907
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| posted on 12/9/14 at 09:43 AM |
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Ride height. General consensus
As I've had the rear bodywork off the car for polishing I thought that now would be the time to sort the ride height.
It's been sitting at 100mm front and rear (measured at the axles) but two up this sinks to 85 r & 95 f.
After my mods to the top shock mounts it now sits 170mm (rear axle) & 120mm (front axle)
Two up that will sink to 150 & 110.
The chassis is based on the book, apart from +4 wide.
Front w/b's are book dims, although a curved shape.
Rear is IRS. My design.
Sump sits flush with chassis bottom, so no protrusion.
Does this sound about right ?
Is it now too high ?
Too low ?
Front to back ratio wrong ?
I welcome all opinions.
Cheers
Paul G
[Edited on 12/9/14 by 907]
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mark chandler
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| posted on 12/9/14 at 10:45 AM |
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4" front, 4.5" rear for me, car has a flat bottom.
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coyoteboy
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| posted on 12/9/14 at 11:24 AM |
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Suspension geometry aside (which you may have to compensate for if possible), don't you want the lowest possible height that doesn't cause
impacts?
Last time I asked about this 100mm seemed to be about the lowest generally recommended for road use.
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19sac65
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| posted on 12/9/14 at 03:59 PM |
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I asked this question to a guy who sets up race cars for a living
I was there over a hour while he explained his science but the basics were ;
The idea is to keep the tyre in contact with the road as vertical as poss
top n bottom w bones pivot up and down in an arc ( not an ark)
The longest they will be is at horizontal with the pivot point ( bush)
With the wheel is hanging down they both get 'longer' to horizontal and then get shorter
So with a level you want the lower wishbone to be appx 5mm higher at the inner bush than the the BJ end
The top wishbone needs to be level or slightly lower at the bush
That way when you corner and the car leans slightly,the lower wishbone moves up - gets longer at the bottom and the top one gets shorter - giving neg
camber
If the lower wishbone were level,or lower at the bush - when you corner it would 'shorten ' and give positive camber - which you dont
want
The same would happen if the top was set with the bush higher than the BJ-as you corner it would lengthen giving pos camber
The rear then sits 15mm higher than the front - measured from the chassis in front of the mount points
Have i explained it clear enough
Made sense to me 
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loggyboy
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| posted on 12/9/14 at 04:07 PM |
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The logic above im sure is sound, but the geometry for every car (even same models!) is different, so there is not obvious answer.
Without a full set up, i've always considered 100mm at front and 120-130 at rear is a good starting point.
[Edited on 12-9-14 by loggyboy]
Mistral Motorsport
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coyoteboy
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| posted on 12/9/14 at 04:35 PM |
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quote:
Originally posted by 19sac65
I asked this question to a guy who sets up race cars for a living
I was there over a hour while he explained his science but the basics were ;
The idea is to keep the tyre in contact with the road as vertical as poss
top n bottom w bones pivot up and down in an arc ( not an ark)
The longest they will be is at horizontal with the pivot point ( bush)
With the wheel is hanging down they both get 'longer' to horizontal and then get shorter
So with a level you want the lower wishbone to be appx 5mm higher at the inner bush than the the BJ end
The top wishbone needs to be level or slightly lower at the bush
That way when you corner and the car leans slightly,the lower wishbone moves up - gets longer at the bottom and the top one gets shorter - giving neg
camber
If the lower wishbone were level,or lower at the bush - when you corner it would 'shorten ' and give positive camber - which you dont
want
The same would happen if the top was set with the bush higher than the BJ-as you corner it would lengthen giving pos camber
The rear then sits 15mm higher than the front - measured from the chassis in front of the mount points
Have i explained it clear enough
Made sense to me
As loggy says, that's fine but that depends on the suspension geometry as I mentioned. Over an average of these cars, seems the average is
100ish but it's a starting point.
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907
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| posted on 12/9/14 at 04:54 PM |
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So far so good then.
Just to add a little more info:-
The front coil overs are set so that the springs are at maximum length.
The rears are compressed about 65%.
Am I right in thinking that if I release the rears and let the spring expand to the same as the fronts then this
will make the springs softer and so lower the rear ?
