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Best chassis for speed runs?

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On 10/12/2019 at 7:56 PM, Yalson said:

Given at 80mph+ the body is probably vibrating a fair bit, moving the mounts further forward would undoubtedly make the nose a lot stiffer. You need to get it as close to the ground as you can, too.

Yeah its not low enough to the ground. It's still at stock TT02 ride height. I may have to limit the shocks and drop it down a bit. OR when I get my new body cut it low to the ground on the front. I also have the front bumper foam so it is up against the front of the body so it keeps the front of the body rigid and not collapsing under the force of the wind at speed.

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10 hours ago, nbTMM said:

Cutting out the rear of the shell so it doesn't act as a parachute and finding the balance between front and rear downforce so it doesn't take off like an airplane is going to matter a lot more than where the body posts are.

True. Also wondering if a little weight on the front could be of some benefit just to get to 100mph. I know it's a bit of a band aid to do that but a little could help?

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4 hours ago, fullspectrum said:

Yeah its not low enough to the ground. It's still at stock TT02 ride height. I may have to limit the shocks and drop it down a bit. OR when I get my new body cut it low to the ground on the front. I also have the front bumper foam so it is up against the front of the body so it keeps the front of the body rigid and not collapsing under the force of the wind at speed.

That would be sensible. RC cars are aerodynamically awkward, given that they are hollow underneath and have no way of generating downforce under the car. The video I linked to earlier, although about 1:1 scale cars and therefore reliant on aero elements you don't have access to, has some good basics on the sort of basic stance the car should have, the reasons why and what can happen if it goes wrong.
 

 

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It's all interesting, but the bit most relevant to 1:10 speed running starts at about 3:45.

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My understanding is that aerodynamics does not scale down perfectly due to how boundary layers act on scale wings and bodies versus 1:1 vehicles. Basically, wings on scale vehicles which are profiled like airplane wings do not work as effectively to create lift/downforce compared to their 1:1 counterparts. That's why you see Lemans/'Pan' style shells with a rear 'scoop' with aggressive attack angle, instead of a separate wing with shallow attack angle and a gap between it and the body for the low pressure zone to form. It's also why RC airplanes have to be extremely light weight or travel at insane scale speeds, because their wings are not very effective at creating lift.

I think most of the downforce on an RC car comes from it simply displacing air, creating a high pressure zone above it, which pushes the shell downwards as long as the air inside the shell is lower pressure. A big scoop on the rear creates yet higher pressure over the rear of the shell as it increases the amount of air displaced, and therefore produces more downforce at the rear, at the cost of higher drag.
If cut out the the rear of the shell, the low pressure zone generated behind the car will suck air out from the inside of the shell and maintain a low pressure zone inside the shell (or at least lower pressure than 'outside'), therefore maintaining downforce. Undertrays/diffusers/splitters on rc cars probably create less actual ground effect under the car and instead just act to better seal up the shell, preventing air from entering the shell and therefore allowing the low pressure zone at the rear to more effectively maintain low pressure inside the entire shell. If too much air enters the shell and too little air is sucked out the back, it becomes pressurised, creating lift and the car takes off.

 

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I would have thought ground effects and areos would have an effect with the speeds we are getting out of these things. Our speed run cars are almost hitting triple figures, with relatively strict rules and the likes of the new Arrma Limitless (which has ground effect diffuser as standard) are hitting over 150mph!!  

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2 hours ago, Wooders28 said:

I would have thought ground effects and areos would have an effect with the speeds we are getting out of these things. Our speed run cars are almost hitting triple figures, with relatively strict rules and the likes of the new Arrma Limitless (which has ground effect diffuser as standard) are hitting over 150mph!!  

Aero yes, to a certain extent. Ground effect, no. Ground effect relies on there being a space under the car which is sealed around the edges in order to create a low-pressure area and effectively suck the car to the ground. This is inctedibly hard to do at small scales, as Lexan bodies flex so much and imperfections to the road surface will be encountered on a 1:1 scale and will affect the cars proportionately. Furthermore, and most importantly, it is impossible to generate ground effect on most 1:10 cars as they do not have a conventional floor.

@nbTMM's suggestion of a low-pressure area under the shell would work if the apertures for the wheelarches were sealed in some way, but as they're not, any pressure differential under there will be minimal, as the lower pressure air under the shell will just suck high pressure air in from outside past the wheels and through the gap between the shell and the road. He is right about the wings on pan cars and buggies creating downforce, but it comes less from aero effects and more from simple Newtonian physics, with air being deflected upwards by the bucket and the car correspondingly being pressed downwards. We had a conversation on aerodynamic effects in buggies on another thread some time ago, where I asked if the wings on buggies actually did anything. Somebody posted a link to a podcast featuring a Schumacher engineer in which he admitted that they made very little actual downforce and that aero on buggies, at least, was more or less pointless due to the enormous compromises involved. But he added that buggy wings are nevertheless vital, as the (considerable) drag they create acts like "the flight of an arrow" in the words of a more eloquent TC poster (@Mad Ax?) and helps keep the buggy in a straight line and adds stability over jumps.

