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dennis68
08-18-2004, 04:35 PM
OK, as we all know I am the new suspension information sponge. I have scoured the net and am at a loss as to how to adjust bump steer. I know what it is and how to measure, but how do you adjust it out? I have adjustable outer tie rods, adjustable center link and a torch (just kidding). My understanding about the tall spindle swap is part of the problem is the tie rods become "too long", in other words the distance between the center link hole for the inner tie rod and the spindle hole for the outer tie rod becomes longer. If I have the ability to shorten that distance, how do I determine what to adjust in the steering linkage to obtain zero bump? Is it just trial and error or is there a method, like tuning, adjust timing first then dial in fuel.

My thoughts were to try and start by putting the distance back to stock with the drum spindles then move the ends up/down and measure change. Is starting with the steering linkage parallel to the LCA a good place to start (happens to be parallel with the ground). Thanks in advance to "the group".

spanky the wondermuffin
08-18-2004, 04:44 PM
there is a section in the steve smith book 'paved track stock car technology',steve smith autosports #239(www.sspubl.com)with bump steer patterns and how to correct them.pg.36-37.jon

MrQuick
08-18-2004, 05:38 PM
remember "EXPERT"..... ex is "X" an unknown factor... spert " and unexpected drip before the shake " :rotfl: ... actually thats all I have to say. oh yeah get the SS books great reading. Hey Jon! Vince

dennis68
08-18-2004, 05:40 PM
Guess I should have put experts and Vince too. :hand:

MrQuick
08-18-2004, 05:51 PM
awwww wise guy?! :slap: nook nook Vin

astroracer
08-18-2004, 06:34 PM
Check out this info...
http://www.longacreracing.com/articles/art.asp?ARTID=13
I am using it along with a suspension program to develope the front suspension for "Bad Ast". It works fairly well. I have the Corvette Rack set up in my study file as they have it described and it's pretty close.
You have a lot of work to do to find out where you're at before you can fix it.
Mark

dennis68
08-18-2004, 09:16 PM
Lots of good info there, thanks a bunch Mark. That will make it sooo easy to get dialed in.

68ls1
08-19-2004, 03:45 AM
Although I posess neither the literary skills nor the technical knowledge to post on corner-carvers.com, I have lurked for quite a while and can say that they really know their sh@t. It's worth the time to go to the site and do some searches and read some current posts. I have learned alot in a short period of time.

Chill, Noel

Norm Peterson
08-19-2004, 05:37 AM
OK, as we all know I am the new suspension information sponge. I have scoured the net and am at a loss as to how to adjust bump steer. I know what it is and how to measure, but how do you adjust it out? I have adjustable outer tie rods, adjustable center link and a torch (just kidding). My understanding about the tall spindle swap is part of the problem is the tie rods become "too long", in other words the distance between the center link hole for the inner tie rod and the spindle hole for the outer tie rod becomes longer. If I have the ability to shorten that distance, how do I determine what to adjust in the steering linkage to obtain zero bump? Is it just trial and error or is there a method, like tuning, adjust timing first then dial in fuel.

My thoughts were to try and start by putting the distance back to stock with the drum spindles then move the ends up/down and measure change. Is starting with the steering linkage parallel to the LCA a good place to start (happens to be parallel with the ground). Thanks in advance to "the group".Bumpsteer is a measure of the "error" between the arcs of the outer tie rod end ball and the curved path of the knuckle at that same height. A length error in the tie rods is one possibility (and if this exists, the centerlink is off, assuming a reasonably sane static toe setting). Bumpsteer also happens when the inclination of the tie rods is incorrect.

Most bumpsteer kits that I'm aware of work by adjusting the height of the outer tie rod end balls relative to the steering arms but don't include an adjustable centerlink. Mostly because the newer cars that the kits fit are R&P steering, I'd guess.

But having centerlink adjustability at my disposal, I'd first set the centerlink such that the tie rod lengths are correct (and the steering wheel centered) and then adjust the outer tierod heights. The tie rods will be nearly, but not exactly, at the same slope as the LCA's at static ride height. Assuming an above-ground front roll center (which means that the UCA's slope downward toward the chassis if the LCA's are truly horizontal), and that the tie rod ball centers are slightly higher than the LBJ ball centers, the tie rods should slope a slight amount downward toward the chassis (smaller slope than the UCA's).

What you're trying to do is angle the tie rods such that a front view line drawn through the tie rod pivot centers intersects the FVIC for the same wheel's suspension at static ride height (or at least approximately that height, since the SVSA length is variable over suspension travel). Katz probably has more detailed information on this, but I think he's still out at Bonneville.

If you've got a below-ground front RC, I'm not at all sure what you'd do for tie rod length, as the SVIC for this case is normally outside the same-side wheel, hence the outer tie rod and knuckle arcs curve in opposite sense. But I suspect that they are a bit long to flatten out the curvature of the outer tie rod ball arc somewhat.

A big note is that LCA and UCA inclinations are not necessarily the visual inclinations of the physical arms - they're lateral lines constructed in the control arm planes as defined by the centers of the chassis side pivots and the balljoints and drawn through the BJ's.

Edit: On looking through the linked Longacre article, it's apparent from the diagram that if the steering arms incorporate Ackerman you have to either compromise on the tie rod length slightly or offset the inner TR end from the line between the chassis points (probably the latter approach is preferable)

Norm

copo69
08-19-2004, 02:54 PM
i use the baer traker system from the baer brakes company and my 1970 chevelle ispancaked i mean on the ground you cant slide a pack of ciggarettes under it and after installing the traker i have no bump steer none zero not any kind so i put them on my camaro wich is 3 drop on all 4 and presto my bump steer was gone .. maybe this will help you also.. :usa:

dennis68
08-19-2004, 04:42 PM
I am running so many non-OE pieces that I am sure of bump steer issues. Without pulling a newbie list of mods I'll briefly sum up the front-end; Howe adjustable center link, Speedway adjustable rod end outer tie rods with 9" swedged tubes for adjusting links, Speedway UCA with solid bushings, Factory reinforced LCA's with solid bushings, 800 lbs custom coils, F body tall spindles, and a quick ratio box.

