 Toe Curve for Semi-trailing arm, From my 914 |
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| groot |
 Nov 14 2005, 02:36 PM
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#1
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Dis member  Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444  |
Some of us have been discussing the 914 rear suspension offline and I measured my toe curve yesterday. I figured I'd share it with the club for discussion.
The axle height scale is in inches and the zero is with the arm level. Attached image(s) 
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| MattR |
Nov 14 2005, 02:45 PM
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#2
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Advanced Member Group: Members Posts: 3,279 Joined: 23-January 04 From: SF Bay Area Member No.: 1,589 Region Association: Northern California |
(IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)
thanks for sharing! Any idea on the front? I'd be curious to know how bad the bump steer really is with stock geometry. |
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| lapuwali |
Nov 14 2005, 03:07 PM
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#3
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Not another one! Group: Benefactors Posts: 4,526 Joined: 1-March 04 From: San Mateo, CA Member No.: 1,743  |
I presume on that graph positive numbers are for bump and negative numbers for droop?
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| groot |
Nov 14 2005, 03:23 PM
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#4
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Dis member Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444 |
Sorry, I meant to explain more, but I had to run out for a sec.
Yes, along the y-axis positive is jounce and negative is rebound.... but it all depends on your starting condition. If you start nose low (the trailing arm attachments to the chassis are lower than the centerline of the wheel) then your zero might be 1 or 1.5.... if you've moved your chassis mounts higher in vehicle your starting point may be -2. And the toe is relative, too. X-axis now... assume you start with zero toe, then look at the change in toe only. For example: if you set your car to zero toe and the axle center line is 1 inch lower than the chassis mounts... your zero is at -1 on the y-axis. As you jounce, the rear suspension toes out first and then goes back through zero toe, then on to toe in. I plan on measuring my front suspension, too, but it would have no relavance to a stock setup (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif) |
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| ChrisFoley |
Nov 14 2005, 10:08 PM
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#5
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I am Tangerine Racing Group: Members Posts: 7,909 Joined: 29-January 03 From: Bolton, CT Member No.: 209 Region Association: None 
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Based on the graph it appears that your car has the axle 3" higher than the pivot when you have 0 toe and that it goes to a maximum of 1/16" of toe out when the axle drops 3" to level. I would infer that a car (such as mine) with the axle level with the pivot and 0 toe static will toe in both in jounce and rebound.
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| groot |
Nov 15 2005, 08:40 AM
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#6
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Dis member Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444 |
No inference required... it's right there on the chart.
Yes, level is a good compromise, but axle slightly low is better (like -0.5) so you minimize the toe change in jounce, even with a slight toe out condition. But, it still is not what you want.... zero toe change and more camber gain is more appropriate. Chris-How did you decide level was appropriate? Do you have some measurements to share, too? BTW... the curve changes depending on the relative heights of the inboard and outboard chassis mounting. |
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| ChrisFoley |
Nov 15 2005, 05:04 PM
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#7
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I am Tangerine Racing Group: Members Posts: 7,909 Joined: 29-January 03 From: Bolton, CT Member No.: 209 Region Association: None
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I did raise the pickups as close to 3" as I could without being anal about it. I'm not actually sure the arms are level now - I was just saying that. I think the axles might be up to 1/2" lower but I would have to measure when I have the car on a good surface. At that point I can take some other measurements as well. One of my next projects may be to incline the pivots to lower the roll center, and move the bearing carrier to restore the static camber. I would be interested in seeing the toe and camber curves for such a change. |
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| Brett W |
Nov 15 2005, 07:50 PM
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#8
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Advanced Member Group: Members Posts: 2,856 Joined: 17-September 03 From: huntsville, al Member No.: 1,169 Region Association: None
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You will increase the scrub by changing the pivot point angles. Have you measured the amount scrub in the stock setup? Have you measured it in your current setup?
Do you know where the roll centers are now and where they go in bump and roll? What is your cg height? Do you know what happens when you increase the distance between the two? You still have to fix the toe change issues with the rear suspension. Kevin is looking to solve the problems. Heard the other day Finch is thinking about coming back to play in FP. You can bet he has analyzed all of these factors. |
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| brant |
Nov 15 2005, 11:30 PM
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#9
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914 Wizard Group: Members Posts: 11,612 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains 
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wow...
great thread Kevin! I'm going to have to do some poking around on the race car now. brant |
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| ChrisFoley |
Nov 16 2005, 07:09 AM
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#10
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I am Tangerine Racing Group: Members Posts: 7,909 Joined: 29-January 03 From: Bolton, CT Member No.: 209 Region Association: None
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All he has to do is snap his fingers to produce the data he needs, lol. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) |
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| groot |
Nov 16 2005, 08:26 AM
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#11
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Dis member Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444 |
I met with Finch the other day for a few hours and had a really good discussion.
