wheel flex?

[ljs96076]

Hello all, I am new to this forum seeking an answer that is being debated by non-engineer-types. I searched the subject but had no success and am not familiar with this sites navigation so sorry if I have missed an easy route to find an answer.

Basically what I am trying to find out is this...
does the wheel on a car actually flex under cornering forces? Every wheel has its designed offset, but is it possible for an offset to be altered during cornering loads and then return to its designed position once cornering forces are no longer applied?

I have been saying that this does not happen on quality engineered/designed wheels. It seems to me that the spokes would crack either at the hub or at the rim of the wheel if such force was applied that could actually push the center of a wheel inwards. This isnt even considering the far less amount of force I am thinking it would take for the tire to be torn from the wheel itself in a corner, much less the tire simply losing grip mid-corner.

Am I wrong here? Does a wheel actually "flex" inward and outward during cornering? This topic came up when considering wheels with spokes that only cleared calipers by 1 or 2 millimeters, and someone thought that the wheel could flex during cornering loads and actually lose that clearance. It is my belief that this is impossible on properly designed wheels.

Does anyone know the truth on this matter?
Thanks in advance
LJ

 

[GregLocock]

Yes they do flex, real car companys do think about it. In practice for other reasons most production car wheels are stiff enough for this to be unimportant compared with flexure elsewhere in the suspension system, but in the past it has certainly been an issue.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JDana]

Stock car racers in the 50s used to cut wheel centers from extra wheels and double them up to prevent breakage.

Another question,
Does the wheel flex only at the bottom, meaning that the wheel is continually flexing/unflexing with each revolution? If so, unless the caliper is positioned at the bottom there wouldn't be a problem.

 

[NickE]

I can confirm (admittedly its anecdotal) that heavy duty Rally wheels will flex enough to cause caliper scraping when used with gravel tires on both pavement and gravel also to certain extent.

I am certain that good sticky tires and lightweight wheels would flex to a significant amount when subjected to high cornering loads. Expecially if the alignment is setup so as to keep the contact patch large during cornering.

As to how much, no clue.

The whole wheel flexes not just the direct line from the contact patch. Thats how hoops work.

Racing / High performance wheels generally dont have center caps because the flex dring competition can cause them to pop off.

 

[rmw]

Our civil engineer peers have a saying. 'If it doesn't deflect (flex) it breaks.' The degree of deflection would vary as a function of the strength and stiffness of the specific wheel.

They all (the ones that did not break) deflect to some degree. With some it might be so slight so as to be practically undetectable. With others....

rmw

 

[ljs96076]

So is the flex of a wheel on a street car, even a high performance one, significant enough to temporarily alter the wheel's offset? Or are we talking about negligible amounts with the greatest flex taking place at the tire sidewalls and suspension bushings?

Thanks again

 

[NickE]

YES, high performance wheels will flex under cornering loads. The higher the load the larger the amount of flex. The whole structure flexes. This flex can be very significant.

(Anecdote to follow
My Compomotive TH2 Rally wheels flex enough on gravel to scrape the calipers on my WRX. They have 2-3mm of static clearance. If I use a 3mm spacer then I get no scraping.

Since the loads placed on wheels are much lower when on gravel I would figure that sticky tires and lightweight highperformance wheels would increase the amount of flex.


ARGGGGG........... How many times?

 

[GregLocock]

Great, I was going to suggest 2-3mm myself.

The distortion in the tyre carcase is much greater than that so it will dominate the change in handling, if that is what is concerning you.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

IMHO, the center section of a steel wheel flexes more than the outer rim.

No one seemed to mention the fact that nearly all high performance wheels are forged or cast aluminum. These are much more rigid than steel wheels. I race off-road in the desert and absolutely no one uses steel wheels except in the bone stock classes.

 

[evelrod]

Interesting aside...this morning the news is the recall of Toyota SUV wheels. It appears they fail due to "wheel flex"!

Fabrico, Al is used because it's cheaper (custom wheels are also the 'in' thing). NASCAR mandates steel only because they are stronger (failure mode is more predictable?). I use heavier aluminum Minilites because I cannot get magnesium wheels any longer at a reasonable cost, same for steel.

