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Spindle flex under cornering....beware!
Posted: Fri Jan 12, 2007 7:27 am
This is for anyone who wants to build a big-brake set and takes track driving seriously. I thought a drawing would be more eloquent in describing the problem. After 70-odd hard laps with my new brakes, I noticed two gouge lines about the middle of the inside surface of both discs, the left one more pronounced (more right turns). The corresponding upright areas were also marked. Dimension A in my car is 4mm!! You understand this means the spindles flex quite a lot when heavily loaded-and I have -2.5* camber!
Moral of story: design your set with about 6mm of clearance! Flex depends on weight of car, tire adhesion and cornering speed.
Posted: Fri Jan 12, 2007 9:50 am
Ive done discs as close as 2mm clearance on race cars here and the spindles dont flex..We killed Kevins car at Kyalami 3 weeks ago and no touch...
I tell you what,if a spindle bends or flexes this much,ITS GOING TO BREAK!!..Im sure of it!
You sure its not a caliper touching and qauging the disc?
Are there witness marks on the upright??
Damn,cant find those close up pics now.........Both these cars heve the big brakes..the 156 even bigger that the gtv...From the wheels you can see I had very little space for the calipers(you recognise them?)cause of the split rim bolts-so the discs were pushed far back almost against the spindles..Ron has also thrashed this car heer,,no touch..
Posted: Fri Jan 12, 2007 9:53 am
Jim,Thinking bout this...your tire is going to take all the flex...trhe spindle is staying right where it is,no flex...MICKE?????????
Posted: Fri Jan 12, 2007 9:54 am
Do you have the famous floating disc setup??
Posted: Fri Jan 12, 2007 11:13 am
Oooohhh....how I'd like you to be right! I took the wheel out, the pads and then the caliper (no sweat, two 19mm bolts) no obstructions anywhere unfortunately. It IS the uprights, they're scuffed right next to the disc 'grooves'. Well, 1400kg take their toll. Now you know why certain models had 31mm (or is it 33?)spindles instead of 29! I will do a 1:1 scale drawing with exact measurements to see exactly how much this flex is; I'm really curious. Thankfully the spindle is forged steel but I don't know how much of this it can take! If we look at it as a system, we have other items too, subject to elastic distortion (not permanent): the bearing and the hub. Also, the bigger the wheel, the higher the exerted torque is on the system during cornering.
All I can say is this was veeeery disturbing!
Posted: Fri Jan 12, 2007 11:28 am
I actually was afraid of this and I think we discussed it with Jim last year. There is a guy in Sweden (not Mz - the other one
) who noticed the same.
After careful and hurting thinkabout it can only be the spindle, shaft or the bearing. The spindle is the only reasonable answer I think.
Someone do a FEM analysis of the part please. I'm really curious even if it's not likely to be a problem for me running street tires on my 900 kg beast and additionally a shortened Spindle (= less flex)
BTW! More clearance is definitely NOT the answer. Just think about your carefully set camber.
Posted: Fri Jan 12, 2007 11:48 am
Hi Micke, camber is 2.5* neg, polyurethane 100% everywhere, full RSR set, 8-pt rollcage, the chassis is spot welded all over the place and the thing corners as flat as can be considering the weight (from various pics people take in track events) And no, I haven't used the in-car recorder yet for g's and stuff (g-tech proRR) I guess the time has come.
You and Mats are ME's here, you do the FEM (I'm an EE so its not my bag!)
I can't see the thick black vertical part of the upright flexing significantly or at all (although there's no such thing as zero flex in these things) Can't do much more about the camber either at this point; this phenomenon should not be happening at all if things were properly sized (= thick spindles!) I judge camber by tire temps and wear - doesn't everybody?
Posted: Fri Jan 12, 2007 2:14 pm
Just to rule out stuff, how do you set your front bearings Jim?
I have heard about spindle flex on our cars from a guy who is racing seriously, and from a lot of other people that they are totally solid.
I'd like to settle this as well, this is bad.
I have VERY tight clearence between the caliper and rim and have had no problems so far.
Posted: Fri Jan 12, 2007 10:22 pm
I have had a steering arm off the upright break off on me as I was braking to a stop on a GTV6..Luckily I was only doing about 10km/h at the time.
This really is concerning here Jim..Ive never believed there might be a problem with this...as you see,Im must say Im still a bit sceptical....
You running them dang fangled floating hats thingy`s??That could explain touching if you are..But I'm almost sure your running tight up discs to the bells..
The ts racer with 400 wheel hp is running 10" wide Good Year slicks,Brembo 350mm discs and BIG brembo calipers...We have never seen signs of upright flex on this either...
