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Subarugears
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Joined: January 03, 2012
Posts: 294
Location: Sydney, Australia
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 Posted: Thu Oct 30, 2014 2:58 am Post subject: Steering Ackerman calculation for SWB buggies |
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Hi all, apologies if this has been covered elsewhere. I searched and searched but found no reference for it for a Manx SWB buggy.
Has anyone done the ACTUAL calculations for the ideal location of the tie rod ends in a 14 1/2" shortened buggy?
The standard VW beetle has the location of the tie rod ends (in the correct spot I presume) along a "V" line from the centre of the rear wheels/axle through to the front ball joints. When the chassis is shortened, this "V" gets shorter and so moves the ideal location of the tie rod ends inwards. It would be great to know exactly how much this is moved inwards so that some billet pieces could be made, allowing the tie rod ends to be relocated and thus correct the Ackerman. This would result in better steering performance and less tyre scrubbing.
If anyone has done it or could point me to the thread, that would be awesome. I have to make these billet pieces anyway, since I am using a non-beetle front hub and am also doing a center steer rack and pinion conversion.
If it's not been done, we could probably do it here on the forum.
Can anyone provide an accurate measurement of wheelbase? I suppose that's published.
Also an accurate measurement of the center to center distance between the top two balljoints.
We can probably do the diagram for a standard beetle and then a SWB one from there.
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BL3Manx
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Location: Northern California
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| Posted: Thu Oct 30, 2014 7:52 am Post subject: |
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Can you clarify what you mean when you say you're going to relocate the ends of the tie rods? Right now the inboard ends of the tie rods are where they have to be to connect to the pittman arm and the outboard tie rod ends are where they need to be to connect to the steering arms on the spindles. Are you going to change the length of the steering arms on the spindles.
What problem are you trying to solve? Are they theoretical problems or something you've experienced driving your buggy. How will shortening your tie rods affect bump steer on a longer travel off road suspension? |
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EVfun
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Joined: April 01, 2012
Posts: 5488
Location: Seattle
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| Posted: Thu Oct 30, 2014 8:09 am Post subject: |
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Ackerman steering describes how the inside tire needs to turn more than the outside tire for a car to turn smoothly. If you draw a line through the rear axle extended out either side of the vehicle then when you turn both front tires should turn slightly different amounts so that both spindles point to a single point on that line through the rear axle. In practice, a perfect ackerman correction results in outside tire wear on the front at the weight shifts to the outside when turning. Because of this, in practice the steering geometry is set up so that the inside tire turns slightly more than it should (points to a spot on the line through the rear axle that is inboard of where the outside front tire points.) That causes it to try and pull a little weight off the outside tire and increases inside tread wear.
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VOLKSWAGNUT
Fastest VW Belt Changer
Joined: October 14, 2007
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Location: Flippin' a Belt........ .... Off-n-On ... NC USA
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| Posted: Thu Oct 30, 2014 9:27 am Post subject: |
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Look here: http://www.thesamba.com/vw/forum/viewtopic.php?t=478234
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Dale M.
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Joined: April 12, 2006
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Location: Just a tiny bit west of Yosemite Valley
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| Posted: Thu Oct 30, 2014 11:32 am Post subject: |
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Personally I never gave it any though on street car as its more or less spend most of its time going fairly straight...
On other hand for auto x car I actually liked the more sever ackerman angle concept to handling and actually voided all the rules by adding a little toe out as a auto-x course is primarily all turns.. Sort of liked the more aggressive pull into corners the sever ackerman angles provide coupled with the tow out...
Dale
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Subarugears
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Joined: January 03, 2012
Posts: 294
Location: Sydney, Australia
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| Posted: Thu Oct 30, 2014 4:36 pm Post subject: |
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removed double post
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Last edited by Subarugears on Thu Oct 30, 2014 8:18 pm; edited 1 time in total |
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Subarugears
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Joined: January 03, 2012
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Location: Sydney, Australia
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| Posted: Thu Oct 30, 2014 4:36 pm Post subject: |
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| BL3Manx wrote: |
Can you clarify what you mean when you say you're going to relocate the ends of the tie rods? Right now the inboard ends of the tie rods are where they have to be to connect to the pittman arm and the outboard tie rod ends are where they need to be to connect to the steering arms on the spindles. Are you going to change the length of the steering arms on the spindles.