If this is the case then it would lessen the difference (at the moment 40mm) between front and back so
that it was perhaps only about 20mm.
I've just got the front mudguards to polish & fit then I'll roll it out and take a photo.
Thanks for the advice so far.
Cheers,
Paul G
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loggyboy
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| posted on 12/9/14 at 05:20 PM |
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The springs wont get softer. The only thing the platforms do is raise and lower the springs resting point.
Mistral Motorsport
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Fred W B
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| posted on 12/9/14 at 05:23 PM |
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Moving the spring seat height will not change the spring rate (how "stiff" it is) at all, only the ride height will change.
Cheers
Fred W B
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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andy188
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| posted on 12/9/14 at 05:47 PM |
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your front springs don't want to be loose/fully extended when the car is sat on the ground, when you go over a pot hole etc your spring will
come off its seat and the shocker will be taking all off the load (guess how I know lol), im still leaning how to set these cars up, been on the road
sine 2009, I jack my chassis up 2" higher than my ride height, then wind my spring platform up till I get the spring with some tension on it
between the spring seats, this is only a rough set up, you can then adjust a few mm to set wishbone angles etc from there, my front is at 120mm, rear
135mm. running 185/70/13tyres, will b changing to 185/60/13 for track days, lowering the car by 15mm approx.
andy
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907
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| posted on 12/9/14 at 09:06 PM |
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quote:
Originally posted by 907
The front coil overs are set so that the springs are at maximum length.
The rears are compressed about 65% of the available adjustment.
Paul G
I chose my words badly. Apologies.
The photo shows my front shock.
The adjustment ring & lock ring are as low as they will go, but the spring is still under a small amount of tension at full droop.
Spring compressors were used to compress the spring to enable the adjustment rings to screwed on.
The white lines show the length of the thread and therefore where the spring seat limits are.
Cheers
Paul G
 
coilover adjustment
p.s.
pleased don't be confused by the wheel laying under the car
[Edited on 13/9/14 by 907]
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JimSpencer
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| posted on 12/9/14 at 11:42 PM |
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Hi
Ok that description means you're running shed loads of preload on the front?
Using the spring platforms to drastically adjust ride height is just plain not a good idea..
Ride height needs to be set with springs and shocks of the right length.
To elaborate
On full droop (suspension hanging down, shocks fully extended) the springs would normally be just loose between the top and bottom platforms - if the
ride height is then wrong adding loads of pre-load isn't going to fix it - change the springs for ones the right length, if they're too
soft change them for harder ones. You may need to change the shock length too.
So you get springs to the right length, adjust the bottom platform so at full droop there is just zero play and this is zero pre-load - wind the
adjuster up and you'd then add pre-load to the spring.
So ride height should now be right using springs and shocks of mechanically the right length.
Preload is a tuning tool for the handling dynamics of the car and although adjusting the shock collars will adjust the ride height the knock on
effects are that you'll either have springs loose at full droopm (not a good idea) or you'll add preload - which May, or May Not be a good
idea depending on what you want the car to do.. which is a different story..
Oh and back to ride height..
As little as you can get away with depending on what roads you have near you.. but about 20mm of rake (rear higher than the front) seems to be a
normal situation.
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brum_gustavo
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| posted on 12/9/14 at 11:47 PM |
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mine sits at 140mm front and 150mm rear. Springs compress about 1/3 of their extended length when loaded. My suspension is more on the soft side,
since its for street use.
I did not shorten my oilpan, which is about 20mm lower than the chassis. The height seems ideal, it will clear most speed bums, but I am in Brazil,
road conditions are far from ideal with a few exceptions. My suspension has about 140mm travel front and rear.
Build diary: http://gustavobrum.wordpress.com/
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907
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| posted on 13/9/14 at 04:50 PM |
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Many thanks for all your replies.
Taking an average of whats been suggested the car now looks like this.
Not too high but slightly higher at the back.
Thanks for all the help.
Paul G
 
Description
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tajgreidotu
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| posted on 14/9/14 at 07:41 PM |
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quote:
Originally posted by 907
Many thanks for all your replies.
Taking an average of whats been suggested the car now looks like this.
Not too high but slightly higher at the back.
Thanks for all the help.
Paul G
Description
Beautiful!!
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