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fullspectrum

Best chassis for speed runs?

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Nice work, @fullspectrum. I'd be interested to know if you felt the new floorpan improved stability and handling at high speed, too. Another piece similar to the one @stew_mac installed at the front of the car to smooth the airflow there might help, although your suspension set-up would seem to preclude fairing over the suspension arms, unless you can get the car to sit a lot lower and have the arms parallel to the ground.

Not sure what effect the shell might be having on your efforts, but again, fairing over the back wheels might help, if you can find a way of securing the fairings that was stiff and secure enough. Fairing over the fronts would probably reduce the amount of steering lock available, but since you're mainly going to be going very quickly in more or less straight lines, that might be an acceptable trade-off for the drag reduction it would give.

For inspiration, here is an article I just read on the fastest car ever to race at Le Mans: 252mph down the Mulsanne Straight in "a clunky, overheating French thing from the late '80s", powered by an engine developed from a Peugeot road car unti introduced in 1974.

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Thanks Yalson.:) my underpan is not as good as Stew's...my first attempt. Im hoping it helps a little. Also Im going to tape the sides of the body down now that the underpan is there to meet it. And Im hoping cutting the back of the body out will help keep pressure lower inside the body shell.

  Now I just need a good day to run it. Im hoping this coming Friday I can give it a go but temps will be cold here in Toronto at 5 degrees Celsius.

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50 minutes ago, fullspectrum said:

Thanks Yalson.:) my underpan is not as good as Stew's...my first attempt. Im hoping it helps a little. Also Im going to tape the sides of the body down now that the underpan is there to meet it. And Im hoping cutting the back of the body out will help keep pressure lower inside the body shell.

  Now I just need a good day to run it. Im hoping this coming Friday I can give it a go but temps will be cold here in Toronto at 5 degrees Celsius.

Taping the shell to the floorpan would help. It will both add stiffness and prevent the underfloor airflow bleeding into the area under the shell. Given that there are four huge gaps in the wheelarches where the differentials can equalise, it may not make any difference, but anything that stiffens the shell up is going to help. Something you might want to consider if you get another shell is trimming it so that the shell overhangs the chassis by a few millimetres. This will create a "skirt", which will help keep the lower-pressure air from bleeding out the sides from underneath the car. The closer to the ground you can get them, the better, but any overhang would help a bit.

Although I don't know if it makes a difference at the speeds where are talking about here, colder air is denser, meaning it is harder for the car to push it out of the way. Downforce-generating components will work better, though, for the same reason. The difference in air temperature might make a couple of mph of difference between winter and summer, though.

Good luck!

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Hahaha...thanks for the input. Im really hoping to get triple digits this time..

If the car doesnt fly away on 4s and I still dont hit 100mph I have two 3s batteries I will put in series for a 6s pass. Hopefully that will be enough to get me over the 100mph hurdle...😀

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5 minutes ago, fullspectrum said:

If the car doesnt fly away on 4s and I still dont hit 100mph I have two 3s batteries I will put in series for a 6s pass. Hopefully that will be enough to get me over the 100mph hurdle...😀

6s might get you over the sound barrier!

It is true that if subtle aero changes don't get you to your target, then there ain't no substitute for cubic inches. Or, in this case, volts. Will your ESC handle that much voltage?

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Yeah I put an Arrma blx185 esc and castle cap pack in the car.😀 

The question is will the 6900kv castle ( maybe fake) motor handle 2-3 passes on 6s...lol. It says max rpm 100,000.....but 22.2v x 6900kv is 153,000 rpm...lol.

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8 hours ago, fullspectrum said:

The question is will the 6900kv castle ( maybe fake) motor handle 2-3 passes on 6s...lol. It says max rpm 100,000.....but 22.2v x 6900kv is 153,000 rpm...lol.

If you need an example for that, according to a source which I read earlier and now cannot find again, the WM P88 was deliberately sent out with its cooling ducts partially taped over to do its record-breaking Le Mans speed run in 1988. The team had already lost 3.5 hours in the pits and effectively decided they were going to go for maximum aerodynamic efficiency to try and break 400kph/250mph, even though they knew it would inevitably grenade the engine and force a retirement.