He believes, as I do, that a 4cyl can win in EP and FP (duh!!). So, if he runs the 914, I wouldn't be surprised to see him in E or FP. We talked a lot about the 914 suspension... and he thinks the suspension is good enough to win without serious redesign required. He has tweaked the 914 suspension on that car, but nothing too drastic. I think I need all the help I can get because I know I'm not the best driver, so I'm going crazy on the suspension. |
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| ChrisFoley |
Nov 16 2005, 09:01 AM
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#12
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I am Tangerine Racing Group: Members Posts: 7,909 Joined: 29-January 03 From: Bolton, CT Member No.: 209 Region Association: None
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Since Mark H. set a lap record and beat Sargis handily I wouldn't be surprised if the CRB gives the FP car a lead trophy or other competition adjustment. (IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif) That will increase the value of modifying the suspension. |
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| groot |
Nov 16 2005, 09:11 AM
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#13
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Dis member Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444 |
Good point!!
Let's hope there are no lead trophies handed out. |
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| groot |
Nov 16 2005, 09:21 AM
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#14
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Dis member Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444 |
I think this is valuable if you're interested in making the production semi-trailing arm work. I was going to make the curves for differences in the height of the outboard attachment point, but I ran out of time.... travel for work (and the new child) makes it hard to work on the car. I probably will not measure it because I'm abondoning the production arm. When someone does measure it, please post it. I'm interested in discussing it. BTW... you don't need a good surface since you don't really care what the zero is... you're just looking for trends and comparative numbers. |
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| Jeroen |
Nov 16 2005, 09:28 AM
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#15
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914 Guru Group: Members Posts: 7,887 Joined: 24-December 02 From: The Netherlands Member No.: 3 Region Association: Europe |
what are you gonna do/build? |
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| Brett W |
Nov 16 2005, 09:29 AM
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#16
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Advanced Member Group: Members Posts: 2,856 Joined: 17-September 03 From: huntsville, al Member No.: 1,169 Region Association: None
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Hence my main complaint with the SCCA. kinda like the US tax system, they penalize you for hardwork and acheivement. The will up the weight or something or shrink the carb size. Just what the FP guys need. Lets hope they don't change the EP rules also. |
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| groot |
Nov 16 2005, 10:17 AM
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#17
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Dis member Group: Members Posts: 894 Joined: 17-December 03 From: Michigan Member No.: 1,444 |
By rule, I have to stay with a semi-trailing arm suspension. So, I'm going to build a new one. Brett was on this path a while ago and I resisted, but looking at the geometry and what I need (more rear grip), there's no way around it..... for me. It sucks, too, because I built and rebuilt this rear a few times now and I'm going to trash it again. But, I like a challenge and this rear suspension is one. |
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| Brett W |
Nov 16 2005, 04:19 PM
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#18
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Advanced Member Group: Members Posts: 2,856 Joined: 17-September 03 From: huntsville, al Member No.: 1,169 Region Association: None
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(IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)
I prevail. Yes the stock trailing arm works but it is far from optimal in a racing situation. A custom built arm is just part of it. |
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| ChrisFoley |
Nov 16 2005, 07:16 PM
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#19
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I am Tangerine Racing Group: Members Posts: 7,909 Joined: 29-January 03 From: Bolton, CT Member No.: 209 Region Association: None
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I took a trailing arm to the shop today. I'm going to fixture it and make camber/toe measurements. Then I can generate 3D plots like the one above, with pivot inclination and axle height as the independent variables. |
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| Thorshammer |
Nov 16 2005, 07:23 PM
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#20
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Senior Member Group: Members Posts: 749 Joined: 11-November 03 Member No.: 1,335 |
Now this is what I am talking about. Finches car (Mr Kirby car) is very stiff, probably in the 400lbs per corner area for spring rate. I believe that to be stiffer than any of us are running.
Erik |
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