Visit The Dirt Forum for More Information

Author Topic:   Flex?
CLBaker25
Dirt Freak

Total posts: 224
posted December 21, 2005 06:34 AM  
I just want to find out if I am thinking of this correctly. The advantage to a car flexing would be the front and rear of the car could be unbalanced and have the overall setup and handling not be nearly as affected. This would make the car better through the corner. Also with a 4 bar car you do not need as much forward bite because the car naturally has a ton of forward bite with the 4 bar suspension.

The main draw back to flex I would think would be less forward bite created by less push coming from the front springs on corner exit. Also I would think there would be an element of inconsistency with a flexible car.

Is this right or wrong thinking?

wfoondirt
Dirt Forum Champ
Total posts: 539
posted December 21, 2005 07:46 AM  
Overall I think you have the right ideas.

A chassis with alot of flex will be more forgiving to track changes and to driver skill. It will also be less reactive to setup changes. The downside would be maintaining long term consistancy a car that is built to flex will begin to flex more over the long term. A rigid car OTOH will be more sensitive to setup changes and driver inputs and will typically last longer.

The biggest reason for dirt cars typically being more flexible than a comparable asphalt car is that the track can change in a relativly short amount of time on dirt and the races are usually fairly short. Thus the need for a larger "sweet spot" on setups.

Kromulous
Dirt Forum Champ
Total posts: 872
posted December 21, 2005 09:53 AM  
I've been thinking about flex in chassis for awhile now, and in Sprint cars its a widley used idea as well. After talking to some Karting people i found the same thing, but they tune with it, via seats and seat posts and axles. More flex, more grip.

One phenomenom i believe is over looked is leverage, if the entire chassis is flexing over on the right front, when it reaches a point where the suspension doesnt travel anymore or the spring rate gets stronger than the flex in the chassis, the chassis flex comes into play.

When this happens i think the chassis flex allows the weight to continue to move over the corner of the car, making a pinning action. It allows the CG to continue moving up and over the corner of the car.

When your cornering, centrifical force is trying throw the car off the track, if the tires can resist the force, the chassis will roll over, the weight is trying to basicly pole vault over the right side tires, using the CG is the pole.

I believe the chassis flex allows the weight to move more without getting to much suspension travel.

another reason for short panhard bars, and front RC's over towards the RF, to create lift after so much travel, while still getting enough body roll to create more of a pinning the tires into the track phenomenom.

I could be over thinking this, i would like to see others view points on this as well.

Krom.

wfoondirt
Dirt Forum Champ
Total posts: 539
posted December 21, 2005 10:24 AM  
First off karts have no suspension so the direct comparision can't really be made. Sprint cars are mostly concerned about weight so I think the fact that they do alot with flex comes as a result of making the chassis light and dealing with the inherant flex.

Basically chassis flex is a big undampened spring. So realistically you can accomplish the same basic effects with the suspension.

A chassis with alot of flex will also be sluggish to driver inputs, some drivers like it some don't. As mentioned above it can also make the chassis insensitive to suspension changes. This is due to the fact that a change in suspension can also change the load paths in the chassis and how it reacts to them (how the chassis will flex). This is a sure fire way to determine if a chassis is "flexed-out."

Another downside to alot of flex is that it can change the geometry of the suspension as it flexes.

All this leads to the use of alot of crutches and can be unpredictable. Its pretty hard to quantify how a chassis will flex and react without alot of track time.

Over the years chassis builders have experimented with extremely flexable cars (floating x-type chassis) and extremely rigid cars (Grt's comp car) and neither saw wide sucess. I think the answer is somewhere in between for a dirt car. With a good driver a rigid car will be faster than a flexable car with the perfect setup for conditions. The problem on dirt lies in the fact that the track can charge rather quickly so its harder to have the perfect setup the entire race.

<psuedo highjack>
Why do you automatically assume that more body roll equates to more "piining the tires to the track"?

PEwaste
Dirt Forum Racer

Total posts: 76
posted December 21, 2005 11:07 AM  
Actually WFO, I'll have to dis-agree with this statement
"First off karts have no suspension so the direct comparision can't really be made. Sprint cars are mostly concerned about weight so I think the fact that they do alot with flex comes as a result of making the chassis light and dealing with the inherant flex."