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11 minutes ago, Yalson said:

If you need an example for that, according to a source which I read earlier and now cannot find again, the WM P88 was deliberately sent out with its cooling ducts partially taped over to do its record-breaking Le Mans speed run in 1988.

Typical. I've now read another page which denies that the claim the the team taped over the ducts for the speed run is true. *Sigh.*

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Ok so I made a little series connector so I can run two 3s in series. I tested it out and at about 3/4 throttle on 6s with the 6900kv motor my front foam BSR tire came apart!  

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9 hours ago, fullspectrum said:

Ok so I made a little series connector so I can run two 3s in series. I tested it out and at about 3/4 throttle on 6s with the 6900kv motor my front foam BSR tire came apart! 

Ah. Well, I can't say I saw that one coming, but now we know that things other than aerodynamics may have issues at close to 100mph. I believe the link that some one posted earlier in the thread about the 200mph+ World Record RC car suggested that when it was doing that speed, the car's wheels were rotating at over 26,000 rpm. And they were specially made 3" carbon fibre rims. So standard diameter Tamiya rims will be doing a fair chunk of that at close to 100mph. The foam tyres may also not be designed for those sorts of tolerances. Anyone? I'm rather out of my areas of (real or imagined) expertise here.

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6S on a 6900kv 3650 motor is asking for trouble. I'd guesstimate that motor is about 600W on 2S. On 6S it is 5400W because 3 times the voltage is 9 times the power. The motor will burn up very quickly. 1000W on a 3650 is about the max you can run for bashing - it'll be hot after running a full pack, even with a decent motor fan. For speed runs you can push it to about 2000W because you're just doing a run and then letting it cool. Above 2000W is entering the danger zone and it will go from dead cold to overheating in a matter of seconds.

Not only that, the rpm of the motor is too high. 6900kv*6cells*3.7v/cell <- i'm being conservative because the current is >200Amps so the batteries will be drooping a lot.
= 153,180rpm

The bearings in a 3650 motor won't like being operated above 50,000rpm for long periods or about 80,000rpm for short periods. Theoretically max power is half of maximum rpm so if you had your gearing set just right you'd only get up to 75,000rpm which I guess is acceptable for very short periods.

I would look for a 3650 motor in the 2500-3500kv range for 6S. That will get you output power around 2000W which is potentially still usable and should last a pass or 2 before getting too hot. You're better off sticking to a higher kv motor on 3S or 4S because you get much higher rpm at the motor for sane (1000-2000W) motor power.

Longer can motors (like 3665, 3674, 4074) will handle more power, but the rpm is much lower for the same power output as a 3650, which is a problem  in a TT02 where you find yourself gearing limited and must have a motor that runs at higher rpms.
 

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Yeah...going to a longer can will reduce my kv and like you said gearing might be an issue although the spur is 49t and pinion is 46t currently so its almost 1:1. 

Im going to do a 3s, 4s and then one 6s pass and see what I get for mph. Im also going to bring different gearing to try out and see if that gets me better numbers. 

The motor says 100,000rpm max and its supposedly a castle motor but I got it shipped from china so it could be a fake.

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Wow lots of cool stuff going on here! 

My thoughts on aero is keep it simple. 1:1 aero does not translate to scale, so a cleaner floor pan to help reduce some of the air getting in then better but no need to go over the top.  the wheels will generate a lot of air movement so best not to cover them and then leave the rear of the body open to let out any of the air. 

I use la maze / wedge shape bodies but modify the rear wind to reduce downforce/drag. you can pitch the rear of the body up so you can get the right amount of rear downforce. 

The body and pan do make a huge difference. also with the suspension and steering setup my cars lock in at around 40mph and then just track straight (unless something goes wrong).

Some of the fastest RC's use the same bodes. A TC6 on 4s 162mph and a serpant 747e on 5s 172mph both use a similar setup. just a simple splitter up front and then some side skirts other than that they are open.  

The motor is a clone. Castle had motors made in China then China companies copied them. they are still very good motors for what they are. i have gone 110mph with that motor on 3s. but that as far as i would push it with volts. the 5700 is good for 4s and i have gone 115mph with a genuine castle 5700kv motor on 4s. 

don't worry about the 100,000 rpm max. you will never get there. just impossible once you add drive train friction and resistance. 