I would suspect the wings on touring cars and pan cars act in the same way, for the same purpose.

I have not seen the Arrma Limitless. I will have to have a look, but if it is anything like the tiny diffusers which someone tried to convince me worked on drifting cars, it really won't work. A diffuser works best when used in conjunction with an underfloor low-pressure area as described above. On it's own it could work, but it would have to be pretty big and mounted within a hair's breadth of the floor to have any effect, with all the problems relating to surface imperfections with it that I have already mentioned.

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Also, aerodynamics does not scale down at all. It always works at 1:1 scale, but with wings and aero devices which are working at considerably below-scale efficiency. Somebody on a previous thread tried to argue that a 1:10 F1 car would act the same as a 1:1 F1 car, as it had scale wings. Unfortunately it doesn't, as the scale F1 does not have the vital underfloor aero devices and it's wings are not a tenth of the size, but 1/1000 of the size, as the dimensions of the wing have been reduced to 1/10 of their original size in all three dimensions.

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Hmm. The Arrma is a strange hybrid indeed and it's interesting that they've scaled it up to 1:7, presumably because bigger cars deal better with the inevitable issues facing scale models trying to achieve high speeds. There is presumably a sweet spot on the scale range, probably a bit bigger than 1:10, where the advantages (higher power-to-weight ratio, lower absolute drag, lower friction losses) and the disadvantages (poor aero response, proportionate reaction to road imperfections and obstructions, instability) are both at their mosrt favourable. Perhaps 1:7 is it?

I am not certain of how effective their diffuser will be, but it's a better effort than anything I've seen at 1:10 scale and they have at least attempted to unify it with some sort of unified aero scheme, at least at the back. The fact I could see daylight under the leading edge doesn't bode well and I doubt it works anywhere near as well as they undoubtedly claim, but somebody has at least thought about it.

The front is a bit of a mess and has no real aero devices at all, since the front wings are just narrower versions of the "buckets" seen on the backs of buggies and have no aerofoil or ground effect shapes. Even worse, the airflow is disrupted just in front of the front axle by a chunky pair of oil-filled shocks, the same devices which render any attempt at aerodynamics in 1:10 buggies pointless.

It is also not a ringing endorsement of the aero package on the Limitless that several of the owners on the first page of images had removed as much of Arrma's attempts at aerodynamics as they could, presumably to save weight, reduce drag and increase speed.

Nice try, but no cigar I fear.

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I ended up buying a GT style body for this TT02 but I would need to tuck all my electronics down low to make it fit. Also this body feels very thin. I decided to keep the Tamiya Raikiri body as I think its low enough and has enough aerodynamics to get me to 100mph, and the quality feels much better.

Im also going to add an under pan made of carbon fiber. I think sealing the bottom should help. Also I may add a few stick on weights to the front of the underpan doesnt work on its own. 

Its my goal to do 100mph with a Tamiya and run a Tamiya body.😀

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Just now, fullspectrum said:

I ended up buying a GT style body for this TT02 but I would need to tuck all my electronics down low to make it fit. Also this body feels very thin. I decided to keep the Tamiya Raikiri body as I think its low enough and has enough aerodynamics to get me to 100mph, and the quality feels much better.

Im also going to add an under pan made of carbon fiber. I think sealing the bottom should help. Also I may add a few stick on weights to the front if the underpan doesnt work on its own. 

Its my goal to do 100mph in a Tamiya with a Tamiya body.

That all seems very sensible. The underpan won't seal the underside on its own (the wheel arches will still allow the pressure inside the outside the car to equalise), but it will help keep things stable. Another thing to do is give the car a slightly nose-down attitude, as that will create downforce from the airflow over the body. It will create drag, too, though, so there is an optimum pitch angle to be found. Weights will obviously help keep the nose down, but will increase net weight and reduce top speed correspondingly. But it may be that the extra stability allows you to unlock performance you were losing out on due to other performance issues. The only way to find out is trial and error, really.

Good luck!

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With regards to the underpan, I found this after a lot of searching: An article from Motorsport magazine from 2002 about the McLaren M23 F1 car.

'They didn't grasp the opportunity in the way Lotus eventually would, but the side-skirts that appeared on M23 during 1975 were the most professional-looking and effective of their type.