Rod

 

[GregLocock]

If I were off roading in the outback I'd prefer steel wheels - they are better at taking impacts, and if push comes to shove you can hammer the dent out.

Designing an aluminium wheel to take the usual sort of damage from square edge pot holes requires a lot of stuff (grin) that does not get put into aftermarket wheels, typically.

Even when you have an aluminium wheel that does not crack under those conditions it will be excedingly difficult to straighten it up again.

However, once you go over (say) 17" wheel diameter a cheap stamped steel wheel becomes unfeasibly heavy, hence the short lived popularity of alloy centres with steel rims.

There's no question on the stiffness issue - once you have an aluminium 16" wheel that has an acceptable life for a production car it will be stiffer than the stamped steel wheel. It will also weigh roughly as much, or in small sizes, more.

Race wheels are different, they do not have to have fatigue lives of the order of 50 million cycles.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

Re: "Al is used because it's cheaper (custom wheels are also the 'in' thing). NASCAR mandates steel only because they are stronger (failure mode is more predictable?). I use heavier aluminum Minilites because I cannot get magnesium wheels any longer at a reasonable cost, same for steel.
Rod"

This could not be farther from reality. Cast aluminum wheels are NOT chaeper than plain steel wheels. Nor have they been considered custom for the last 25 years.

Nascar is almost as low-tech as racing gets. Basing an overall view of anything on their standards is nothing short of silly.

Cast and forged wheels are much more rigid, strong, and reliable than plain steel wheels.

Try looking at ANY steel wheel on ANY construction or ANY military vehicle. They are of completely different design and much, much heavier than a standard steel wheel.

In case you did not notice nearly all motorcycles, cars, trucks, big rigs, and even cement trucks, come with aluminum wheels. And it's NOT because they look cool or they are cheaper.

A standard steel wheel bends like butter off-road.

You don't need to believe me, just take a good look around.

 

[RobWard]

2 observations (from a casual observer):

1) In an extreme environment (i.e. off road)if my life depended on it I'd go with steel. It's more ductile and a failure isn't as likely to be catastrophic. (As Mr Locock said)

2) I was very surprised when I picked up my brother's alloy wheel off his Westfield. It weighed a LOT more than the steel spare, which to my mind was a bit daft(actually, I think Mr Locock said that as well). Maybe it was more rigid, but I thought that reducing the unsprung mass was the important thing. My brother wanted the look more than the performance anyway, so it wasn't an issue. Hell, at least he didn't fit spinners.

Then again, what do I know?! Fortunately the people who design the cars I drive know more than I ever will.


(PS apologies if this is a rambling reply, but the wife is watching something about Trinny and Suzanna, and I've been reaching for the rum...)

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[GregLocock]

"Cast and forged wheels are much more rigid, strong, and reliable than plain steel wheels."

No they aren't more reliable, and they crack, whereas steel wheels bend. Hey I only used to design and test them for a living, you obviously know more than me.

"Try looking at ANY steel wheel on ANY construction or ANY military vehicle. They are of completely different design and much, much heavier than a standard steel wheel."

Um yes, if you maker them much heavier then they can be stronger. I doubt anyone would argue.

"In case you did not notice nearly all motorcycles, cars, trucks, big rigs, and even cement trucks, come with aluminum wheels. And it's NOT because they look cool or they are cheaper."

In Australia we generally use steel wheels except for bikes and cars.

"A standard steel wheel bends like butter off-road."

Somewhat true, and a standard alloy wheel breaks. An offroad steel wheel is fine.

In the Australian Outback the standard bushbashing vehicle is a Toyota Troopcarrier. I won't go so far as to say that they never run alloys, but if you look at this page all but one are on steels.

http://images.google.com.au/images?...-a&rls=org.mozilla:en-US:official&sa=N&tab=wi

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

The flex and weight has a lot to do with design and construction technique as well as material.

Pressing, the technique normally used to make steel wheels is limited re depth of ribs, so flex control is limited, but pressed steel it tough and unlikely to break. On a poor design, fatigue cracks around the bolt holes can occur. I know because I have cracked OEM steel wheels.