A kart, even with no suspention still can be compared to a modified or latemodel chassis. When you "soften" a car up to flex, what you are doing is letting the chassis work more and obsorb more force. This allows the suspention to work a little better. Flex also allows a car to have a bigger sweet spot, so that if you miss the setup a little you still are decent. If you take a look at some of the top modified chassis you will notice that they are very soft on the front end, build wise. Thin tubing, not a lot of stuff up front. A top modified chassis builder reccomends that you scale his chassis after every 2 races because they flex so much. You can also get a chassis that is too rigid and won't flex and work. That is why the square tube cars are going by the wayside.

Kromulous
Dirt Forum Champ
Total posts: 872
posted December 21, 2005 11:57 AM  
Well i dont necessarily say more roll, more pinning action, but if the RC's and the moment arms are set up for it, it can.

If the CG moves up and over to the right enough, it should create more of the pinning action.

Like i said above my thinking may be off on this, i'm not sure, just kind looking for the answers myself. Seems like flexable Mod chassis is where its at these days.

The sweet spot theory is a good one as well, i know a Sprint car guy that has an old sckool Crew chief and he can not stand the new flexable cars because the changes dont translate enough of a change to the handling.

Krom.

CLBaker25
Dirt Freak

Total posts: 224
posted December 21, 2005 12:16 PM  
I guess that I was more thinking about the advantage in an unbalanced setup (standard drit track car) where the front end and the back of the car want to roll at different angles chassis flex would allow each end to roll a different amount.

wfoondirt
Dirt Forum Champ
Total posts: 539
posted December 21, 2005 12:44 PM  
PE, did you read my whole post? You pretty much regurgitated what I said. What I meant by the car and sprint references is that they don't make the cars flex because it's nesessarily better but for other reasons. Karts because they have no other suspension and sprints for weight.

Krom, I guess I'm not really understanding what you triing to get at with the pinning reference.

I aggree that most builders are build pretty flexible cars these days. Where I disaggree is the reason behind it. I see two reasons for it. Number one is for weight and rear percentage, dropping weight in the front improves both. Number two is too make the cars have a bigger sweet spot for a wide range of tracks/drivers. You have to remember that chassis builders have to sell cars, and the average racer isn't really sharp on setup. Most average racers put the sheet setup on and drive it. If a builder built a car that required alot of chassis knowledge and a good driver to be able to run good they wouldn't sell alot of cars. So instead they don't build the best car they can they build a car that will work ok for most drivers and most tracks. See where I'm going with this?

CLBaker, What you described is why a flexable chassis has a bigger sweet spot.

[This message has been edited by wfoondirt (edited December 21, 2005).]

PEwaste
Dirt Forum Racer

Total posts: 76
posted December 21, 2005 01:02 PM  
Your wrong on the karts deal.

Karts in the mid 90's were built with 1 1/8 tubing, thick walled. Now they are building them with 1 1/4 tubing, even 1 3/8 and thick walled tubing, with different characteristics to create flex. A race car that flexes will be easier to set up and hard to miss on.

The chassis I refer to as a Modified doesn't seem to have a problem winning.

Kromulous
Dirt Forum Champ
Total posts: 872
posted December 21, 2005 01:05 PM  
I'm with ya, CLbaker's post hit on it pretty good. I think were going in the same direction, just taking different rods to get there. I tend to have a odd way of thinking about things, its scary in there LOL.

I follow you as well Wfoondirt, i believe your spot on with the "House set up" deal, and the sweet spot theory.

Our car i notice, which is really stiff, is real fickle on what it likes and when.

Krom.


Kromulous
Dirt Forum Champ
Total posts: 872
posted December 21, 2005 01:07 PM  
quote:
Originally posted by PEwaste:
Your wrong on the karts deal.

Karts in the mid 90's were built with 1 1/8 tubing, thick walled. Now they are building them with 1 1/4 tubing, even 1 3/8 and thick walled tubing, with different characteristics to create flex. A race car that flexes will be easier to set up and hard to miss on.

The chassis I refer to as a Modified doesn't seem to have a problem winning.


We had a Kart in the late 90's made by Bandit that was made with thin wall tube on the right side for more flex. Especially made for the dirt track down there in Daytona, thing was always tight seemed like.

Krom.


wfoondirt
Dirt Forum Champ
Total posts: 539
posted December 21, 2005 01:26 PM  
quote:
Originally posted by PEwaste:
A race car that flexes will be easier to set up and hard to miss on.

