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Learning how to build Lotus Seven replicas...together!
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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 12:05 am 
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Joined: November 15, 2009, 9:58 pm
Posts: 424
Location: Port Angeles. Wa
The SIMPILIST bump steer setup is a mirror and a laser!!! No chasing a dial indicator. I can easily get within a few thousandths easily. To find out how long the tie rod should be, keep turning the steering wheel to the left when working on the right side and keep correcting toe. The laser dot on the vertical line (reflected from the mirror) will not curve when the tie rod length is correct. High or low rack vs steering arm height will be an angled line. The laser/mirror/vertical line on target vertical line is SO easy to analyze!!!!!
I had a glass shop cut a 4"x 8" mirror that I double sided taped to a piece of aluminum sheet that can be put on a spindle hub bolt/stud or clamped to the rotor. Set the car at ride height or anything parallel to ride height. Pull the shock/pushrod so the spindle can be lifted by hand. Shine the laser onto the center of the mirror when at rough ride height from just behind (or in front of front wheel centerline - a line thru both spindles) about 3' away. Setup a surface with a vertical line that picks up the laser beam (maybe 3+' out). Lift the spindle up and down to show bump steer!!!! Its that simple. I will post pictures if wanted!!!!!!

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My build log viewtopic.php?f=36&t=10658&start=0 NOW NAMED =The Wycked 7

My other build log viewtopic.php?f=18&t=15162 The Skayt'R6


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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 7:27 am 
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Joined: September 22, 2005, 8:12 am
Posts: 1800
Location: 4AGE in S.E. Michigan
Spitfun
I see that you are in Michigan. Your profile does not have a location, but you are welcome to borrow my bump steer gauge. Might be worth a short trip if you are within a couple hours of South Lyon. You probably would still need to make a spline mounting board, since I have a 4 hole lug pattern Vs 5. If interested just send a PM.
Davew


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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 2:24 pm 
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Joined: October 24, 2008, 2:13 pm
Posts: 4537
Location: Carlsbad, California, USA
@Kartracer47

I for one, would love to see photos of your setup.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 2:52 pm 
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Joined: December 29, 2007, 10:41 pm
Posts: 955
Location: Vancouver, BC
Lonnie,
I think this is what kartracer is trying to describe:

Image

The mirror is parallel to the hub flange, and if you shine a laser at it from a nominal distance it should reflect back along a straight line on the wall/board/flat surface that you've placed farther away than the source of the laser, if the bump steer (at ride height, body roll, etc) is as close to zero as possible. If not, the laser reflection will wander away from the straight line you've drawn on the wall etc. The turning left thing had me stumped for a minute, but it makes sense as you're lengthening the theoretical tie rod once you adjust the toe back to what you desire. Once you know the length that works you can then modify your steering rack or find a suitable one with the correct inner tie rod end length (and rack width).


NOTE: I have the mirror (blue line) rotated to show a toe change, the mirror should be parallel to the line on the right side at ride height (or when you achieve zero bump steer)


Last edited by cs3tcr on June 8, 2021, 3:33 pm, edited 1 time in total.

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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 3:28 pm 
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Joined: October 19, 2012, 9:25 pm
Posts: 3315
Location: Summerville, SC
What I learned working on the SR20 seven was there is a happy place somewhere in 3D space for a rack that's otherwise too long.

It took me a couple of days scratching my head to find it, but that car, then the MGB powered 7 both got full length Miata racks without bump steer.

Its much easier to find the happy place with the chassis resting on blocks putting wheels at full droop.
Take the springs off the shocks, or remove shocks completely to make cycling the wheels through full range of movement easier.
Wood blocks and stout rope to tie the rack in place during mockup.

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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 7:44 pm 
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Joined: October 24, 2008, 2:13 pm
Posts: 4537
Location: Carlsbad, California, USA
@cs3tcr

Thanks so much for that illustration. I think I'm 90% there now. I'll probably get the last 10% when I actually have to do the analysis on my build.

I'm not quite there on the turning aspect: turning left while checking the right side. My build is front steer, rack and pinion, so turning left involves "pulling" the right side of the rack and tie rod into the center of the car making it "shorter" in a sense. Everything stays the same size, of course, it just moves. I'm probably not visualizing something correctly.

I think I need to think about that a little.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 8:26 pm 
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Joined: January 2, 2009, 1:45 pm
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Location: Halifax, Nova Scotia
Lonnie, when you 'turn left' you are moving the inner tie rod end left, effectively shortening the rack (at least on the right side). When you find the spot with no (or minimal) bump steer, doubling the distance between the inner tie-rod end joint (or the right hand end of the rack) and the centreline of the car gives required rack length. It is a neat and elegant trick - kudos to Kartracer!

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Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601


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 Post subject: Re: Bump Steer
PostPosted: June 8, 2021, 9:25 pm 
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Joined: August 28, 2010, 7:53 am
Posts: 286
I recommend the the OP build a bump steer gauge and play around with it. A big part of building your own car is learning and understanding - lasers and/or dial indicators are great tools to see what is really happening. If you can minimize bump steer for 2" bump and 2" rebound, you'll be happy.
Anti-dive geometry can cause an "S" shaped bump steer curve, so compromises must be made. With little to no anti-dive, bump steer can be very close to zero.


