Confused about chasis flex!
 

So I seem to be confusing myself as to what is actually needed as far a chasis flex is concerned.

I have been told that with rubber tires you want a lot of chasis flex as they have less traction than foams.

I have also been told that you want a stiff chasis on carpet as it is higher traction than asphault.

My confusion is I run on carpet with rubber tires! Do I just pick a middle setting and play from there?

How does one determine the level of traction as a particular track or is is subjective. I have no basis for comparison.

Depending on how many options you car has for flex, you will likely tune which ever end of the car is giving you issues. A lot of flex can hurt highspeed steering, to much can cause a low speed push, to much front and not enough rear can lead to oversteer, etc.
Most of the newer cars are very balanced flex wise, and you can tune by adding/removing screws on the top deck.

I am confused about chassis flex in general. Why would you not want the car to be perfectly stiff (consistent and predictable), and then rely on the shocks/springs/damping to adjust traction?

It seems counter-intuitive to me to have what could be and unpredictable 'spring' or possible inconsistency of flex, in the middle of the car to add traction - then twiddle with the other suspension bits to also adjust traction????

Thoughts on this? (vehicle engineering types - please pontificate! :) )

This idea of tuning "one end" of a chassis' flex always perplexes me.

The end of a chassis you are tuning is only flexing relative to the tyres on the ground at the opposite end of the car (the middle of the car is not attached to anything), so it's basically impossible to isolate the actual flex from one end of the car to the other.

I can see how you might get some additional effects if, say, the front end of the chassis is flexing relative to the steering mechanism; but fundamentally, a flexy chassis will generate more grip and be less responsive, and a stiffer one will generate less grip and be more responsive.

I'm of the opinion that if your car stops getting quicker as the grip level increases at a track, then the chassis is too soft. I used to run a TOP Photon, and the standard 6-cell chassis was awesome on the first run of the day (rubber carpet). By the fourth run of the day it was grip rolling. Going one grade stiffer on the top deck made the car slower in round one, but better in the finals, which is where it counts.

Here's my thoughts on chassis flex. We have driven the state of development of R/C vehicle chassis to an extremely refined level. The tuning options and resolution of them is incredible.

Stiffer was better for a long time in following with standard race car practice from full size thinking. This was especially true during the foam tire touring car era. Grip available was so high that a flexy car was pure evil if not set correctly, or at the very least off pace.

Now that rubber tire is the standard we have to factor in tire loading and heat along with setting the car for driver and circuit. Load the tires too hard, traction goes away. Not enough, car is off pace or breaks traction easily. Traction level going up over the course of an event (primarily carpet but asphalt builds in relation to weather) means you are always chasing the correct setting for optimal tire loading.

With the highly refined level of tuning now possible from most chassis, finding the correct setting can be difficult. Getting right up to that 'magic' point where the car is truly neutral the entire run is very difficult. With a stiffer chassis, driver input is also much more direct and if you're off enough on settings, a car that is good early in the run may 'go away' by the end of the run.

One other factor that complicates what we do with model racing cars is feedback. Quite simply, we do not sit in our cars. We do not even 'feel' what the car is doing. Drivers tend to use that term to describe how the car reacts to input on track but there is no physical feedback from the chassis. Without that feedback we do not know when a car is near the limit of traction. We simply react to it after our eyes see it and it's processed by our brain and hands.

Long story short, a more flexible chassis with finer tuning results in better performance. This is because it is easier to drive near the limit of traction without going too far and overloading the tire. Think of the flexible chassis as a two way filter, both for input from the driver and the racing surface.

I am running a Xray T3'11 with 17.5, stock timing, and rubber tires. I had some trouble getting around the tight 180 degree turn last week. The only scres not installed are the steering post standoffs.

I am also running a T3'10 VTA setup. and that car seemed too loose on the rear end at mid corner/corner entry. No steering standoff or rear chasis braces on that rig.

If that helps.

So you basically setup a car for the surface you will be running on and fine tune from there? I guess my reason for asking was the inital confusion on what intial setup is recommended for carpet/asphault.

Additionally, Xray has so many options for stiffness, chasis flex, hard arms, hard hubs, etc... I just can find a reference on how each part changes the car. If someone told me that with a carpet car, you need to set it up stiff. I would do it and vice versa.