Ulti foams are good for 172mph. the same wheels i use. free running the car is not the same as running it so weird things can happen with harmonics which you wouldn't get under load. 

 

 

looking forward to seeing you crack the 100 with this bad boy! 

 

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That is sick Stewart! Cant wait to see that run!

Ive been intrigued by what speeds guys have been getting with the 1/14 WlToys cars these days but I need another car like a hole in the head...lol.

So tomorrow is the day I will try my TT02 again going for 100mph. I also will give my high center of gravity slash another go for 100. The weather is cold and rainy today but tomorrow looks ok...just very cold. Probably 0 to 5 degrees Celsius. Going to a new road too. Less cars and longer but not very wide. Happy Halloween guys and wish me luck!

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Just need a front splitter/undertray and a motor fan and I'm ready to get in on the action ;) 2S, LRP 3.0t 11600kv motor to start. 'Legal' gearing as well (gear cover unmodified). Will try it on 90t spur/41t pinion first which I suspect is super conservative - peak power around 100-110kmh but should still have enough power to wind out past that. I have gone 100kmh on a 7.5t trackstar motor on 2S which is only 410Watts, lancia delta shell too lol (aerodynamics of a brick). This motor is 800Watts on 2S, so I don't see why it can't be good for just shy of 150km/h with the correct gearing. Twice the power, 1.5x the top speed - square law.
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Wow that thing looks awesome Stew!

Well I went out to my new spot today and did some runs. It was cold and very windy and the TT02 didnt do well today.

I had trouble seeing the car and lining it up and drove off the road a bunch of times just lining it up. The 6900kv motor on 6s with my Arrma blx185 was kinda lackluster probably because it was overgeared. The motor didnt get fried however. My aerodynamics werent good enough and it did a backflip on one run. 

On other runs I wiped out into the weeds and I lost the car on a few different runs.  I lost a dogbone on one run. The best I got was 76mph. So I swapped the 5700 kv motor back in and on that run I lost a wheel...hahahaha. Not a good day for my TT02.

  But my high gravity slash 4x4 did well today. It was a reversal of luck with my cars. After putting 2 million weight diff fluid in the rear, putting a wing on, cutting the back of the body out and installing Arrma Hoons tires on it she was planted on the road. It didnt go airbourne once...which surprised me. I forgot to put foam on the front bumper to support the front of the body so it dragged on the ground at speed but I still managed a personal best of 93MPH. So the Traxxas saved the day from total disappointment....lol.

So after all the running down the road and looking in the weeds for tires and my cars Im pretty tired to say the least. Im glad I made it to 93mph but 7 more mph to 100 eluded me. Then my GOPro died so that was it...hahaha.

At this point Im not sure where to go with my TT02. Im wondering if the BLX185 slowed it down compared to my OcDay cheap 120amp esc which got me to 90mph. And the 6900kv motor may not have the torque. Should I keep the 6900kv motor in and drop the gearing? Or should I go back to the 5900kv motor? 

 

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On 10/31/2019 at 1:51 AM, stew_mac said:

My thoughts on aero is keep it simple. 1:1 aero does not translate to scale, so a cleaner floor pan to help reduce some of the air getting in then better but no need to go over the top.  the wheels will generate a lot of air movement so best not to cover them and then leave the rear of the body open to let out any of the air.

I'm not sure what you mean by "the wheels will generate a lot of air movement". The wheels do generate a certain amount of unwanted aerodynamic effect, but that is due to their interaction with the airflow around the car. If you can isloate them from that airflow, then all of that drag will be eliminated. This is why the Thibault Le Mans Chevron – with its fully enclosed front AND rear wheels – was said to be so dynamically stable and aerodynamically slippery. Indeed, many Le Mans racers have had fairings over the rear wheels as it cuts drag and increases torsional stiffness for no real penalty. Well, it makes tyre changes more difficult, but that's not a penalty you will have to deal with. Once the wheels are faired in, the only interaction with the outside world those wheels are having is with air which has come under the car and up through the gaps cut into the floorpan for them. This air will by definition be turbulent already, so it isn't going to make much difference if it interacts with the wheels too.

