losi xxx

[xx4-nutter]

thanks phil ill have a go with that and see how it goes. ive just had a good close look at my dads xxx-cr hybrid that was paul rotherhams and it has the modified parts on it lol aswell as this brace between the servo and cells done ( which ive never noticed before lol )

so whos going to the regional sunday at duhram ? i hear there is only 60 booked in??!!

Garry D.

[greeny]

just added 30 grams to the front end of my XXX hoping it will cure the onpower LACK of steering problem!!!! and i correct should this work??

thanx jordan

[tonymon]

going back to the weight thing, one thing u have to remember with our toys (lol) is that we are dealing with scale cars with the same gravitational effects as a normal car and huge power to weight ratio (already mentioned i know).

[Lee]

Very true, its the same with speed, speed is a constant regardless of the scale of the car.

Also the first thing our 2wd cars want to do is flip them selves over due to the power, and the weight distribution, possibly dan or chris could tell us if the X6 is any better?

I have added 25g just in front of my CR and its not to ride the bumps better, its to stop the front wheels lifting when i came of the banking at bury whilst trying to get the power down.

I agree with northy that the shocks should do the work and a lighter car is ultimatley faster, but on the other hand the immense power we have to hand, combined with the layout of the cars, sometimes a little weight is needed.

[paulj]

I can think of one class of racing where some controlled chassis flex is deemed a good thing...

MotoGP

look at the problems Yamaha ran into at the start of the '06 season with chassis setup. They stiffened the chassis(frame) from '05 and picked up loads of chatter problems making the bike very difficult even for Rossi to get the best out of it. They then couldn't run the latest 16.5, at least I think it was that size, front Michelins as this gave them more grip and increased the chatter problems.
The answer? They went back to the '05 chassis with modified pickups to suit the '06 running gear and Rossi got very close to pulling the rabbit out of the hat, oh for 0.005 seconds at Valencia...

No full scale race or rally team are going to add weight to the chassis unless they can build it below the minimum weight for the catagory and then play with ballast to fine tune the setup. But I've yet to come across a full scale catagory that has 1700g weight limit including onboard driver

Northy you are comparing apples with oranges

[shinytopman]

Originally Posted by jimmy

weight adds stability and slows the reactions - which on our crazy-fast cars is a good thing for lesser drivers like myself

Good point and if you put that into context of scale speed then F1 or even WRC teams may well be adding weight !

[SHY]

This is a very complicated issue, and setting up race cars is an endless process of compromising. I will get back with a more in-depth answer. But for now, I just wanna say:

-I think Northy is absolutely right. Any race car in any class will normally be faster when it's as close to the min. weight limit as possible.

Now, one thing that significally separates R/C racing from 1:1 racing is that we in many (not all) classes have power AND run-time in excess. As long as legal we can for instance add monster wings without effecting top speed (no drag penalty).

When push comes to shove it's very much about tyre load. This is our only contact with the ground. More grip = faster.

OK. So what we basically want to achieve is as much tyre load as possible, without having to much mass to carry. That would generate too high G-forces in corners especially. Plus chassis roll. We can achieve this (F1 is extreme in that respect) by generating downforce (and/or ground effect). Since our toy cars are small, full-scale aerodynamical principles don't apply. It's for us simply a matter of:
-shoving air up (and therein generating downforce)
-how the car cuts through the air (jumping and cornering)

In order for a R/C wing to do any good, it must be both steep and large. A tiny, flat aerofoil wing will do no good but look good...

Basically then we would want a bigass wing both front & rear, as long as it is legal. (I'd really like to see in a wind tunnell test how much turbulence the rear wing would suffer from the front one btw)

Now, putting too much downforce on the chassis can also be negative, as it loads the suspension. Have you guys seen how we do it for 1:8 TR? The rear of the shell is resting directly on the rear hubs! Brilliant! Tons of downforce straight onto the rear wheels, without compressing the suspension. I'd like to see that on a buggy as well!

NB! I'm not saying here that adding more weight to the car can't be faster. I'm sure it's true, depending on the track, grip etc. Especially since we have both power and runtime to be able to carry more weight around. But what I want to point out is that it's not the best way to do it. If car A has the same amount of tyre load from high downforce and weighs 1400 grams. And car B has the same tyre load from chassis weights (and minimal wings) and weighs 1600 grams. Then car A should easily both out-accelerate, have higher cornering speed, and be less likely to roll over.