If you were to ask a full scale designer about chassis flex they would say it is bad. But they have hours in simulators and sophisticated suspension designs to over come compliance problems. As racers we have very little time to absolutely dial in the ultimate set up and rather old fashioned suspension design so we can use chassis flex to help. We can be 1lb out on the spring 20 cst out on the oil 0.5mm out on the position of the shock for the ideal configuration, but chassis flex will overcome all that and get a car that we can drive around the track in the quickest possible way.

So, chassis flex is bad but it is a compromise and a way of achieving balance easily.

An interesting point is that now MotoGP are talking about bike frame flex...so maybe someone is looking at RC:)

I am also "frustrated" with chassis flex, as a former 1/1 scale person, but do agree about the true feedback and adjustability we lack when compared to full scale.

Here's another thing I am wondering about, 3* or so of rear toe. Why so much? never came close to that in full scale. Anyone have a "good" explanation, other than its possible, it has a positive enough effect, vs the negative drawbacks, so use it.

Thanks.
:)

You my friend are exactly right.

The main explanation for so much toe is this, we don't really need to worry about top speed. Very few of us run on tracks where we can even hit terminal velocity much less spend much time at it. Therefore, we run very high drag wings and lots of toe to maximize grip, corner speed and acceleration at the expense of reducing a top speed that we will never reach anyway.

Plus tire wear is much less of an issue and our power to weight ratios are much higher than most 1:1 racing.

My understanding is the chassi flex is it helps in two ways. Firstly softer chassi just like softer suspension increases grip, and a combination of a softer chassi allows for slightly stiffer suspension which provides a more responsive car with the same level of grip.

The second advantage is a softer chassi helps deal with the high frequency vibrations which the suspension cannot absorb and it transmitted into the chassi.

If you go in a modern day very stiff car with poor shocks there are a lot of high frequency vibrations which are transmitted throughout the chassi and can be felt by the passengers. This can result in less stability under load in corners. With great shocks this is much less of a problem but we don't have that in RC as of yet. This could be the next RC evolution.

Also keep in mind most bumps on RC tracks are much bigger than they are in 1/1.

Chassis flex has been apart of the RC design philosophy for as long as I can remember. Understanding how chassis flex can help is very important, but I have found that each chassis has different characteristics. In the case of a top deck and the chassis keep in mind that both of those items effect the overall ability for the chassis to flex. Flex in the chassis exists in real scale cars as well, but is exaggerated in these scale cars. It is hard to see it but a chassis that does not flex will break!

Chassis flex is used as a tuning tool and makes a very large difference in how the car reacts to many surfaces. Some cars have a very predictable amount of flex and the flex is very linear. Other cars (The ones with multiple top decks) tend to flex in a progressive and less predictable manor.

Most modern RC cars (specifically Touring cars) are design for lower bite surfaces using rubber tires. These chassis need to flex in order to roll and generate the traction required for the tires that exists in the market today.

I recently made a small change to my top deck and this resulted in a huge change in the flex of the car. I felt this change immediately and has been great. But these types of changes don't always work the way you might think. It is best to try different scenarios to find what works best for you on the surface you run on.

Keep in mind that the components attached to the chassis also effect the flex. This would include the bulkeds, motor mount, servo mounts, arm holders, and steering components. When you fiddle with these components they will effect the flex of the car as well.

Like any setup change chassis flex can go too far and start to create inefficiencies within the car. Even though you can make chassis changes in the front middle and rear this will effect the entire car.

I agree and disagree. Certainly flex is an integral part of touring cars, but flex does not gain traction. It gains you compliance. Traction is a result of a chemical reaction between the tyre and the surface. That cannot be changed by flex.

For example, 2 identical TCX's but one has a flexible top deck the other as a stiff top deck. They will both have equal traction of the start line. In the turn it is the car that maximizes the potential slip angle of the tyre that will corner faster.

Flex allows us to achieve balance quickly.

Playing about with the top decks of cars is a very inexpensive way of dialing the car to the track and your driving style and is highly recommended to all racers.

Care to show that reaction in a balanced equation? The force of resistance between tire and road is called Friction, its a physical property not a chemical one. F = ma. 4 years of uni science degree at work here, my $40K was not wasted LMAO.

And friction can be changed by flex, in both positive and negative manner. Think of it this way, a steel ball and a rubber ball, one is going to have much more rolling resistance (friction) than the other because of material flex, the same goes with chassis, if a chassis flexes under certain conditions it can generate more friction in one part of the car and less in another.

IMO its a bit of everything. The softening of rubber tire is a chemical reaction to the compound, but flex is a physical change.