That large underbody area (previously considered a design no-no) was paying increasing dividends, too, especially when the sidepods were extended to the rear wheels in 1975. Suspension movement was consistently reduced to keep the track/car interface as close and as consistent as possible. And a NACA duct was let into the floor under the driver to create an area of negative pressure inside the cockpit.

"We weren't particularly aware of ground effect in 1973," admits Coppuck. "We knew we were generating downforce from the underside, but had no accurate measurement. All we could do was try an idea and see if the driver liked it. We had, though, always found that bigger was better. The bigger the plan area, the more downforce."'

I do not quite understand why a large, flat floor creates downforce, but I can only presume that it somehow streamlines and accelerates the airflow under the car, creating a low-pressure area underneath. Albeit one nowhere near as marked as the ones produced by the venturi tunnels under full ground effect cars like the Lotus 79.

Anyway, bedtime. Hope some of that is useful to you.

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18 hours ago, Yalson said:

Ground effect relies on there being a space under the car which is sealed around the edges in order to create a low-pressure area and effectively suck the car to the ground.

For the most effective use, yes, but you can prove ground effects / venturi effect by blowing across a piece of A4 paper, and it'll lift, proving a negative force exists. 

18 hours ago, Yalson said:

Somebody posted a link to a podcast featuring a Schumacher engineer

That was me!😂

Trish also said, after wind tunnel testing, that body design was negligible to aeros ,as an Off Road car has too much furniture, shocks and towers etc. 

The furniture is all under the body on an on road car, so presume body design has some effect? (@100mph)

 

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On 10/20/2019 at 6:53 PM, Wooders28 said:

For the most effective use, yes, but you can prove ground effects / venturi effect by blowing across a piece of A4 paper, and it'll lift, proving a negative force exists. 

That was me!😂

Trish also said, after wind tunnel testing, that body design was negligible to aeros ,as an Off Road car has too much furniture, shocks and towers etc. 

The furniture is all under the body on an on road car, so presume body design has some effect? (@100mph)

 

Not sure what you mean by blowing across the paper.

The ground effect/venturi won't make it lift as you suggest, though. The lower pressure in the expandng section of a venturi can be used for downforce, but won't make a piece of paper lift. There are other effects that will, though.

Body design is the only thing that will really affect downforce on a circuit car. It needs to be slippery to reduce drag, and will generate a certain amount of downforce if you set it at a few degrees of nose-down attitude. The wings on the back are for stability more than downforce, but they generate a lot of drag. If you can get it close to the floor then it will help with CoG and handling, but most of the other supposed aero effects are mainly window dressing until you start getting up past 60mph or so. If somebody could fashion a really stiff car shell with proper wheel wells, a sealed floorpan, side skirts that sealed a layer of air under the car, a very low nose, faired-in rear arches, some sort of arrangement to seal the airflow off from entering the front wheel wells when the car was travelling in a straight line (soft brushes?), and maybe a proper aerofoil rear wing, then it might have a noticeable effect on straight-line speed.

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8 hours ago, Yalson said:

Not sure what you mean by blowing across the paper

Try it, our lecturer used it to show the effect. If you hold a piece of paper with both hands (in the corners) ,the other end drops down. If you then blow across the top, length wise, the dropped end starts to lift. 

8 hours ago, Yalson said:

mainly window dressing until you start getting up past 60mph or so

Agreed, and you'll need to be hitting that kind of speed to be in our top ten, going to have to start thinking about a more slippery body for my off road speed cars! 🤔

8 hours ago, Yalson said:

If somebody could fashion a really stiff car shell with proper wheel wells, a sealed floorpan,

I think that's exactly what @stew_mac has done with his TT02, almost 120mph speed run car, layering carbon fibre onto the lexan body (if i remember right?)

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9 hours ago, Yalson said:

Not sure what you mean by blowing across the paper.

The ground effect/venturi won't make it lift as you suggest, though. The lower pressure in the expandng section of a venturi can be used for downforce, but won't make a piece of paper lift. There are other effects that will, though.

Try blowing an air gun into a open plastic bag, or under a sheet of paper held 1cm above a table. The effect is opposite to what you'd expect. The bag gets sucked closed, and the paper is sucked down onto the table.

You can get the same effect with a flat bottom RC car, although I think the effects of dealing with the fact that the "inside" of an RC car shell is rather leaky (wheel arches and other gaps) makes much more of a difference than ground effects alone.

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12 hours ago, Wooders28 said:

Try it, our lecturer used it to show the effect. If you hold a piece of paper with both hands (in the corners) ,the other end drops down. If you then blow across the top, length wise, the dropped end starts to lift.

That's not a venturi or ground effect, though. That's the same process as takes place above and below an aerofoil wing. The accelerated air over the curved surface of the top of the paper creates a low-pressure area which sucks the paper upwards – if the curved area had been solid then it would have had the same effect on the whole unit, as it does on aircraft, which is what generates lift and flight.

