otherwise It would have made things a whole lot more difficult. I will stick with the plastic spools for now as they seem to spin true enough, and I like the fact that the plastic is a bit of a cushion in the drive train.

Next I need to make/install a belt tensioner to go just behind the front pulley then the center driveline is done. The stock belt tensioner sat on the top deck but with the smaller pulley the belt hugs the top deck. I'd rather not put the tensioner on the loaded side of the belt but there is no other option, so it will have to do.

[RC10th]

Here's roughly what it will look like.

I will use the smaller higher KV motor first to see what it will do. I'm pretty sure the ESC has basic data logging so I can see max motor RPM, max ESC temp, and max motor temp.

I was surprised a 1/8 motor fits under the body, it looks pretty stout in there!

[NomadRacer]

Here you go, mate. Sorry for the delay in getting the link. I couldn't remember the exact wording of the thread title last night and the search function was no help.

http://www.rctech.net/forum/electric-road/889041-speed-run-electric-all-classes-welcomed.html

[RC10th]

Cool link, looks like lots of people are doing it.

I got a little bit done as I've spaced the motor mount and top deck 2mm to fit a 1/8 motor. If I can mill a 1mm deep motor pocket into the chassis I can reduce the spacers down to 1mm.

I originally wasn't going to use the stock suspension and make something else up but decided against it in favor of cheap off the shelf parts. I've also fitted some aluminum suspension blocks to the front but still have to fit some to the rear once I decide on the rear toe. The rear hubs are 2 degree and I don't see a 0 degree option for them, the car comes with 3 degrees total rear toe if you include the blocks. More toe makes the car more stable on power, less toe free's the car up which may not be a good thing.

My choices are,
2* blocks and flip the hubs for 0* total toe
3* blocks and flip the hubs for 1* total toe
0* blocks for 2* total toe

I'm thinking 1* toe in as a start?

I still haven't made the front tensioner as I want to use the larger pulley they make instead of the small bearing tensioner, but I have to work out how to adapt it as the car it's used for mounts completely different.

[RC10th]

I figured out how I'm going to mount the batteries, just gotta order some bits now to make a mount.

The chassis will accept up to 8s using two normal sized lipos, but I think that will be WAY over powered.

Gonna be fun I think

[Lonestar]

Re Toe... I think you're playing with fire if you aren't going at least 2.5 or 3 deg... that's a lot of power to lay down to the ground...

Cool project

[NomadRacer]

Hey mate, looking good! I agree about the toe needing to be 2.5 -3 degree for stability.

[RC10th]

Thanks guys, it is coming along. Hopefully it stays in one piece long enough and not burst into flames before an all out speed attempt, lol.

The toe thing has me scratching my head as I agree 2 - 3 degrees will help drive the car straight under acceleration. My concern is the high speed stuff as the tires will be scrubbing possibly causing the rear to float, I'm running spools to avoid any tire uploading. I was thinking at speed when aero plays a role the side dams will help keep the car straight with 0* toe, I guess it would still do the same thing with rear toe. I could try both on a lower voltage to see which works better, would be interesting.

I'm quite curious as to how quick this thing will be.

[Lonestar]

Thanks guys, it is coming along. Hopefully it stays in one piece long enough and not burst into flames before an all out speed attempt, lol.

The toe thing has me scratching my head as I agree 2 - 3 degrees will help drive the car straight under acceleration. My concern is the high speed stuff as the tires will be scrubbing possibly causing the rear to float, I'm running spools to avoid any tire uploading. I was thinking at speed when aero plays a role the side dams will help keep the car straight with 0* toe, I guess it would still do the same thing with rear toe. I could try both on a lower voltage to see which works better, would be interesting.

I'm quite curious as to how quick this thing will be.

will you be on foams or rubbers? Both are anyway used to 2.5-ish degrees of scrub at 60mph+ speeds... I wouldn't think this is an area of concern - at least not the main one

[RC10th]

I will be using BSR purple foams as they seem to be the tire of choice.

[romulus22]

Very cool. Look forward to watching your progress. Top speed is much harder than I thought it'd be. I gave up after I couldn't keep my car from taking flight. Best of luck in your venture.

[RC10th]

Thanks, it will interesting as I have no idea what to expect. I think aero is the key above a certain MPH.

I found your thread but it's a shame all the pics were deleted.

http://www.rc10talk.com/viewtopic.php?f=37&t=33177

A local guy who ran his touring car with 4000kv and 4s said he could never get top speed out of his as it would blow the belted rubber tires apart. He 'd just throw the battery in and go without any set up.

I will start out slow and boring with 7.4v gradually increasing to 14.8v or 22.2v. At least this way I can monitor the data and hopefully address any issues as they arise.

[Timmahhh]

Cool build! I helped a friend of mine with a project like this. It ended up as an Xray T4 with a Tekin RX8 and 2650kv 8th scale motor with lots of ESC timing, almost equally sized pinion and spur, and a protek 4s battery. Originally it had a novak 5.5t 540 and 3s, which was good for I believe 67mph (according to gps on an old phone), and fairly easy to control and set up. Once we put in the 8th scale , it was a whole different story. It would get to approximately the same speed at 1/4 throttle (no gps measurements were done with the 8th scale electronics though), and then soon after, either blow over, have a significant failure (tires exploding, belts shredding, etc) or some combination of both. From a power perspective, 100mph would have been pretty easy really, but getting there without an issue, and having a good place to do it was the problem. It probably never got much above 80 before he sold it, but it was definitely fun while it lasted