Ff = µFn would be a better example then yours. µ changes as the tire gets softer, and the more the car flexes, the normal force increases on the outer tires.

How does this all work in terms of how much flex you want/need?

Don't ask me. :lol:

Could not agree more

Even that is a physical change, think of it this way, water changing from ice to steam to liquid water again is still water, its chemical structure has not been altered, its still H2O, the same is of rubber, friction causes heat which changes the physical properties of the rubber, making it more elastic, but there is no chemical change in its chemical formula, it sill remains CH2=C(CH3)CH=CH2 polymerized.

Incorrect about the normal force. The peak normal force applied to the tire is a function of weight transfer only and does not change with a more flexible chassis. Total weight transfer is only a result of the geometry of the car (WB, track, roll centers) and the location of the mass relative to this geometry. The rate of weight transfer is influenced by all sorts of things like roll stiffness, damping and chassis compliance (which feeds into roll stiffness). The reason that chassis flex makes the car easier on its tires is because you are changing the F/R roll stiffness distribution as you roll and reducing the rate at which weight is transferred.

I'm intrigued.

So if I had a car turning right, you're saying a car with more flex wouldn't distribute more weight to the outer tires(FR RL RR)?

I don't know if it works or not, but ive put conical washers under the top deck screws to increase surface area load to stiffen a chassis.

It is a very small thing to do though, maybe it would not be noticed on track.

But, because my top and bottom chassis is quite thin yet I race with foams, I thought I would try it and they have stayed on the car for 3 seasons now...

If you're turning right the outer tyres are LF and LR.

Chassis flex won't change the total weight transferred, it will just change how it is distributed between the front and rear tyres, and how fast it is transferred.

+1 A more flexible chassis gives less abrupt transfers of weight, because the flex in the chassis acts against inertia. So i guess you could say it keeps the weight on the inside wheels longer. Much like what an anti roll bar does, as load is placed on the outside wheels, and upward force is applied to the anti roll bar, due to its shape and mounting position, the upward force is converted into torsional force which applies a downward force on the inside wheel. So as a chassis flexes it some of that force pushes the inside wheels back down. You cannot escape inertia, but if you understand how it works, you can use some of its properties to your advantage.

perhaps I shouldn't make plain statements (of course friction) but in an interview with pirelli, they were explaining the "complex chemical reactions" that occur between the surface of the tyre and the tarmac. Also this article makes reference to it

(trackpedia.com/wiki/Advanced_grip)

and these are interesting readings

(insideracingtechnology.com/tirebkexerpt1.htm)

and

(brachengineering.com/publications/wheelslipmodel1.pdf)

+2 Some Kart racers undo the bolts on the chassis in damp and on long tracks to aid compliance. The only problem with our chassis flex is that at some point the chassis has to come back to equilibrium.

This article is just plain wrong and kinetics explains why there is no reaction between the rubber and the bitumen pavement. To further illustrate this point, old tire rubber is used as a viscosity modifying agent in a number of bitumen related products like rubber bitumen binder.

When you make an alloy of 2 metals you have a physical change in the crystalline arrangement of the atoms not a chemical one, bitumen and rubber interact the same way. Oh and with kinetics the ways to speed up a reaction are pressure and heat, so the chances of a reaction happening at the low temperatures and external pressures of our tires is almost non existent if there is no chemical reaction at 200 dec C.

I want my car to be more stable under throttle.. more or less flex?

:weird:

I think you've got that backwards there.

With an anti-roll bar, as the chassis rolls, an upwards force is put into the end of the swaybar on the outside of the car. The swaybar transmits that into an upward force on the arm, and eventually the wheel/tire, on the inside of the car.

-Mike

Correct, an anti-roll bar actually 'lifts' the inside wheel of your car in a turn, which reduces the amount of roll your chassis experiences.

The reason more flex makes a car easier to drive is because the front and rear of your car do not have the exact same roll centre, or the same spring rates, or dampening rates, etc. The difference between the front and rear roll rates of the car at turn in, mid corner, and on exit is constantly changing; this difference in roll is equalized through chassis flex. The rate at which these two amounts of roll are averaged out is controlled by the chassis' flex.
As the grip level of a track increases the amount of force exerted on the chassis through a corner is increased. If the chassis allows for to much flex the front and rear roll rates will be able to fluctuate too much, so the chassis needs to be stiffened for more consistent control. The opposite is also true when the grip level is decreasing.


