What about it for some reason your chassis points aren't quite where you said they were?Quote:
Originally Posted by Tim.Wright
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What about it for some reason your chassis points aren't quite where you said they were?Quote:
Originally Posted by Tim.Wright
That's a valid reason, but in practice we've never experienced that much warpage. As long as you weld the frame up on a table (jigged to the table) and then stress relieve the welds before pulling it off everything usually seems to line up pretty well.Quote:
Originally Posted by ChristianChalliner
You want to watch out for "not quite right" chassis points because some (say toe link mounts, etc.) can have a relatively large [and usually unexpected] impact on kinematics.
1. Correcting the chassis point locations with this joint will only move the outboard point location out-of-specQuote:
Originally Posted by ChristianChalliner
2. A steel control arm can be flexed a mm or two to fit if required. If your build inaccuracy is greater than that you've got bigger problems.
3. Errors in the longitudinal (X) direction for the chassis pickup points are not super critical.
1. Unless you can adjust the length of each arm independently in which case it will end up back where it started :)
2. I agree
3. I agree again.
Not trying to say you're wrong and we decided not to do it in the end anyway, I just thought it was an interesting idea worth looking at that's all.
Christian,
A bit out of context bit I feel it worth to share this. I mentioned this earlier in some previous posts.
If you have a double wishbone suspension and your top wishbones are adjustable with rod ends on the inboard and/or outboard side, you will agree that you can have the same static camber with a large wishbone (large in the y direction) and a little amount of camber shims OR with a short wishbone and a big amount of camber shims.
Same static camber but different KPI angle and (although slightly) different scrub radius.
I learnt the hard way many years ago that within other parameters one of the biggest influence a different KPI could have (I say could because it depends on the suspension geometry) is the wheel / spring motion ratio. When you know that the wheel rate is the spring rate / MR square.
If you have that kind of suspension adjustability worth to try different KPI on the track for a first car or for a prototype. That is if your car is not finished 2 weeks before the competition.....
But if you do change the KPI value by several degrees make sure you also change you spring stiffness accordingly so you do not change too many inputs at the same time (even worse without knowing it)
Also in case of KPI or Caster or Ackerman change do not forget to check your bumpsteer too.
Christian, if you are 3D printing your intake plenum, be aware that untreated it will be very porous! Make sure you know how you are going to get around this. If you plan to use paint / lacquer, make sure you test its compatibility with your 3D printed material before spraying up your final piece, or you may have the privilege of seeing it chemically disintegrate before your very eyes.. !
Claude, doesn't this depend on where your pushrod / pullrod is mounted? If mounted on the bottom wishbone / lower upright, I can't picture the mechanism you've described being very significant at all.. Out of curiosity, what kind of trouble were you seeing when this happened to you in the past?Quote:
Originally Posted by Claude Rouelle
CWA, Yeah we're aware of it :) The company who is printing it for us has a treatment they apply to seal it, we found out the hard way last year!
CWA,
Significant? Could be big, could be small? Just try. Use Catia or Solid works or any good kinematics software. Change the KPI angle keeping everything else (scrub radius, track, static camber etc...) the same for a given roll or heave motion input and look at your wheel vertical movement Vs damper movement motion ratio.
With a shorter top wishbone (and more camber shims to keep the same static camber) for a given amount of chassis heave
- the trajectory of your top wishbone outboard point will be different
- the wheel center z and y movement for a given amount of chassis heave will be different
- the camber variation for a given amount of heave will bee different
- the trajectory of your pushrod pickup point of bottom wishbone (or on the upright) OR the trajectory of your pullrod pickup point of top wishbone (or on the upright)
all these accumulated little things will give you a different damper movement for the same amount of chassis heave
We changed the KPI mainly for camber variation in heave and in steering reasons. There was any "troubles" other than the car did not react the way we thought it was going to react simply because we did not have the wheel rate (or the wheel rate curve) we thought we had. In fact if I remember well by changing the KPI from 9 to 12 degrees, it changed the motion ration 12 % ...which changed the wheel rate by 25 %
PS Ds you see the long thread I wrote last 03-13 about all the causes of the compliance and how to eliminate its cause instead of working on its consequence? It is mainly because of your questions in your previous thread that I wrote that.
Since all is quite on the Forum front, I thought I'd add this quick note.Quote:
Originally Posted by ChristianChalliner
The MAIN ADVANTAGE of the "wishbone-shaped" wishbones shown below is that they allow the upright-BJs to be closer to the centre-plane of the wheel, whilst also having a wide mounting base at the chassis, than is possible with the more conventional "A-shaped" A-arms.
At the front-wheels this enables better steering-geometry, including greater possible steering range (ie. lock-to-lock), with less loads on the steering-linkage during braking. At the rear-wheels it similarly puts less loads on the BJs and toe-link during longitudinal accelerations. In both cases, and together with the wider possible chassis mounting base, this arrangement can greatly reduce adverse compliance problems. Especially the VERY-BAD-FOR-HANDLING TOE-OUT compliances!
http://www.britishracecar.com/JohnDi...ell-004-CA.jpg
Of course, there are many detail variations possible for such "Y-shaped" wishbones. Search the prior art for inspiration.
Z
Not only the dynamics is the matter but the wheel package should be also concerned especially in 10" wheels with A shaped wishbones.