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Bump steer spacer vs moving the control arm pivot point


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Forgive me if I use the wrong terminology here.  I also know this question will be a little "vague" as far as suspension design and geometry goes but here it is...

 

So I have seen a lot of people using bump steer spacers to change the angle of the lower control arm on lowered vehicles.  

 

I have also seen a few setups where the control arm pivot point has been moved up to achieve what seems to be the same effect.

 

My questions:

 

Which way would be recommended?  

 

Are the bump steer spacers just easier to install OR is it actually better than changing the location of the control arm pivot point?

 

If changing the vertical location of the lower control arm pivot point would there be any benefit to moving it inwards or outwards to get better geometry relative to  the stock steering rack?

 

I know there is a LOT more to the front suspension geometry than this but I figured this was a good place to start.

 

If anybody knows of a good forum thread, website or book to help me really study up on this type of stuff that would be great!

 

Thanks in advance for the helpful responses!

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Then measure and input.  I have WinGeo3 from Mitchell and spent 40 hours measuring and entering the S30 dimensions.  Its not a trivial thing if you want any kind of accuracy.

That was my plan for the weekend!  Do you have any pictures/drawings with dimensions that would speed up the process for me?

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I can offer some tips - accuracy matters - measure to the 32nd of an inch or .0325 if possible.  Measure from or two a known flat surface.  Use plumb bobs and lasers (if you have them).  Cut the heads off bolts (M8, M10, M12, M14, etc.) and mill one end to an exact center point.  Put these bolts into whatever you're measuring to give the center of the mounting hole. Check the straightness of parts before measuring to make sure you're not measuring a bent part (that one set me back a day).  Clean everything.  Remove the springs and sway bars and reassemble the suspension.

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Knowledge is power. And Clark is right about the FSM giving you the data needed to plot the chassis. Now that I'm working on vintage Porsches, the shop manual gives all the chassis and suspension measurements. It took me about an hour to enter it all. Models for the 901 chassis are a dome a dozen because, in 1964, Porsche cared about the racers.

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Since I am restricted by SVRA rules on moving the pivot point of the LCAs there is very little I can do other than adding spacers on the struts.

 

Maybe I'm missing something and one of you knows how I can make adjustments that fit the rules.

 

That said, I readily agree that improving my driving would have a greater impact than any bumpsteer corrections.
 

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The old cheater move for 510s was to bend the steer knuckles and then (I still have a hard time believing this part) claim "accident damage" if it was contested. Really silly if true, but that's the story I've heard from different sources, and apparently they let that slide. Never seen a set of Z steer knuckles done that way, but you can still buy the 510 ones from DP:

http://dpracing.co/images/DPR15.jpg

 

How much it matters depends on how bumpy the tracks are where you race and how low the car is. My car was a real handful at Buttonwillow, after moving the pivots was pretty well transformed. No more sawing at the wheel to keep a straight line, no bumpsteer induced emergency lane change maneuver over one particular bump on that track.

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I can't say I have ever had an emergency lane change or handling issue that was caused by bouncing the car (up or down). Once I put the race car back together later this spring (engine, tranny and diff) I will corner balance, align and check the bumpsteer

 

I have been sectioning my times now that I have an Aim Solo and tying that to the GoPro made seeing the error of my ways much easier. Getting better tire temp data is this year's project. That and growing bigger balls.

Edited by gnosez
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You have been given you a lot of good advice.  A lot of what will make you faster depends on you and how you do things.  Are you the feeling type or the analytical type.  Do you need to find things out on your own or do you do well with coaching.  If you know those things it can help you find the resources you need.

 

That aside one thing that is often overlooked is a good notebook and set of notes.  You should have everything in these.  From car setup to all lap times and printouts from the AIM.  I keep detailed notes of what the weather was and what the track and air temp was when I was running.  This might seem like overkill but as you have more time later in the week you can look over your notes and think about what you did and how the car was working.  Many times you may find something you didn't think about when you are pressed for time.  And when at the track you have a reference you can use for what to do, which is assuming that you are making setup changes.  Even things as simple as what starting tire pressures to run.  And what is the best hot pressure I should be at.  With the gopro you can get a cheap little program (race render) that lets you overlay data onto the video.  And if you have a couple of cameras (one that can see what you do and another for where you are on the track) you can see areas from the data where you can improve.

 

To answer your question about the investment to map the chassis for building a suspension model it probably won't make you any faster at first.  But if you know what to do with the data or where to ask the right questions (wonder where that could be!?) it can help make you a lot more consistent and help you with what to do next.  Each car is just different enough there's no one size fits all approach.  But as Clark mentioned you want the arms close to flat or pointing down when looking at the tire just a little.  And if you have actual suspension model you can see what each end of the car has for various suspension curves and map this back against track data.  This would be used to try and set the cars alignment and ride height in the best spot for the track.  This sometimes will make you faster but generally it makes you more consistent and leads to better tire life.  

 

 

 

 

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The old cheater move for 510s was to bend the steer knuckles and then (I still have a hard time believing this part) claim "accident damage" if it was contested. 

 

It's not cheating when allowed in the rules :ph34r:   But I do think there's a legal way this can be done with the spacers.  If they had an angle to them you'd move the steering arm.  I can find no spot in the SCCA rules that defines how the spacer must be made.  You also have the steering rack bushings that can be made different thicknesses to change the position of the rack.  

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