Please see: http://users.telenet.be/elvo/9/1.html
(main page: http://users.telenet.be/elvo/ (this is a good page, pretty nice semi technical explanations of the basics of chassis dynamics))

IMHO, w/out getting too technical, the reason "flex" is engineered into rc touring cars is due to the crappy suspension. The things that pass as shocks do not react quick enough.

I've played a lot with different chassis and top decks on a few different cars and in general I've found that chassis flex is a lot like shock oil, the more flex the more grip to a point but it also works the tyres harder.

That said, recently I've been running a kyoshos tf6 and it is stiff as all hell but still generates crazy grip. Go figure, I think it might be down to the velvet shocks :D

Time to bring this thread back from the dead:D

Been about 3 years since the last post, but what's your consensus on chassis flex of TCs these days? Out of the box, is it designed more for mod than stock classes?

On our medium-high grip tight indoor carpet track we run a little harder damper oil and HPI silver springs on our TRF418 and with the center screw in on the top plate to allow for less flex.

Others run a little softer damper oil and Yokomo springs (softer) and the center screw removed, compared to us.

Guess its about driving style, but to us the softer setup with more flex provides maybe an easier car to drive, but also a lazy car, that tends to "hang" in the carpet in med-high grip situations.

The "harder" setup, on our track, provides in our hands, a car that rotates faster.

I currently run a Losi Type R touring car which is very stiff and doesn't seem to be the worse for it. Watching the car go around the track it looks largely the same as other cars. If there is one handling characteristic I would pin on the extremely hard chassis, it would be that the car has a much harder low-speed rotation than most cars where the car pivots right on the nose. This can be adjusted, of course, and I do tend to run a little more front sauce than most TC racers for a given traction level, but I like the car and would not consider the stiff chassis a handicap.

Chassis Flex now there is a subject near and dear to my heart.

What chassis flex does is allow you to change the stiffness of a spring that connects the front and rear suspension. The effect being to either soften or stiffen the overall response of the chassis and affect the distribution of the weight transfer between the front and rear. It does absolutely nothing to increase or decrease the total weight transfer from the inside to outside wheels. What it does affect is how that weight transfer is distributed between the front and rear tires. Depending on how you have your front and rear suspension setup that can be a good thing or bad. So stiffer chassis the more connected the front and rear suspension are and the softer the less… Flex also affects the overall response, stiffer the quicker the car responds, softer well you get the idea.

The biggest reason chassis flex is needed is because these cars we race are so complex most racers do not fully understand how to adjust the handling by changing springs, anti-roll bars, caster, camber, camber gain, roll centres…. There are more than enough ways to adjust the handling using what I just mentioned without throwing in another variable. In reality you can get a much better handling car by leaving the flex alone and adjusting the suspension geometry to achieve the desired result. IMHO

It would be interesting to build a car and tune it by chassis flex alone. I have a hunch that a 4wd touring car could be made into a competitive racer without touching the geometry and shocks.

I thought that another big problem is that our suspension and steering has too much slop compared with the real thing.
On the other hand, try removing all the slop, you'll end up with an undrivable car.

reminds me of the times where I went crazy trying to get all the slop out of my M03. The chassis is pretty stiff for it's size too. After all that, I came to the conclusion that the minis run best with all that slop :lol:

The key seems to be to let the chassis and shocks work together for the surface it is running on. If the chassis reaches its max flex during cornering, the load to the suspension will increase suddenly, and may go beyond the tire's grip. Too soft, and the suspension will not be able to put down the proper load on the tires at the right timing.

Inpuressa, your thoughts on the stiff TRF418 and a flexible chassis car like the ARC R10?

I drove my 418 and a few buddies at my track have the ARC which I drove too. They have different handling characteristics but both handle fantastic, just different feels. Installed the Samix on the 418 and curious how it will be.

imo, the 418 and having the samix conversion will give you the full spectrum to adjust to many situations. I should have gave the stock 418 chassis more time with the 17.5 class on high grip. The samix allowed the car to be driven hard. With faster motors 13.5 and below, the flexibility of the samix should make the car easier to drive. The samix 418 should feel similar to the ARC.

Our asphalt track is a small technical layout, but enough grip to run 1/12th. If the condition is right, we even experienced traction roll. In conditions like this, it seems that a stiffer chassis can get better lap times. That's why I am experimenting on the ARC to get the chassis a little stiffer w/o resorting to an alum chassis.