Casting, the technique normally used in OEM aluminium wheels gives a lot more scope re rigidity of the design, but is more brittle. They are more prone to breakage from impact. I know bwcause I have broken aftermarket aluminium wheels from pot hole impacts.

Weight depends on a number of factors, not the least of which is size.

Racing wheels are another story as the environment is much more controlled and cost is not such an issue.

By the way, Fabrico obviously doesn't know the eng-tips addage, RIR.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Gee, guys...I guess that leaves me with...

"Nascar is almost as low-tech as racing gets. Basing an overall view of anything on their standards is nothing short of silly."

Well, I guess that's one point of view...you will get one heck of an argument from most folks, though. NASCAR uses steel wheels because of MANY failures with earlier trys at alloy wheels, cast or forged. I have only broken one steel wheel and that was my fault (recycled an old wheel by accident) BUT I have a STACK of Al wheels that are cracked and in some cases broken...but hey, what do I know, I only race the things. I still use aluminum in spite of it's shortcomings in my class of racing simply because it is all that is commercially available to me at a price I can afford.
Oh yeah, for the F1 fanatics among us, they don't us Al either...to heavy.

"This could not be farther from reality. Cast aluminum wheels are NOT chaeper than plain steel wheels. Nor have they been considered custom for the last 25 years."

I don't know about that, either. Market demands cause a lot of funny phenoms...Fads are fads, even 25 year old ones. It is not cheaper to run off a bunch of steel wheels and add on a silly plastic hubcap that no one will buy...market driven engineering?
For that matter, my first " street wheels" were a set of Keystones on a Hypo Ford in 1964! My first real mags were a set of American magnesium things that cost a fortune...they really did look cool. Sure wish I could find a set of Minilite magnesium wheels that are not all crusted up and useless. Oh well.

Rod

 

[ljs96076]

Interesting stuff guys, thanks a bunch.

But the motion of flex I am looking to find out about is relative to the offset of the wheels. Is it possible/common/likely that lateral forces can be exerted on a sporstcar/passenger car cast or forged wheel to the point that the actual center portion moves further into, or further out of, the outer rim of the wheel a significant amount (more than 1-2mm). My belief is that a tire will lose grip (approximately at 0.9 to 1.0 g's) before the wheel will ever have enough force applied to it to alter its offset with cornering forces applied. I believe the amount of deflection incurred prior to reaching tire-grip-breeching force would be absorbed in the suspension bushings as well as springs and shocks as the weight of the car transfers down onto them.

I see NickE claims his rally car suffers this flex causing the rotors to be scraped by the spokes, but I am not convinced that the wheel isnt flexing at the wheel studs, cousing motion in the plane of the camber of the wheel during cornering, so while I do not disregard his input, I simply would like more insight on this matter from a design perspective.

Sorry to repost, just wanted to try to keep the discussion focused on the specific type of flex motion that concerns me.

Thanks again

 

[GregLocock]

You'll actually get a camber-like change rather than an offset-like change, ie the bottom of the wheel will tilt one way, the top the other.

As we've said, 2 or 3 mm is quite believable.

Since you seem to want us to do the work, why don't you get a steel rim, and a dial indicator, and a hydraulic press and apply the corner weight to the rim?





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ljs96076]

I'm not so concerned with steel wheels as much as I am looking into this idea for cast/forged alloy wheels. Certainly if I had the means or equipment to actually test a forged or cast alloy wheel, I would. But I do not and thought I had as good a chance to find someone with some experience in wheel design here as anywhere else. Thanks for everyone's input.

 

[1450racing]

They certainly flex to some degree. I took my road car to the track about 10 years ago, and fitted it with fairly wide slick tires and lightweight spun aluminum Keizer wheels. I had approximately .090" of clearance between my front calipers and the wheels themselves. During one particular corner on the track I'd hear a horrible rubbing noise, so I returned to the paddock and after inspection I found the wheels deflected enough to rub the calipers and smear the aluminum somewhat. Sitting still there was still clearance. With a forged or cast wheel it probably would not have been an issue, and I suspect that with a less grippy tire they would not have rubbed either.

Bob