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Mahler9th's Avatar
 
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I am going to do it on the roll-off platforms I use for CB and alignment work. I will measure the ride height and attach my string set up to the car. Then I will just lower the car by turning the perches on the front.

I think I have enough range to do all of this without removing either spring.

Then I will use my jack under the front and bring the car up in increments, taking toe measurements along the way.

Then plot a graph.

Then I will use shims to make changes. Probably different thicknesses left and right to get two curves per go 'round.
I already have the shims-- very thin.

Then measure again.

Once I get the targeted curve shape, I will be done.

The key will be to establish a targeted curve shape. I won't be trying to minimize toe change.

I will gather some perspective from some now-retired racing professionals familiar with 911 set up, and target a curve shape based on that.

The part that is likely to suck is measuring the toe with the stings (actually fishing line). I will be measuring to about 1/32 of an inch. I typically set toe to 1/6 out on each side at static ride height.
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Old 04-06-2018, 01:58 PM
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Doing it by moving the spring platforms and jacking sounds easier than what I did.
Old 04-07-2018, 04:05 AM
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I think the gussets serve a purpose. There have been a couple of comments about wanting the steering arm to bend in an impact, and that the gusset prevents it. I've seen a few wheel impacts first hand on struts without the gussets, each time the strut tube kinked and buckled at the bottom of the spindle sleeve, the arm didn't deform. The arm is much thicker and stronger than the tube so I wouldn't be too concerned about bending the arm.

The purpose I see for the gusset is to reduce the deflection of the tie rod bolt. Bump steer kits replace the stock tie rod end with a relatively long thin bolt and a stack of spacers. There has to be more deflection in that configuration because the load is going through the bolt before it ever gets to the arm, and the arm is orders of magnitude stiffer than the bolt. The bolt is cantilevered and will be in bending. The gusset will minimize the deflection of the bolt by putting it in double shear. I doubt any of us are sensitive enough to feel the deflection, but it's happening. Over time the cyclic loading could cause a fatigue failure of the bolt. I have seen one of the bump steer kits fail this way, but the owner put the bolt in upside down so the bending was happening at the threaded part of the bolt which created a stress concentration where the part was bending. It's probably not a huge risk considering how many kits are out there without gussets, but when was the last time you replaced those bolts? On some kits its a custom machined part, on others (like ERP) it's a standard bolt. I like the ones with a standard bolt because it's easy and cheap to replace.
Old 04-25-2018, 08:59 AM
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Regarding bump steer, I will start a thread when I do my project later this year.

Regarding the gussets, I think it is important to consider the range of parts/mods that folks have. In my area, it is quite common to have spindles raised for low race cars running 16 inch wheels. This is perhaps less common in other areas, where 17 and even 18 inch wheels might be more typically employed. Those set ups could have the spindles raised by a much, much greater amount.

So I think that difference should be taken into account.

Regarding bump steer kits... the only ones I trust are the Eisenlohr kit (ERP; this is the one I have, purchased via SmartRacing Products/Jerry Woods Motorsports), and one that may have been designed by Jerry Woods Motorsports. I am not sure if JWM have a kit that is currently available to all.

My ERP kit was purchased over a decade ago. It came with a high grade bolts. I think they now come with aerospace bolts, and I have changed mine to an aerospace bolt.

I have been in touch with ERP about replacement parts and maintenance.

I trust my kit without gussets, but again, my spindle height is not as extreme as many I have seen.

As I always say, the answer in many cases is "it depends," and it may be very beneficial to work with local resources with pro experience on these types of things.

And again, when I refer to ERP I mean Eisenlohr Racing Products. There is another company with those initials, but while they have some great/fantastic products, they DO NOT have pro racing experience.
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Last edited by Mahler9th; 04-25-2018 at 09:21 AM..
Old 04-25-2018, 09:17 AM
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One of the more elegant solutions I've seen to this problem. Rather than bending the arm down (which changes Ackerman) or putting in a bunch of spacers and a long bolt (which has issues discussed above) this car separated the spindle and steering arm completely. Then the custom steering arm location is optimized to reduce the bumpsteer. You can still add spacers if needed but you wont need a 2" stack of them to get the geometry corrected.

Old 04-25-2018, 09:18 AM
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As you said Mike, with 16" wheels you can only raised the spindles about 30mm so it would present less challenges that cars with 17-18" wheels and higher spindles.

another form of gusset that reduces the bending of the bolt

Old 04-25-2018, 09:25 AM
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Deflection? I think you guys are seriously overestimating the forces involved in steering the car.

I've had the steering arm on one of my struts bend twice in incidents. The strut tube never kinked or buckled because the steering arm is supported by the entire diameter of the strut tube just like the spindle. In fact, the strut tube would be much more likely to kink or buckle with the gusset in place as it is welded to the tube in one small spot.
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Old 04-25-2018, 09:55 PM
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Yes, deflection. The same reason we all replace the stock tie rods with turbo tie rods, and replace all of the rubber bushings in our suspensions with bearings.