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I carbon fibre my body for to increase strength. But this body is for 130 - 140mph. 
 

I have gone 100mph with just using some lexan heated up and folded into shape to support the sides of the body. 
 

 

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Here is an under tray and splitter I made. 
 

to be honest I ran 110mph with a basically stock TT02RR just with the right body.

6FE806EA-A976-4975-BDBF-9A6C91D225B1.jpeg

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On 10/23/2019 at 10:22 AM, stew_mac said:

Here is an under tray and splitter I made. 
 

to be honest I ran 110mph with a basically stock TT02RR just with the right body.

6FE806EA-A976-4975-BDBF-9A6C91D225B1.jpeg

Nice. Does it make a noticeable difference to the stability and handling? And have you considered taking the aero components and further, such as fairing in the wheels and adding side skirts? The skirts would be difficult, as I'm not really sure what would form an effective seal at those speeds, but it might be worth thinking about. The fairings would be easier as the body seems a decent stretch wider than the chassis and you could presumably just not cut the wheelarches out to add stiffness and help seal the bodyshell apertures. It might be trickier at the front, but you do seem to have clearance there and it might be enough if you're trying to keep the car in a dead-straight line. I don't know where you do your speed runs, but fairing in the front arches will depend on how much space you have and how flat it is. If it is an empty enough area, do what they do for 1:1 speed record attempts: mark it out and sweep the marked area free of stones, leaves, dust and other detritus. Because if you hit even a 5mm pebble at 130mph with a 1:10 car, the sudden change in pitch attitude might be enough to flip it.

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@stew_mac Would it also be possible to fair over the suspension arms? I notice in your picture that the suspension arms do not appear to drop below the line of your carbon floor panels, meaning it might be possible to add a stiff piece of flat lexan to the floor of the car to cover the underside of the front suspension, effectively joining up the curves formed by the cut-outs on the front section and the mid sections fitted either side of the car's "spine". You'd still bleed airflow into the wheel cut-outs, but it would tidy the airflow under the car and presumably increase the low-pressure area. I can't work out from this image if you could do the same with the rear arms, but if you could then that would help tidy airflow, too.

You could also tape over the screw heads and holes in the chassis with very thin tape. To be honest, it probably would not make that much difference, but anything that smooths the airflow or prevents higher-pressure air bleeding into lower-pressure areas would be an incremental help, and they all eventually add up.

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Hmmm. Just found this on the interwebs, about a Chevron chassis modified to run in the C2 class at the 1987 Le Mans race with fully enclosed wheels.

Not the most successful of machines, as it retired fairly early on while running dead last, but the article does have some quite interesting things to say about the wheel fairings:

"At 18:20 local time, the car was back on track with Andre Heinrich at the wheel. Heinrich ran long enough to warrant a fuel stop, something which didn't occur often since the extremely slippery machine used up to 25% less fuel than the other cars in its class.

Additionally, the fully enclosed wheels made it incredibly stable at high speeds, a characteristic which was especially beneficial in fast corners like Indianapolis."

The resulting car was fairly odd-looking, but the faired arches were seemingly pretty effective in terms of reducing drag, even if it was otherwise fairly hopeless.

744945173_ThibaultChevronB36REDUCED.thumb.jpg.76e11957eb8c19bbad43138e6eee536b.jpg

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On 10/23/2019 at 5:22 AM, stew_mac said:

Here is an under tray and splitter I made. 
 

to be honest I ran 110mph with a basically stock TT02RR just with the right body.

6FE806EA-A976-4975-BDBF-9A6C91D225B1.jpeg

Thats awesome thanks for the pics Stew!

 I made a crappy attempt at a carbon fiber underpan..actually "side wings" like yours. At any rate Im running out of time here in Toronto before its too cold/snow hits. Also my biggest obstacle is working 60 hrs a week and my kids eating up my time. Shopping, music lessons, Taekwondo...it never ends...hahahaha😣

So with my shabby underpan, front bumper weights and a counterfit(I think?) Castle 6900kv motor and new Arrma BLX185 esc......we will see what happens. Im guessing another blowover but hopefully I can hit 100mph.

 I also did some work on my Slash. I put 2 million viscosity diff fluid in essentially locking the rear end and put some Arrma Hoons on it. Also duct taped the inside of the body. I also came to the realization that the Traxxas gyro stabilization was turned up too high so that may have been part of my problem.   So both cars did 90mph. Can I make it to 100? We will know next Friday...

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Here's a few pictures of the underpan/wings, new Castle($40 from China...maybe fake)6900kv motor, Arrma Blx185 esc with Castle cap pack, weights on bumper and rear of body cut out.

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