What problem are you trying to solve? Are they theoretical problems or something you've experienced driving your buggy. How will shortening your tie rods affect bump steer on a longer travel off road suspension? |
Let me preface this by saying this is all planning, not an actual issue I have personally encountered driving my buggy, as it's still in the build.
I do not have VW beetle front hubs, I have Subaru WRX front hubs (to provide the AWD to the front wheels) modified to accept VW ball joints and the upper and lower trailing arms. They do not have the same location for the tie rod ends as VW front hubs, so I have to make my own brackets for the WRX front hubs that will locate the tierod ends in the best possible location in three dimensions; up/down, in/out and fore/aft.
I also do not have a VW beetle steering box. I have a reversed Daewoo/G.M. rack and pinion that is center steer and will be mounted to the VW beam in the appropriate location to eliminate/minimise bump steer. We have that part covered - getting the tierods straight, even and located at the best height with the tierod end location that I will manufacture.
My issue is in determining how far in/out and fore/aft I should make the bracket and hole for the tierod ends. I could copy the VW beetle setup from their front hubs as a starting point. However, on a SWB buggy, this becomes compromised because the Ackerman has changed do to the shortened pan.
So I wanted to know if anyone has calculated how much further inwards the tierod end holes should be moved in order to correct the Ackerman.
It seems to me that everyone just leaves it as it is and lives with it.
(by the way it looks like its 5.9mm inwards on each side)....
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Dale M.
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Subarugears
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Location: Sydney, Australia
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| Posted: Thu Oct 30, 2014 8:14 pm Post subject: |
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I don't need lessons in Ackerman, I understand it.
I need to know if anyone has actually done it for the beetle and SWB buggy.
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Subarugears
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Joined: January 03, 2012
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Location: Sydney, Australia
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| Posted: Thu Oct 30, 2014 8:14 pm Post subject: |
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I don't need lessons in Ackerman, I understand it.
I need to know if anyone has actually done it for the beetle and SWB buggy.
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joescoolcustoms
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Joined: August 08, 2006
Posts: 9054
Location: West By God Virginia
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| Posted: Thu Oct 30, 2014 8:37 pm Post subject: |
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| Dale M. wrote: |
Personally I never gave it any though on street car as its more or less spend most of its time going fairly straight...
On other hand for auto x car I actually liked the more sever ackerman angle concept to handling and actually voided all the rules by adding a little toe out as a auto-x course is primarily all turns.. Sort of liked the more aggressive pull into corners the sever ackerman angles provide coupled with the tow out...
Dale |
When you say "voided all the rules", do you mean actual written rules or suggested typical setup rules? (have never auto X'ed, so I do not know). Sounds like it would help.
I have a relative who ran bias tires on the rear and radials on the front in order to get his front engine V8 to help turn.
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Subarugears
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Location: Sydney, Australia
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| Posted: Thu Oct 30, 2014 9:00 pm Post subject: |
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he he I have a cousing who has one of them Manxes with a Porch engine.
It lifts the front wheels.
Seems legit 
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Iguana
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| Posted: Thu Oct 30, 2014 10:49 pm Post subject: |
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We went through all this when we were doing the R&P conversions and I am pretty sure Wes modelled it all up in CAD.
I will ask him for you and see if he still has it. I have the output somewhere but by the time I got it it was an Excel Spread Sheet and a set CAD drawings for an adjustable Jigs that took the guess work out of it. |
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Subarugears
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Location: Sydney, Australia
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| Posted: Thu Oct 30, 2014 11:22 pm Post subject: |
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| Iguana wrote: |
We went through all this when we were doing the R&P conversions and I am pretty sure Wes modelled it all up in CAD.
I will ask him for you and see if he still has it. I have the output somewhere but by the time I got it it was an Excel Spread Sheet and a set CAD drawings for an adjustable Jigs that took the guess work out of it. |
Wes has only done the bumpsteer calcs to locate the R&P....would be great if you could get them as I can't get an answer out of him.
From our last conversation he did not do the Ackerman calcs for an SWB pan...but was alert enough to point out to me that I needed to do this for my setup. Which is a good start.