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 Post subject: Re: Bump Steer
PostPosted: June 9, 2021, 8:29 am 
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Joined: September 2, 2013, 9:12 am
Posts: 117
Location: Michigan
I am trying to wrap my head around this. I found this video online: https://www.youtube.com/watch?v=oYoiq1GTrpI Seems a bit crude,but does get the job done. Am I correct in saying I can get most of the bump steer out by adjusting the end links up or down to try to get in the center of the arc? I bought some heim joints and spacers that will allow me to lower or raise the tie rod ends where as with the standard ends I was unable to do that (easily anyway). Also,why can't you just mount the laser to one of the wheel studs and just point it toward a steady-mounted piece of paper to get the points needed?

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 Post subject: Re: Bump Steer
PostPosted: June 9, 2021, 9:13 am 
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You can reduce the bumpsteer around ride height but not eliminate it for the whole range of motion if the inner rack balls are not where they need to be.

All you really need is a 12x24 inch piece of mdf, a couple hinges, a 24 inch 2x4, and a couple bricks.
Attach the 2x4 to the mdf with the hinges near the ends of the 2x4.

Remove the coilover, support the car at ride height, straighten the steering, lean the mdf against the tire below just below the center (notch the mdf center if the center cap sticks out too far from the wheel)
The mfd touches the tire at the front and rear. Place bricks on the 2x4 to keep it from moving.
Raise the tire as if you hit a bump. If a gap appears between the mdf and one tire sidewall (front or rear), you have bump steer.

Shim to minimize bumpsteer around ride height. This should also reduce droopsteer around ride height.
No smoke, no mirrors, no lasers but to be fair, those methods can be more accurate though the devil is in the details. I’d say they cost more than wood but I’m not so sure anymore. If all your doing is trying to reduce it, knowing the exact amount isn't critical. Eyeballing works.

Lots of goods comments on the scrub and all. We could make improvements all day but the shimmage is a good start. Looks like the turn signals would have to be moved and custom fender mounts made to go with new offset rims to fix the scrub :cheers:

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 Post subject: Re: Bump Steer
PostPosted: June 9, 2021, 7:36 pm 
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Joined: August 28, 2010, 7:53 am
Posts: 286
Spit - the track width changes with bump/rebound, so mounting a laser to the wheel stud wont discern between track width changes and bump steer.

I have had good luck with the $10 dial indicator and MDF, but never tried a laser. If i have time, ill take a pic of my bumpsteer gauge that i made from scrap.


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 Post subject: Re: Bump Steer
PostPosted: June 10, 2021, 7:09 am 
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Joined: January 2, 2009, 1:45 pm
Posts: 1299
Location: Halifax, Nova Scotia
This is an extract from my build log 7 (blush) years ago. The bracket is the one used to support the upright when measuring for the a-arms, held steady by a wedge driven between the rotor and the caliper. I was projecting the laser onto a piece of wood about 5 feet away and got 1/16th inch bump steer at first cut ... easy-peasy

(photo to follow ...) (edit:done)


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File comment: Of course, I suppose I should convert that 1/16th bump steer to an angle ...
Bumpsteer setup 20140316 (Small).jpg
Bumpsteer setup 20140316 (Small).jpg [ 92.59 KiB | Viewed 216 times ]

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Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601
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 Post subject: Re: Bump Steer
PostPosted: June 10, 2021, 5:51 pm 
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Joined: February 22, 2017, 10:47 am
Posts: 17
Location: Raleigh NC
Since I haven't seen it mentioned yet, and since this is a LOWcost forum, a good place to get a dirt cheap laser pointer is your local pet store. That, plus some scrap wood/duct tape/chewing gum can make a reasonably accurate bump steer gauge.

:shock: <--- My cats, when they see the red dot magically appear on the carpet next to them.

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 Post subject: Re: Bump Steer
PostPosted: June 10, 2021, 6:56 pm 
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Joined: August 28, 2010, 7:53 am
Posts: 286
Here is a pic of my gauge that i made from scrap. The base is 5/8 thick aluminum, though 3/4 ply of MDF would work just as well.
The "U" shaped part pivots on the "pillow block" (steel rides on aluminum, no bearings or bushings) It needs to be rigid, so i used steel scrap (3/4" round with 1/8" wall) Cheap import dial indicator on one side and a 1/4" steel rod on the other. There is an aluminum plate that bolts to the hub...MDF would be fine too, as long as it is flat. Took about 2 hrs to build. Had it at least 10 years. Several friends have borrowed it too.


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Bump steer gauge.jpg
Bump steer gauge.jpg [ 235.36 KiB | Viewed 191 times ]
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 Post subject: Re: Bump Steer
PostPosted: June 13, 2021, 11:57 am 
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Joined: December 27, 2013, 12:17 pm
Posts: 13
spitfun wrote:
Well,thanks for all the responses guys, I really appreciate it. I think I will start with converting over to spherical rod ends,that will give me more adjustment up and down-hopefully that tames it,if not, then I will look into moving the rack as a second option.

What thickness is the plate where steering tie rod is fitted - 8-10-12mm??
I consider some similar setup....


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