I use la maze / wedge shape bodies but modify the rear wind to reduce downforce/drag. you can pitch the rear of the body up so you can get the right amount of rear downforce. 

The body and pan do make a huge difference. also with the suspension and steering setup my cars lock in at around 40mph and then just track straight (unless something goes wrong).

So it does make a difference? That's really interesting. I have often thought about this from a theoretical point of view, but I have never had the chance to actually try it out. I'm glad it's working for you. I may sit down soon and try and come up with a list of every aero element which might have a noticeable influence on RC speed running and see if anyone fancies trying to put them all together to see what difference the sum total would make. 

 

 

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The problem with pushing things too far, such as making a fully enclosed flat bottom car that is incredibly low to the ground is that while it may make significant downforce, it may also be aerodynamically unstable vs ride height. I.e. it may change from generating significant downforce to significant lift if the chassis hits a bump and pitches the front up a few mm. We don't actually need significant downforce as RC tyres generate adequate traction with just the weight of the car. What we are aiming for is an aero package which does not create lift.

Running an insanely low ride height may also make the car susceptible to rubbing the ground or the wheels rubbing the body which will unsettle it. Keep in mind that the road is effectively incredibly bumpy compared to a 1:1 car and we are trying to drive at 1600kmh+ scale speeds.

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3 hours ago, nbTMM said:

The problem with pushing things too far, such as making a fully enclosed flat bottom car that is incredibly low to the ground is that while it may make significant downforce, it may also be aerodynamically unstable vs ride height. I.e. it may change from generating significant downforce to significant lift if the chassis hits a bump and pitches the front up a few mm. We don't actually need significant downforce as RC tyres generate adequate traction with just the weight of the car. What we are aiming for is an aero package which does not create lift.

Running an insanely low ride height may also make the car susceptible to rubbing the ground or the wheels rubbing the body which will unsettle it. Keep in mind that the road is effectively incredibly bumpy compared to a 1:1 car and we are trying to drive at 1600kmh+ scale speeds.

This is true. I mentioned the problem with 1:10 cars hitting 1:1 scale pebbles in a previous post. Very low ride height is always preferable if you can achieve it, as it bring other advantages beyond the aerodynamic ones: increased stability, lower centre of gravity, better handling etc.

HOWEVER, running a car with a 2mm ride height is obviously impractical in the real world for the reasons you mention. A Ford Focus RS (to pick a car at random) is 147cm tall. If it ran over a 1cm pebble at 70mph the car's stance would not even move, as its tyre would almost certainly have enough ability to deform to absorb the impact without passing it up through the car's suspension. A true 1:10 scale Focus RS would be 14.7cm tall... and I think everyone can see where this is going. 1cm would be nearly 7% of the scale Focus's total height and would be the equivalent of the real Focus hitting a 10cm rock at 70mph. In fact, the 1:10 scale hit would be even more dangerous, as the smaller tyres do not deform in a scale fashion and would certainly alter the car's pitch angle, which may be what causes the flipovers which @fullspectrum referenced in an earlier post. The nose of the car gets lifted, the airflow gets underneath, downforce switches to lift and it takes off.

There aren't many 10cm rocks lying about on Britain's motorways, for obvious reasons. But there are a LOT of 1cm pebbles pretty much everywhere we want to run RC. The 202mph RC world record run which was linked to earlier in this thread took place on a disused airstrip, which would have been as flat as an RC surface is going to get. But to run at those speeds the strip would have had to to be swept by hand (as 1:1 speed record courses are) to remove all those scale 10cm rocks. That is obviously not practical for most people running on roads and other public spaces, so a degree of ride height and suspension has to be dialled in.

As for downforce, you are right: to go fast in a straight line we don't need that much of it. This is why most of the things I have mentioned recently in this thread have been less about downforce and more about reducing drag and increasing stability, which really are issues with 1:10 speed runs. The undertrays do increase downforce, but they do so in a way which does not increase drag. They are doing the job of keeping the car "planted" and tracking straight over minor blemishes and irregularities. You could do the same job with a big bucket wing, like off-road buggies have, but while this will act like "the flight of an arrow", arrow flights keep arrows flying straight by adding a source of drag at the tail. These big bucket wings do not create significant downforce, but beyond a certain point, the drag generated will become so great that it will start to rotate the car like a pedal, lifting the nose etc etc. You know where that leads.