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Dale M.
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| Posted: Fri Oct 31, 2014 7:11 am Post subject: |
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| joescoolcustoms wrote: |
| Dale M. wrote: |
Personally I never gave it any though on street car as its more or less spend most of its time going fairly straight...
On other hand for auto x car I actually liked the more sever ackerman angle concept to handling and actually voided all the rules by adding a little toe out as a auto-x course is primarily all turns.. Sort of liked the more aggressive pull into corners the sever ackerman angles provide coupled with the tow out...
Dale |
When you say "voided all the rules", do you mean actual written rules or suggested typical setup rules? (have never auto X'ed, so I do not know). Sounds like it would help.
I have a relative who ran bias tires on the rear and radials on the front in order to get his front engine V8 to help turn. |
When you shorten wheel base the inside tire has actually more turn angle (ackerman angle) than it did when vehicle had longer wheel base... By actually using a bit of toe out it increases the turn angle even more, and cause inside wheel to be more aggressive into turn then when car has long wheel base... Pretty common thing to use a bit of toe out in dedicated auto-x cars... Not good for street and track cars though (sunday racers) ...
Dale
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GS guy
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| Posted: Fri Oct 31, 2014 9:46 am Post subject: |
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Seems like a simple geometry/trig problem? Couple of triangles with corners at the king pin centers and rear differential. We know the wheelbase, just need to determine the king-pin side to side dimension.
To me, the biggest unknown is this line from the Wiki page: "Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low-speed manoeuvres."
Which way modern cars "adjust" the ackerman for best performance and handling might be the toughest part to design in?
I'm thinking buggies have so little weight over the front tires anyway, any scrub caused by the incorrect ackerman is probably inconsequential. But I agree with wanting to get it at least close in the overall design.
Jeff
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Subarugears
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| Posted: Fri Oct 31, 2014 2:48 pm Post subject: |
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I agree with you there Jeff.
I have done the squares and triangles and worked it out.
BUT I have been using a balljoint center width of 1308mm (51.5 in) as that was the "front track" published for beetles.
I don't know if this is correct as my buggy is over 1,000kms away.
If someone could do an accurate measurement of a balljoint beetle at ride height, center of top balljoint to center of top balljoint, then I could confirm the calcs.
Happy to post the images up on here so it can be seen and shared for anyone wanting to do the same.
Been having an interesting discussion with fellow VWers here about modern steering geometry. Thye obviously use some different principles as well. VW Superbeetles with rack and pinion steering have the tierod ends in front of the balljoints and almost on the same plane left-to-right. WRX's have the same. These would seem to be outside of the Ackerman geometry.
I think a can of steering whupass has opened up.
Just trying to get it as correct as I can. I'm sure it will steer well anyway, buggy dynamics being what they are.
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GS guy
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| Posted: Fri Oct 31, 2014 4:52 pm Post subject: |
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I dug in my old front suspension files and found the Baja Wes bump-steer spreadsheet data. Wes had developed a cool Excel spreadsheet that calculated bump steer for a VW front end with a rack and pinion fitted. You could manipulate the inner tie rod end/rack locations and see what happened to the bump steer characteristics. From the spreadsheet printout, here are the VW dimensions Wes provided:
Total Width - Top BJ to Top BJ: 1170mm
Total Width - Bottom BJ to Bottom BJ: 1205mm
Total Width - Tie Rod Ends (at spindles): 1100mm
Spacing Upper to Lower Torsion Tubes: 150mm
Spacing Upper BJ to Lower BJ: 167mm
(Not sure what camber or toe is implied with these numbers.)
This is the best data I have on the BJ front end. Might help?
Jeff
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Subarugears
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| Posted: Fri Oct 31, 2014 9:45 pm Post subject: |
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I have Wes' spreadsheet but didn't spot this.
I will do the diagrams with those balljoint widths that you posted in order to calculate the Ackerman. Thankyou
Again, I am not calculating bumpsteer, I have already used his model to make a rack and pinion clamp the minimises/eliminates bumpsteer. It's about correcting Ackerman.
I will post up the diagrams later and also show the models of the rack and pinion clamps that are going on the Subarugears WRManX as part of this.
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