I believe you might get flex in the upright ,not in the actual spindle though...It lies in the wrong plane??
Posted: Fri Jan 12, 2007 11:35 pm
Mats, I set the bearings the old fashioned way, not as AR says. I tighten a bit, use the big brass hammer to settle while turning....tighten until resistance is felt, loosen to where the pin goes and thats it. The result of this is no perceptible free play when grabbing the wheel from top+bottom and trying to wiggle, even after 1000km (I habitually check for this every time the car is raised, as I feel the bearings are working very hard with all the weight and neg.camber)I then make it a point to change bearings every 1.5 years or so...since I've had one melt and snap the end of the spindle off (they welded together) Fortunately, this happened in slow city traffic at a stoplight...after a crazy track summer weekend a few years ago, the wheel was held by the caliper...)
Barry, the reason I say spindle flex is I can't imagine the huge upright flexing itself although at this point nobody can rule anything out. I don't have floating discs, I ain't rich yet!
Another thing I have to account for is temperature effects. I have measured 120*C on the wheel between the studs. The alloy bell was off-scale for my laser temp gun (Over 250*C), forget the discs! The calipers (alloy) are also mounted on hefty alloy brackets....lots of variables there! Going sci-fi here, I'd love to see this system operating on a big lathe...that would tell a lot about temp effects but no flex info.
Well, glad the euro/dollar is favorable for us, have to order a new set of Wilwood HD's, damn! Anyone can suggest a low-price source that does export?
Posted: Fri Jan 12, 2007 11:54 pm
Well I've seen spindles snap off before, and the reason was the same as Jim's, bad bearings I believe.
Now about the flex, kind of worrying, but haven't seen the problem myself even when I had wilwoods.
I go to some tracks which have some banked high speed turns, like maybe 160km/h turns, ok so maybe not THAT fast.
But 4mm due to flex alone seems very high for something which looks pretty sturdy. Perhaps it has to do with the expansion as well, as you said? That would make more sense. So basically you have a few components to the equation.
BTW, isn't there an run-out (wobble) tolerance allowed for the whole disc/bearing assembly?
My guess is, this is not just a case of spindle flex alone.
Posted: Sat Jan 13, 2007 1:19 am
Jim,With you there on the snapped spindle end...180km/h,Porsche 944 turbo cup racer,felt something strange and tapped off.It broke the caliper mount at 160km/h and the wheel went under the car..eventually stopped with the wheel,what was left of it,still under the car....bad body damage though...
Zman,You would need a lot of material at high temps to distort even 1 mm at the disc..I don't think heat is the major player distortion wise here...
For removing finger prints at the source,yea,definitely..
So,Whats the conclusion here...??Im in for little to no distorsion suspension wise....
Posted: Sat Jan 13, 2007 1:42 am
You right maaan. Linear expansion assuming steel (alpha at 13 X 10^-6) would require pretty huge delta T for even 0.001m change in length.
Posted: Sat Jan 13, 2007 2:37 am
1) someone do a FEM (on the DeDion as well). I'm not allowed. I promised my professor never to do any FEM to get through university (as U can guess this was NOT my strong side)
2) simple test. Jack up the car. Put on two rims without tires. Pull the lower side of the rims together with suitable tool. Measure force and deflection. Post here. Same in the rear except easier with DeDion off the car.
Posted: Sat Jan 13, 2007 11:28 am
Common guys good old fashion math and mechanics will do here, FEM could be possible, but no time. Job is killing me >55 hour work load/week. And FEM is for pushing project leaders over the edge to get my wright
Gives nice pictures but is very time consuming and very misleading, when not done properly.
Your bearing adjustment is done the same way by me, had a wheel bearing freeze up in Aremberg corner on a warm-up round in the first year on the ring. Since then I set the bearings pretty loose and change every 2 years.
Back on topic:
Flex depends on weight of car, tire adhesion and cornering speed.
You also forgot outside wheel diameter and ET of your rims. Center of gravity, roll center and distance between the wheels (l to r)
I'm not so sure if it does matter how much static camber you set. The highest force will be present during cornering and if the camber is set right you will use you complete tire width. And then the load could be assumed somewhere in the middle of the tire...... I run 4 deg!
As a system three things will happen:
1) your bearing axle will bend.
2) your upright will deform into an S-shape. (torque implied by the bending axle)
3) bearing deformation, which I think is neglect able.
Jim. do a good detailed drawing with sizes and we will do a fast sum on the matter. I still think 4 mm is much, but that is a gut feeling