But... there's a limit (rules) to how big wings we can use. So once that's maximum, there may still be more to gain by adding weight. But that's in this class - because grip is often low. For track racing with high levels of grip extra weight is without question slower. Except when you're driving in rain (low grip).

[I'll be back with more...]

[Chris Doughty]

another thing to note....

making the 'chassis' heavier is actually making the weight percentage of un-sprung mass 'less'

sprung 750g - unsprung 750g (50/50 sprung/unsprung)
sprung 1000g - unspring 750g (can't work it out, but sprung mass is higher percentage)

rubbish figures and example I know but I don't have time for details...

whats your htoughts?

[Dunc]

Originally Posted by DoughtyUK.net

another thing to note....

making the 'chassis' heavier is actually making the weight percentage of un-sprung mass 'less'

sprung 750g - unsprung 750g (50/50 sprung/unsprung)
sprung 1000g - unspring 750g (can't work it out, but sprung mass is higher percentage)

rubbish figures and example I know but I don't have time for details...

whats your htoughts?

That's an interesting point Chris, certainly not something I'd thought of. So by increasing the sprung mass of the vehicle you cause a reduction in the apparent (or relative) unsprung mass. Whether this actually reduces the impact of the unsprung mass I'm not sure....

Btw, the ratio for your second example is 57/43

[paulj]

bikes ain't that good an example either as centrifugal force has a lot of effect on handling and direction change. Yamaha run their motor in the reverse direction to Honda and pretty much everybody else in MotoGP to counteract some of the centrifugal force genrated by the wheels. This improve direction change at the cost of top end power as they need to use extra gears to get everything turning the correct way at the gearbox end. One of the reasons the Yamaha can hold a tighter line through a series of corners and and get up the inside of oponents into a corner.

I'm not sure if this would have much effect with RC cars due to the lower forces involved but it should be different in belt as compared to shaft drive 4wd?

F1 engineers would happily pay the penalty of extra weight if it could consistantly produce faster lap times
I'd be suprised if the old Brabham 'fan car' was below the weight limit of the time but as it almost literally 'blew' everybody away on the track with it's lap times I don't think the engineers cared about the excess weight too much. Mind you the FIA quickly found an excuse to ban it however dubious the reasons.

Chris it sounds sense to me, but that don't mean much A lower unsprung weight should make the suspension work better but you still have to find the ideal settings to go with a lower weight. We don't have the benefit of suspension dyno's and real time suspension data logging or much in the way of sophisticated damping either...

[SHY]

@Chris: My initial thought is that for OR it's not significant enough. Even for 1:8 TR, which I know well from 12 years of racing and lots of testing, it's not really noticeable on the laptimes. Also lightening the drive-line (rotating mass) is theoretically supposed to give better laptimes, but again: not measurable. What is defo measurable is lightenting the car 100-200 grams (in total)... very noticeable when driving, and very noticeable on the laptimes! Racing is very much about cornering speed - that's where you win or loose IMHO.

@paulj: you mention motoGP and the centrifugal force of the wheels. Wouldn't that produce more grip? (=good stuff). You also mention 'fan cars'. IMHO here you can't put weight into consideration as this was so effective that it outweighted the possible extra weight of the fan & motor. Those (like the Chapparal) were 'vacuum cleaners' on 4 wheels, which actually sucked themselves to the ground = higher cornerings speeds and G-force recordings than ever before. The stupid thing with this solution was that they picked up all the debris on the track and blew it in the face of the drivers coming from behind.

I'd like to add another thought (before I forget). As we have this tremendous speed & power, and these really tiny tires which is our only link to the ground - I think that this is the uttermost important room for improvement in laptimes. I think that these could very well be more sophisticated: tubing, wowen in steel/etc, tyre pressure, balanced rims etc. The tires/wheels we are now using are really 'toy solutions' as I see it (unbalanced both horisontally and vertically). But then again - it might not be significant enough, so only one way to find out... Balancing them with lead etc will take away the worst vibrations and help some, but there is still more to gain I believe.

