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Author: Subject: Rear wishbone head scratching
J666AYP

posted on 13/8/19 at 08:13 PM Reply With Quote
Rear wishbone head scratching

Hi gents,

Just going over the rear suspension on the project. As I have pretty much thrown the book plans in the bin and set it on fire I'm now left with a headache trying to sort out the rear end.

The chassis was built for a live rear axle and means I'm using the diff from the merc donor and planning IRS I have built a frame/cage for holding the diff. My plan is to use this frame to mount my rear suspension mounts on... seems simple so far yeah...

The bottom of the frame won't be a problem for the mounts, however to use shorter top wishbones on the top I will need to stagger the mounts (one mount will be about 2" further back than the other. Will this cause a problem? Or can I just use equal length rear wishbones to make the whole process simple? I know unequal length top/bottom wishbones is preferred but will it really make that much difference when there is less than 5" of overall suspension travel?

Or would having a lower wishbone and multiple point top arms be a better option?

Jay

[Edited on 13/8/19 by J666AYP]

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907

posted on 14/8/19 at 02:30 AM Reply With Quote
quote:
Originally posted by J666AYP
Hi gents,


I have built a frame/cage for holding the diff.

The bottom of the frame won't be a problem for the mounts, however to use shorter top wishbones on the top I will need to stagger the mounts (one mount will be about 2" further back than the other. Will this cause a problem? Or can I just use equal length rear wishbones to make the whole process simple? I know unequal length top/bottom wishbones is preferred but will it really make that much difference when there is less than 5" of overall suspension travel?

Jay

[Edited on 13/8/19 by J666AYP]




IMHO

Difficult to say with no photo's (did look at your archive) but I would not stagger, or have equal length.

I would build out from the top of the "diff box" (triangulated) to make the box wider at the top and mount the top wishbone on that. \[_]/

My own rear top wishbones are the same length as my front tops although they have rod ends instead of the Transit link.

Don't laugh, but I made a scale model of my suspension with lollypop sticks. I said don't laugh.


Put some pics up and you'll get a more accurate response.


Paul G





Member of the Suttol Owners Club, the MX5 Owners Club and the BMMC

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Camber Dave

posted on 14/8/19 at 07:35 AM Reply With Quote
Ideally both front and rear lower wishbones should be as long as practical, mounted parallel to the car centre line and parallel to the ground.

The upper wishbones dictate the geometry and therefore the handling characteristics of the suspension.
A rear upper arm should be about ¾ the length or the lower one and have a slight angle up to the wheel. This keeps the wheel upright as the car rolls in the corners, but also fairly upright as the car squats during traction.

If your upper arm has different ‘leg’ lengths it will rotate about an axis angled to the lower one. This will jam up if using bushes and with rose joints, introduce a complex alignment to the rear upright. It will probably break an arm as it is introducing a twisting force to the upright.

As 907 says, remake the diff frame to get both arms on the same axis. Of course both could be angled to the centreline BUT model it somehow to check camber change in bump and roll but also more importantly toe change.

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SPYDER

posted on 14/8/19 at 06:22 PM Reply With Quote
Useful visualisation/calculation tool...

CLICK HERE

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AdamR20

posted on 14/8/19 at 07:45 PM Reply With Quote
http://www.vsusp.com is great too You can just copy / paste the URL to 'save' a setup.
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J666AYP

posted on 15/8/19 at 09:44 PM Reply With Quote
Hi gents,

Thanks for the info, would have responded sooner but work has been trying to ruin my life again.

I have added to the diff cage to accommodate unequal length wishbones, now I'm just waiting on the brackets to turn up in the post so I can start fixing them in place. Have taken this time to do abit of strengthening and triangulation to help me in the future. Will update with photos on my build page as/when I get things moving.

Jay

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