The ERP bump steer kit is designed for 20mm raised spindles and uses a small 1.5" spacer. They have sold 1000+ of them and you rarely hear about any issues, so obviously they are robust in most situations. If you are running 16" wheels you can only raise the spindles 30mm before the ball joint hits the rim of the wheel. If you are raising your spindles more than that and running longer spacers it's not a bad idea to support the bolt. The bolt is essentially a cantilevered beam: when calculating the deflection of a beam the length (length of the bolt in this case) is cubed. Increasing the length of that 1.5" bolt just a half an inch to 2" doesn't seem like much of a change, but it more than doubles the deflection. Double the length of that bolt to 3" and it will deflect 8 times more than the original.
Old 04-26-2018, 06:58 AM
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We are talking about one of these cars:

1979 SC
1986 Carrera 3.6 L+

So I doubt he has his spindles raised more than 30mm. Again, the amount of deflection here is super small since the steering has such small forces applied to the steering arms. Also, the bolt is holding the spacers in tension so it not just a bolt hanging out in the wind....

The bump steer spacer kit cannot be equated to rubber suspension bushings. The loads are orders of magnitudes different.
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Old 04-26-2018, 06:24 PM
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interesting topic, i can speak from experience with wider track rubber and maxed out caster you can exploded a alfa gtv steering box while putting around in the paddocks or turning the wheel stationary in the garage. then you be scrambling to pick up all the little ball bearings. :-) i would imagine the load are pretty high when the car is not rolling fast enough if it can split a steering box case.

some of the early rsr had bent steering arms to address bumpsteer from the factory as i recall. im not sure how they did this modification since folks have bent arms themselves and some have experience fracturing of the arms.

if the loads on the tire is up over 3000N static or steering wheel @ 400Nm then it is significant as these two research papers have suggested.

SAGE Journals: Your gateway to world-class journal research

https://pdfs.semanticscholar.org/4d26/00804309cff90a3695d39a36c4b1cf739977.pdf
Old 04-27-2018, 07:21 AM
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It's also not just the tire forces you need to think about. Ditch hooking a curb is going to load things a lot more then steady state cornering. Or spinning off track and catching the wheel on something at full lock is going to load things even further and has a good chance of breaking something.

But lets do some simple math. If we assume most people can exert 50lb on a 360mm steering wheel that works out to just shy of 30ft-lb of torque. With a standard 17.78-1 steering ratio that's 525ft-lb at the steering rack. If we assume the standard tie rod end has 0 leverage arm, that 525lbf at the tie rod end. The Rebel Racing Bump steer kit I have uses a 14mm stud. This gives a I= .00450 in^4. If we use 29,000 ksi for the Youngs Modulus of steel, a 2" spacer will result in .011" of flex in the bolt, a 3" spacer 0.036", and a 4" spacer 0.086" but it is really a lot more complicated then this as the bolts have a large spacer around them making their effective diameter roughly 18mm. This makes the I= .0123 in^4 and brings the deflection down to .004", .013", and .031" with 2", 3" or 4" spacers respectively. This still isn't the whole picture though as the steering arm will also be torquing adding additional flex to they system and this is much harder to model and my assumption of 50lb is likely on the low side of what the absolute limits of driver wheel grip could be. There will also be some torsion bar effect in the steering column and LOTS of movement in the rubber donut where the steering shaft connects to the steering rack if that hasn't been replaced with something harder.

So as with most things, the answer is it depends. Given these calculations, it would appear that a 2" spacer isn't a big deal in single shear, a 3" is marginal, and a 4" you probably don't want to drive without double shear brackets.
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Last edited by Evan Fullerton; 04-27-2018 at 12:00 PM..
Old 04-27-2018, 11:54 AM
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I cringe a little bit when I see the tie rod ends with 2" of spacer and the bolt in single shear. I think Carrol Smith has a whole chapter about this in one of his books.
Old 05-05-2018, 04:51 AM
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Quote:
Originally Posted by Matt Romanowski View Post
I cringe a little bit when I see the tie rod ends with 2" of spacer and the bolt in single shear. I think Carrol Smith has a whole chapter about this in one of his books.
yes agreed. i would warrant caution with long extended leverage on single shear on a bolt especially without the tapered fitting. there are bumpsteer kits and there are bumpsteer kits. many are just straight bolts without the proper tapered shank for the that matches the eye hole angle in the steering arm. one can imagine the movement involved by just using a standard bolt. i believe rebel has the proper tapper in their bolt seats to eliminate any play.
Old 05-07-2018, 09:56 AM
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Setting your drop link length is not terribly difficult. But it is time consuming to do it right with precision and repeatability. A decent bump steer gauge is not terribly expensive either. The Deco brand gauge at Speedway Motors has a nice instruction manual to show you how to go about it.

https://www.speedwaymotors.com/shop/bump-steer-gauges~8-15231

However, with the large stud spacing of the 911 hubs (130mm = 5.12"), you will need have to open up the mounting holes a bit if you were to use a hub plate that has 4-1/2" to 5" lug spacing
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Old 05-07-2018, 10:48 AM
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you can rig a bumpsteer guage with an L bracket, plywood and a hand drill. or just a magnet and pointer.

its massively time consuming and you can tailor to you preference on the zero setting to see which way to stagger your toe in or out.

easiest way is to just shim it up the same dimension of the amount that you have lowered your ride height to. it will be close enough since we are dealing with a road and not formula car right? :-)
Old 05-08-2018, 05:11 AM
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