The Baer trackers are kind of a gimmick as they do not fix the problem (not to mention they outrageously priced) of bump steer in the lowered A body with tall spindle conversion-the tie rods need to come up, not down. I am running something similar only because I have inner tie rod adjustability as well so I can tune it out completely.

68fusion
08-21-2004, 07:47 PM
just my 2 cents....used to work on late model car here in NC and most LMSC are built off of camaro pick up points- all susp parts are fab now and they have some pretty trick center links with slugs(cnc $) to adjust the gain or bump. I dont know how the "howe" is adjustable and to what extent but it took a long time to get right. Several companies make a bump plate ( fastens to wheel, you put a contraption with two dials against the plate and travel the susp. ) We would check the gain(from ride height) at 1in and 2in. sometimes 2.5 if it looked good at 2. Cant remember the numbers we shot for but know it never went to ZERO. Most of the time we were more concerned with the R front and left only checked an inch or so. The slugs would give the adjustment we needed to keep bump from " dragging/pushing the wheel" during travel. But then Protouring wants left and right turns...will take a little getting used to. :icon996:
I think your gonna have bump one way or another just try to find a happy medium.

Dont even get me talking about shocks...whew.. :naughty:

Salt Racer
08-24-2004, 03:16 PM
While what Norm said is true, it is rather difficult to find true tie rod centerline and FVIC location on existing suspension.

Even if you lay out every pick up points on CAD and build suspension accordingly, something will be off due to heat distortion from welding, machining tolerance, etc. Checking the finished product with bumpsteer gage is the only way to minimize bumpsteer.

One thing you'll have to be careful about typical bumpsteer gage is the toe span. Most have dial gages spaced apart 16" or so. 1/8" bumpsteer at 16" span will measure 0.203" with 26" tall tires. Keeping this in mind, you should try to minimize bumpsteer as much as you can if the span on your bumpsteer gage is significantly less than that of the tire diameter.

I thought twice about posting the info below in public forum, but I feel that EVERY suspension manufacturers should at least get the bumpsteer curve right, and if it helps some street rod chassis manufacturers to improve their products, that'll be a good thing so here it goes...

Toe-out on both bump/rebound - tie rods too long, height correct
Toe-in on both bump/rebound - tie rods too short, height correct
Toe-out on bump, Toe-in on rebound - outer tie rod too low, length correct
Toe-in on bump, Toe-out on rebound - outer tie rod too high, length correct

All of the above assume even amount of bumpsteer on bump & rebound (eg, 0.25" toe-out on bump, 0.25" or toe-in on rebound for condition #3). There are situations where both tie rod length and height are incorrect, in which case the magnitude of bumpsteer will be uneven.
Also, this is for front steer. Conditions will reverse on rear steer.

I wouldn't call those bumpsteer kits gimmicks, but simply bolting them on will not reduce bumpsteer unless you really luck out. Personally, I really don't trust those machined studs included in bumpsteer kits. Not sure about expensive Baer kit, but I highly doubt studs in cheap kits have rolled threads and proper heat treatment. Blowing up an engine will ruin your day, but losing steering could ruin your life.

I'm somewhat safety conscious, but I'm also cheap. So for my personal rides, I normally bore out the steering arm holes with either 5/8" ream or mill, and use Grade 8 5/8" bolts and spacers instead. You can see an example in the pic below (it's the front susp on my Riv). Tie rod is visible behind the swaybar link. Rod ends are 5/8" x 5/8" 4130 non-teflon/kevler liner right now, but they'll be replaced with NHBB rod ends soon.

https://static1.pt-content.com/images/noimg.gif

I was in a big hurry when I did this, and didn't have bumpsteer gage handy so I just used a 26-inch long angle iron and a tape measure. Turned out the tie rods are too short on this car, as the tires toe-out roughly 1/16" @ 26" span on both 2.25" bump and rebound (rear steer). Tie rods need to be almost 2" longer to correct this, but there really isn't any room right now. Narrowed drag link isn't an option due to oil pan clearance, but I'll be able to bend the steering arms and increase the tie rod length at expense of reduced Ackermann angle once disc brakes are on. As it is, it's still a big improvement over stock bumpsteer curve (1/4" toe-out on bump, 1/8" toe-in on rebound). Although I increased caster to +6.0 degrees, the spacer thickness still ended up at 1.054", which is a bit longer than I like.

It's unrelated, but notice that bump stop has been trimmed and rounded so that ride rate will increase progressively when suspension bottoms out. If you just trim the bumpstops and don't reshape them, sudden and massive increase in ride rate will result. This is something you really don't want at the limit.

I forgot what the bumpsteer curve was like on A-body with tall knuckle conversion, but airrj1 has a nice graph on his personal website.

K

dennis68
08-24-2004, 03:27 PM
Nice post!! Well written and I like the simplified trouble guilde to overall length/height. Thank You.

airrj1
08-24-2004, 08:09 PM
Denny,

If you didn't see it before here is my results: http://www.buffnet.net/~airrj/Bump_Steer.htm

Don't take them as gospel, this was my first attempt. I just got the car on the road today, and I will be starting to try to dial in the bump steer soon.

dennis68
08-24-2004, 08:13 PM
Hey RJ, if you don't have one yet, get that Howe centerlink. What a nice piece of linkage. I like the fact that each piece can be replaced separately.