The chassis I refer to as a Modified doesn't seem to have a problem winning.


This is pretty much the whole point of what I'm triing to explain. I never said a car with alot of flex can't win races. Thats the whole point of a chassis builder building them that way. So more drivers can win on more tracks with the same setup. But a more rigid car with a perfect setup for conditions and a good driver will be faster than one that flexes alot. But on dirt that is more difficult to accomplish than on asphalt. Thus the reason dirt cars are typically more flexible than asphalt cars.


spock
Dirt Roller

Total posts: 5
posted December 21, 2005 03:47 PM  
The idea that karts do not have a suspension is not entirely correct. The chassis itself is a spring. Many different materials are used to make chassis from, with different tubing even on the same chassis. The main reason for a change in chassis design is a change in tyre technology. As tyres improve, the chassis needs to improve to keep up. Also the power levels of the motors has been the minor reason for changes. A good 100cc ICA motor will put out 25+ hp. and turn over 19,000 rpm. Generally, the stiffer you make it, the more grip you have. Softer you make it, the less grip. Rear axles are a major tuning tool on a kart. Different materials, diameters, even hub length are are used as tuning devices. On rubbered up tracks, you go to soft axles and short hubs to free up the chassis. Many times the hub will extend out past the end of the axle. Softer chassis have a shorter life span and are genrally easier to tune. A wider sweet spot if you will. Some of the soft modifieds out there will have a short useful life on rough tacky tracks, as they are constantly in a state of motion. After 20 races or so, you'll need a welder handy. Stiffer cars may not be as fast, but will have a much longer life span. Any car will work in a certain set of conditions. Getting a car to work in a wider range of conditions and be consistant would seem to be a better chassis to work with than one that won last week, and finished 10th this week just because the track changed a little.

20crew
Dirt Full Roller

Total posts: 51
posted December 21, 2005 04:28 PM  
I seen an interview of a east coast mod chassis builder not to long ago he said they were testing a chassis with flexible joints in it (certain spots ) to help the car flex.

Ego Racing
Dirt Forum Champ
Total posts: 771
posted December 21, 2005 06:14 PM  
Spock is correct, the cart chassis is desighned as an adjustible spring and the driver is the damper or shock. The ONLY reason they are built that way is because they have NO suspension on the cart in the way of STD shocks and springs.
Every chassis should have some flex, to much and the car will get inconsistant and tubing will start breaking for being worked, to little and when in an accident the driver takes to much impact. The last was the case with NASCAR when the drivers were getting hurt every wreck, the cars were so stiff they were transmitting the impact to the drivers. If it is to soft the car could colapse in a wreck. It will also wear out quicker.
As for a chassis with joints in it, they have tried it with late models for several years, GRT and TNT both had greaseable joints in the chassis at one time and both have moved away from it because there was no way to predict or control when the joints moved or how far they moved.

Dman
Dirt Freak

Total posts: 280
posted December 22, 2005 07:47 AM  
In this whole discussion no one has mentioned the fact that sprints and carts are built of chromoly and 99 percent of mods are built of mild steel and not all are DOM.
The flexibility and fatigue are totally un-predictable as the quality control of this material is marginal. DOM tubing is of coarse better but will not flex consistantly and fatigue more with each "flex".These cars are all welded which creates more fatigue and brittle issues as these cars are not Consistantly welded (not by machine). So there are too many inconsistancies that come into play here to support the flex theory. A spring or shock change is more beneficial,consistant or effective on a rigid chassis.

cozmo
Dirt Maniac

Total posts: 110
posted December 22, 2005 08:02 AM  
Our Northeast mods are built with a lot of flex in the chassi and when they are new they are mad fast. When they age your changes to set up get drastic till the car does not respond to any changes no mater what you throw at it. A chassi lasts for about one season racing once a week. We had a worn out chassi that would run good for about 8 laps then fall way off. Got a new chassi had no problems.

coz

CLBaker25
Dirt Freak

Total posts: 224
posted December 23, 2005 06:43 AM  
How would you build a mod frame to flex? Where would you have it flex at? Would you build a solid rear half of the car and let the front flex? Or would you let the whole car flex?

Thanks for the info!