[Chris Doughty]

imagine a rippled 20 meter stretch of track.

both cars with 'nice' suspension setups on them for the cars weight

1 'barge' that weighs a lot

1 'feather' that is very light

which one would you imagine to be more 'stable' going through this section?

with regard to 'ripples' I mean small bumps that could be absorbed by suspension movement

heavy cars slowing down corner speed I completely agree with, but in off-road its not all about corners

[Dunc]

Originally Posted by SHY

Refer to post above

First off: back with more?! There's plenty to mull over here already

I agree with you that a conventional wing, i.e. aerofoil, scaled down to our off-road vehicles would be next to useless due to the lack of size and speed to generate any meaningful downforce.

I don't quite agree with the mechanism you suggest for generating downforce, i.e. "shoving air up". My personal opinion is that our wings simply act as obstructions to the flow, i.e. drag devices. They cause separation of the air flow behind them (like flow around a cylinder) and therefore a pressure drag. Because the wings are mounted relatively high up, this drag generates a torque on the chassis which ultimately manifests as increased force on the rear wheels. The air being pushed up is just a result of their shape; it's not the main mechanism. The larger the steep sections at the rear of a wing, the more pressure drag you will generate and hence "downforce". I think the large sides of the wings also aids lateral stability, i.e. helps to combat snaking, particularly at low speeds when accelerating out of corners. The drag mechanism I've described will be less important under such conditions as it requires a certain amount of velocity to have any meaningful effect. This is all just guess work on my part by the way!

Very interesting point about loading the suspension, although I'm not sure how practical it would be to mount to the hubs instead on our buggies. Nor whether it's particularly detrimental, as we're not that reliant on aerodynamic effects. Intriguing all the same.....

It's true that increased weight is preferable via aerodynamic effects rather than increased mass, but because our wings are actually very inefficient, adding mass is the only practical way around it. Just my opinion.

I enjoyed reading your comments SHY, you made some interesting and valid points. I look forward to the next instalment

[Dunc]

Originally Posted by DoughtyUK.net

imagine a rippled 20 meter stretch of track.

both cars with 'nice' suspension setups on them for the cars weight

1 'barge' that weighs a lot

1 'feather' that is very light

which one would you imagine to be more 'stable' going through this section?

with regard to 'ripples' I mean small bumps that could be absorbed by suspension movement

heavy cars slowing down corner speed I completely agree with, but in off-road its not all about corners

That's exactly what I was trying to imply in some of my previous posts. I just wasn't able to think of such a nice analogy!

[bigred5765]

but in essence were talking about racing in 3d offroad not 2d onroad, were jumps and bumps have a effect on the motion of the vehicle as well,and i think the whole idea of adding the weight was just to help produce front end grip and add steering, which i think can be done in the most buy other methods,
even though we run 20grms upfront in the xxxcr, and a wing

[strobe]

What are the minimum weights for 2wd and 4wd cars to race at regional and national events.

[paulj]

One thought springs to mind here which way does the motor spin in a 2wd? I've not got one to hand here, but depending on the direction of rotation it would want to either lift the front wheels or push them into the ground I'm sure this must have been thought about in 2wd transmission design? Mind you there are forces from the gears and diff spinning in the box and the pinion/spur too any ideas?

[Col]

Paul, good point. When a 2WD car is in the air (mid-jump) and you accalerate, the front wheels lift. When thought about in context to some of the previous posts, this seems to reduce the weight on the front on the car therefore reducing steering abitily. However, I suppose this also has a major benefit in that weight on the rear wheels is increased (therefore grip?) when accelerating.

[SHY]

Much obliged Dunc! You talk about the 'side dams' on the wing: As I see it they mainly for OR do one thing: support & strengthen the 'main wing'. For a closed shell track car they do a lot for stability, we play around with this. But over and behind the wheels of an open wheel car there's a real mess... massive turbulence (look at the 'barge boards' on F1 cars to 'shield' the rest of aero stuff on the car). Unless we can have some molded carbon wings we have to use lexan with side stiffeners. I'm not saying it does nothing, but I bet we could not feel the difference from a stiff enough wing without side dams. (Just guessing! Rules also regulate this. If they were A LOT larger though). For track we used to be allowed to stiffen up the sides of the shell (not anymore). My findings were that flex was actually better as I could feel when the car was close to the limit. With a super stiff shell it 'just went'...

