Slimy38
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| posted on 3/6/24 at 07:00 AM |
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Wheel clearance below bodywork
I will start by pointing out that I'm not building a 7, it's a Healey replica that is full body. My question is around clearance for the
front wheels. I built up the suspension over the weekend, the first time everything has come together. Apart from some shocking toe problems (my rack
extenders are too long), the one question I have was related to the front wheel clearance under the arches.
As it's fixed bodywork I don't have the luxury of moving arches, so I was wondering what clearance is considered ok? There's about 1.5
inches when the car is settled on the wheels. To achieve this I've had to wind my spring seat all the way out. The clearance is fine when I turn
from lock to lock so there's nothing there, it's just height that I'm wondering about.
They're also on the thicker springs, I don't know the actual spring rate (I bought them second hand) but they're thicker than the ones
I have for the rear.
I'll try and post a photo shortly, I just need to get the photos uploaded.
(Edit). I should add that the lower arm is horizontal as it is, which I think sounds about right? Or should there be a slight drop on it? The damper
is 13" eye to eye which I believe is right?
[Edited on 3/6/24 by Slimy38]
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Fred W B
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| posted on 3/6/24 at 08:36 AM |
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You need to check you have clearance at full bump, that is with the springs removed and the coilovers sat on the bump stops.
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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Slimy38
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| posted on 3/6/24 at 09:44 AM |
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quote:
Originally posted by Fred W B
You need to check you have clearance at full bump, that is with the springs removed and the coilovers sat on the bump stops.
I don't think that would ever be possible, that would put the wheels above the bonnet surface and the chassis on the floor. But then that
suggests the coilover is just too short?
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Fred W B
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| posted on 3/6/24 at 10:22 AM |
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The wheel travel is controlled by the coilover travel. It does seem you need less travel on the coilover, or the body is mounted too low.
I wrote the below in response to another question elsewhere, but it applies here
To understand what is going on:
Assemble front end complete but without springs fitted.
Let car down to the bump stop on the coil overs, checking nothing else limits travel in the system.
Measure from a point of the chassis at the front axle line to the ground = Dimension A
Jack car up to the top limit of the coil over travel, checking nothing else limits travel in the system. Use a piece of card under the wheel to check
when it just lifts of the ground
Measure from the point of the chassis to the ground = Dimension B
Subtract Dimension A from Dimension B = total suspension travel
Refit the front springs (of a known rate) and snug the coil over adjuster up until the spring is just held in place.
Let the car down until it sits at a ride height.
Measure from the point of the chassis to the ground = Dimension C.
Difference between dimension B and dimension C = static deflection. You want static deflection to be about 40 to 50 percent of total travel.
The percentage difference between actual deflection and desired deflection gives an indication on how to change the spring rate.
You need to be happy with the ground clearance dimension A at the fully deflected state. If not something else needs to change, not just the spring
rate.
The spring length needs to be such that the coilover reaches the bump stop before the springs go coilbound in compression.
The ride height can be adjusted by moving the spring perch but this does not change the spring rate, only the ride height.
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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Slimy38
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| posted on 3/6/24 at 10:40 AM |
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Some very useful guidance there, I will follow the procedure and see what I'm working with. Just on this one though;
quote:
Originally posted by Fred W B
The spring length needs to be such that the coilover reaches the bump stop before the springs go coilbound in compression.
I can guarantee the spring will be coilbound before it gets to the bump stop. I've just had another look and while there is only a short amount
of travel, it's still more than the gaps between the spring coils. I'll get the spring off and start with the bump stop measurements, see
what's going on there.
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Fred W B
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| posted on 3/6/24 at 10:47 AM |
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Some people fit nylon packers under the bump stops to reduce the coilover travel in a pinch.
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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Slimy38
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| posted on 3/6/24 at 12:16 PM |
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quote:
Originally posted by Fred W B
Some people fit nylon packers under the bump stops to reduce the coilover travel in a pinch.
That might be an idea, thanks. To be honest I'm more worried about how little movement there is under extension, with the spring wound up so far
it's already compromising the ability of my car to drop into a pothole. And we all know how important that is nowadays...
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Slimy38
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| posted on 3/6/24 at 05:12 PM |
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Well, that was certainly an experience!! As expected, I can't actually drop the car down to it's bump stop because I'd never be able to
get the jack underneath to lift it back up again.
However, I did put the wheel on a fixed platform, and got the measurements. Dropping the car on to it's bump stop left me with 5 inches
clearance, and lifting it to a point where the wheel skids on the floor got me 10 inches. So that is 5 inches of suspension travel.
Staying with the lifted wheel at the same height, dropping it down on the spring brought it down to 7 inches. So that's 3 inches of deflection,
with a total travel of 5 inches that's me up to about 60% of deflection, so I guess I need a 20% stiffer spring?
I'm not overly worried about that for now though, I can go stiffer if needed but I kind of want a soft-ish ride. What's more worrying is the
bump stop ride height. Given the platform for the wheel was 2.5 inches high, it means the chassis would be 2.5 inches off the ground if it hit the
bump stop. The front valance is 1.5 inches below that, and the sump is about the same. So basically if the car did pop a spring or I decided to really
go for the apex, I'm losing the body and the engine in a big streak of oil and GRP on the tarmac. And that's all ignoring the damage
it'll do to the wheel arch on the way down/up;
I guess it's new damper time. How do I work out what length I need open and closed?
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Slimy38
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| posted on 3/6/24 at 06:39 PM |
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The other option that has been suggested is moving the mounting point. I don't want to move the mounting point on the suspension arm as I think
that would increase stresses. But I could replace the bracket on the chassis. That would take care of the ride height, the over-extended coilover and
the bump stop height all at once...
Anyone suggest a reason why I shouldn't start down this path?
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adithorp
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| posted on 3/6/24 at 07:28 PM |
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What's your chassis? Surely the suspension is designed with ride height based on the suspension design. Calculations should start from that.
Once you've sorted that then the body should be positioned accordingly not the other way around.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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Slimy38
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| posted on 3/6/24 at 09:12 PM |
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quote:
Originally posted by adithorp
What's your chassis? Surely the suspension is designed with ride height based on the suspension design. Calculations should start from that.
Once you've sorted that then the body should be positioned accordingly not the other way around.
Chassis is Haynes with the Saturn mods for MX5. There's not much option for body positioning though, it has mounting edges that fit to the
chassis bottom edge. The only adjustment I've needed to do was shorten the length by a couple of inches, the height is as it's been
designed.
I am wondering whether the body design requires the whole chassis to lift up, or if mine is sitting too low because of the coilovers.
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