Kromulous
Dirt Forum Champ
Total posts: 872
posted December 23, 2005 07:09 AM  
From the way some look nowdays, the rear clip, and from the fire wall forward are made to flex.

The idea of running unbalanced set-ups front to rear, and allowing the frame to flex so both can do it is a good one. Never thought about it but that makes total since.

Krom.

Dman
Dirt Freak

Total posts: 280
posted December 23, 2005 07:10 AM  
To answer yor question most mods tend to flex/twist most from the dash bar forward as there isn't as much tubing/triangulation up front. So you can figure an early chevelle clip will flex at that point more than a 72 up chassis (metrics included).Also age and rust come into factor here.This could explain why that 2003 Dirtworks felt better than a 2005 (Dirtworks guys don't get tweeked here.It is just an example).

dirtbuster
Dirt Forum Champ
Total posts: 2039
posted December 23, 2005 07:13 AM  
We had a DW9 chassis which seems to be notorious for being stiffer than most cars. And more so than the DW8 for sure. We ran this car for 2 years and I can tell you it was the most consistent car on the scales. Every week we would set it on the scales it would be within 5# of the week before, our old DW8 was never like that, it changed almost weekly. Now on the track was a different story. The 8 was very forgiving on the track, and consistent. The 9 you had to stay on top of it. When you hit the setup right it was a rocketship, but if you were off it was a handful, most of the time we would hit it right for one end of the track but be off a little on the other end and so overall we would end with an average finish. Thatís where the flex comes in, it may not be the absolutley fastest but it allows the chassis to work over a wider range of conditions. Condidtions which you cant adjust for once the race starts.

I would try to make it flex through the whole car. To make it flex you would need to take out as much of the triangulation as you can avoiding the cage around the driver obviously. If you look at Shaw cars (I think) they have hardly any if any at all triangulated bars along the right side. I have heard these cars have a tendency to bow the rights side frame rails because they flex so much.

washeduptoo
Dirt Maniac

Total posts: 181
posted December 23, 2005 07:49 AM  
When you are talking about trianglated bars are you talking about the bars run down vertical down to the frame rail or what bars are you talking about? Sorry to ask a question that I should know, but I don't. Thanks if you can help me understand what yall are talking about.

CLBaker25
Dirt Freak

Total posts: 224
posted December 23, 2005 07:50 AM  
To create the flex in the front end would you use moly or DOM and put in less bars or go with electric weld tubing which flexes naturally?

dirtbuster
Dirt Forum Champ
Total posts: 2039
posted December 23, 2005 07:55 AM  
washeduptoo
if you look at the right side of the chassis you have horizontal bars and vertical bars then usually some angled bars from top to bottom. that is what i mean by triangulated, anything that runs at an angle will try to stiffen the chassis more than just a vertical/horizontal bar. You cant take them all out, you need some of them for strenght especially around the driver to protect him.


Kromulous
Dirt Forum Champ
Total posts: 872
posted December 23, 2005 08:17 AM  
Chrome moly has more memory, it will flex and then try to return to its natural resting point.

Its a good and bad deal, if you weld chrome moly and if its under a load after its welded it will try to find its resting point, so it creates a stress from the get go. It can be manipulated to help make things do certain things as well. So thats not totally bad.

DOM does not have the same level of memory, it will flex and only return if other forces wants it to. It does have some memory just not alot.


Krom.

Dman
Dirt Freak

Total posts: 280
posted December 23, 2005 08:50 AM  
Chromolly requires tig welding, tight fits and normalizing to be its best.You can mig it but it will crack around the welds with age (and flex)sooner than mild steel. It is tempermental to heat from the welding process. Chromolly is what sprinters, dragsters and some other race cars (we don't talk about here) are built with where weight is a major consideration and cost doesn't matter.But even chromolly loses its temper and fatigues eventually, just not as quickly as mild steel.Chromolly is harder to straighten after its is bent.I have seen more chromolly snap than bend. Just remember,
The more a chassis flexes the sooner it becomes junk and you end up chasing your tail to set up the car.

dirtbuster
Dirt Forum Champ
Total posts: 2039
posted December 24, 2005 07:26 PM  
i think that is why alot of the Skyrocket guys get a new car every year. Which is also why there is a waiting list to get a new one unless you already have one. Not that they wont keep working after a season but might not be as good as they were originally.

Back to the Archives