As I said, I will get back with more in-depth. I'm not an engineer, so I had to read up a lot to understand. My very good & very intelligent (sadly for him not very good looking ) friend Elvo and I had a very interesting discussion thread on myTSN some years ago. Luckily I kept it, so I'll make a pdf outta it and post it on the net. Before discussing aerodynamics (or more correctly: fluid dynamics) there's a lot of basics to learn first.

Still, theory can be interesting enough, but it's more fruitful to do simple on-track tests with the stopwatch. I only care about how it feels to drive and what laptimes it makes.

First of all let me tell you that track is not that different from OR. It's not 2D vs. 3D, forget that. The supspension on track cars is just as imporant, and the oil is just as black after a few runs, I can guarantee that! The asphalt tracks are far from 2D! And after doing my first 1:10 OR race I must say that the driving style is also not that different - the basic idea is the same IMHO.

Now - turbulence may very well be a more correct explanation of how our wings work, but that's not that important to know in detail. I'm used to using a lot the "Gurney" which does the same, and is very effective. What matters is that a wider, taller, and steeper wing creates more tyre load (a front wing gives more load on the front tyres).

@Chris: 'the float'... This is where I'm on shaky ground, I admit. Still I believe this is a matter of suspension settings combined with not total weight but rather the distribution of it. If they have the same shocks/springs the heavy one might float better and don't bounce around. But if the lighter car had softer shocks/springs it should handle it just as well (keep the wheels on the ground). And be faster on other parts of the track. The actual load on each tire still will be higher of course for the heavier one, which creates more traction. It will on the other hand have to push forward more mass and hence spin more. All in all it will depend on the track so it just has to be tested in each case I believe. Let's make a test and plot it in a diagram!!!

It seems to me that the cars have for a long time been a bit 'frozen' as to placement of battery cells and the motor. But now Chazz and others are finally working on that... Mr. Sideburn's ELCO seems very efficient as well! As I see it, if you go for a mid-motor 2WD and maybe at one track find it has to little rear traction - why not place 1-2 cells behind the rear axle? It's not rocket science to do that...

Mind you guys: you don't wanna end up with a fragile swiss cheese here, so we gotta find (for the believers in low weight) a way to reduce weight without reducing durability. I did my first ever 1:10 OR race last weekend and I went from 6 to 5 cells in both 2WD and 4WD. Better to drive & faster for me. Shorter runtime though, but no problem. I'm gonna try with 4 cells in 2WD as well... I'm furthermore thinking about spreading the cells more also, who says they must be one happy family always? (when I used 5 I had a foam spacer between 2 and 3, worked great). Compared to 1:1 cars our mass is kinda 'lumped together'. As to suspension setting in 2WD I used the softest AE springs and Losi #20 oil. I'd preferred even softer as it got very bumpy. I have a feeling that what's available is dialled in for 6 cells and overweight cars. When the weather is cold the viscosity is also in effect quite higher ('temperature stable' my AE).

Oh! And let me give Northy some credit for also pointing out that it's not necessarily about lowering weight per se. It can be a process of 1) lowering the total 2) putting it where more ideal. Balance is crucial for any race car. This is my goal, the problem is more how to reduce weight - these things are way too heavy and still quite slender. And to finish first, first you have to finish... So for me it's for now a question of testing all kinds of wings front & rear and playing around with 4-5-6 cells and placement.

[Dunc]

SHY, keep it coming!

I am qualified in fluid/aerodynamics, so this aspect does interest me quite a lot. I'm more out of my depth on the dynamics, i.e. suspension and geometry, side of things, so keen to learn more. With off-road it's clearly the latter that is most important in setting up a well-balanced car.

As I suggested before if the air flow separates behind an object, causing increased pressure drag, the flow will be turbulent by nature. I'm pretty sure boundary layer effects (be it laminar or turbulent) are relatively inconsequential with our buggies.

I think an important point to remember with on-road racing, particularly 1/8 IC where they run Le Mans LMP style shells, is that the whole body acts as an aerodynamic device. This combined with increased speeds brings aerodynamic affects more to the fore regarding setup.