A flat bottom, lower ride height and perhaps a smaller rear "kick-up" wing (as @nbTMM has engineered with the shell in the pics above) or Gurney flap, and cutting out the rear panel of the body will significantly aid stability and reduce drag. If you want to reduce it further, use as slippery a body as possible, cover the wheels, and don't trim the area of the body behind the rear wheels back so far, to give you a longer "tail". You will need a couple of degrees of negative pitch (nose-down attitude) to keep the nose down and stop the car flipping, which you can with get either through suspension set-up or with adjustable body mounts.

How much of all of the above you want to do is obviously down to the individual, but it all helps.

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21 hours ago, nbTMM said:

Just need a front splitter/undertray and a motor fan and I'm ready to get in on the action.

I have gone 100kmh on a 7.5t trackstar motor on 2S which is only 410Watts, lancia delta shell too lol (aerodynamics of a brick).

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I love what you've done with the tail of this 962, @nbTMM. One of this issues with removing the rear panel of a car is that the tail it is going to lose a lot of stiffness and become "flappy". The upshot of this is that beyond a certain speed it may start to vibrate in the airflow like a flag. That is absolutely not going to happen with your shell!

The Lancia Delta Integrale, in both 1:1 and 1:10 forms, is an amazing car with many great qualities. Sadly, slippery aerodynamics is not one of them, as you rightly point out. Then again, rally cars are rarely configured for low drag. Mostly it's about high downforce.

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On ‎9‎/‎10‎/‎2019 at 2:44 AM, fullspectrum said:

For my Boomerang which I managed to get to 49MPH I used a high 5900kv motor because you cant go tall on the pinion because of the gearbox. And tell me this...why is my Boomerang straight as an arrow without a gyro? It's like night and day between the Boomerang (a buggy for dirt) and my TT02 (built low for the street) I'm baffled...

Because some things get meaner as they get older B)

It may have something to do with the boomerang having a tranverse mounted motor, whereas the TT-02 is longitudinally mounted. The boomerang transfers the power to rear axle without any torque twist (Just tries to pick the front end off the ground), but because of the layout of the TT-02 transmission, the turning force being put through the centre transmission is trying to twist the differentials around the centre prop. Using very powerful motors this is going to try and lift one front wheel off the ground and burying the rear wheel on the opposing side into the ground as the chassis twists.

 

 

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19 hours ago, MadInventor said:

Because some things get meaner as they get older B)

It may have something to do with the boomerang having a tranverse mounted motor, whereas the TT-02 is longitudinally mounted. The boomerang transfers the power to rear axle without any torque twist (Just tries to pick the front end off the ground), but because of the layout of the TT-02 transmission, the turning force being put through the centre transmission is trying to twist the differentials around the centre prop. Using very powerful motors this is going to try and lift one front wheel off the ground and burying the rear wheel on the opposing side into the ground as the chassis twists.

 

 

It may also be partly down to the Boomerang's greater suspension travel and larger wheels. Cars with these features can roll over imperfections and small obstacles in the road better than touring cars with smaller wheels and limited suspension travel. If you imagine a Mini (with 10" wheels and relatively limited suspension travel) and an SUV (with 16" wheels, higher ground clearance and greater suspension travel) both hitting a paving slab laid in the road at 40mph, you can imagine the difference in response. This is also why cars in larger scales tend to cope with road imperfections better.

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On 11/2/2019 at 8:09 PM, MadInventor said:

Because some things get meaner as they get older B)

It may have something to do with the boomerang having a tranverse mounted motor, whereas the TT-02 is longitudinally mounted. The boomerang transfers the power to rear axle without any torque twist (Just tries to pick the front end off the ground), but because of the layout of the TT-02 transmission, the turning force being put through the centre transmission is trying to twist the differentials around the centre prop. Using very powerful motors this is going to try and lift one front wheel off the ground and burying the rear wheel on the opposing side into the ground as the chassis twists.

 

 

Actually the TT02 is going straight now. I was running it with toe in which made it super twitchy...now corrected with toe out. So once it gets up to speed it pretty well goes straight.

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