Trevor Booth
Lifetime Supporter
Bob,
100% correct as your two lower links are co planar and you should not get bump steer.
100% correct as your two lower links are co planar and you should not get bump steer.
rear suspension setup
- Thread starter egoman
- Start date
Trevor Booth
Lifetime Supporter
Jack,
the configuration of suspension design is the designers personal choice whether it be reversed lowe A arm or twp parallel lower links.
Current trend is an upper and lower wishbone with a toe link so that bump steer can be introduced. Much like a front suspension with the toe link replacing the steering arm.
You could change a reversed lower A arm to two parallel links and it has been done before. The before and after configuration would dictate what effect it may have on the cars handling
the configuration of suspension design is the designers personal choice whether it be reversed lowe A arm or twp parallel lower links.
Current trend is an upper and lower wishbone with a toe link so that bump steer can be introduced. Much like a front suspension with the toe link replacing the steering arm.
You could change a reversed lower A arm to two parallel links and it has been done before. The before and after configuration would dictate what effect it may have on the cars handling
Thanks for your reply, Trevor.
I asked because I own a sports racer someone built using uprights+suspension from a mid-1970's Formula Atlantic(Lola) which originally used the reversed lower a-arm rear suspension locating link. In my car, they replaced it with parallel lower links, leaving the rest of the rear suspension the same as original. It seems to work fine(i.e., no apparent toe change during suspension travel).
A followup question: is there any reason to run caster in such a rear suspension(parallel lower links, single upper link, and U/L trailing arms)since you don't need to correct for toe change?
Jack
I asked because I own a sports racer someone built using uprights+suspension from a mid-1970's Formula Atlantic(Lola) which originally used the reversed lower a-arm rear suspension locating link. In my car, they replaced it with parallel lower links, leaving the rest of the rear suspension the same as original. It seems to work fine(i.e., no apparent toe change during suspension travel).
A followup question: is there any reason to run caster in such a rear suspension(parallel lower links, single upper link, and U/L trailing arms)since you don't need to correct for toe change?
Jack
Trevor Booth
Lifetime Supporter
Jack,
no particular reason to run caster unless space considerations
and not a good idea unless inboard pickups for lower links are also castered
( see Bob P post)
I have converted a few early racers to parallel links for people.
every body has the "best" idea so they make their own choice.
no particular reason to run caster unless space considerations
and not a good idea unless inboard pickups for lower links are also castered
( see Bob P post)
I have converted a few early racers to parallel links for people.
every body has the "best" idea so they make their own choice.
Trevor Booth
Lifetime Supporter
Jack,
yes they were better as the original Rev A was bad to start with.
Having said that, either type can give the right solution that the designer is looking for.
me personally I prefer a system that can provide control of bump steer. Bump steer is not a bad thing , excesssive can be bad.
Drivers preference, vehicle layout etc etc all go to make up the "package" i.e. you cant just look at one part in isolation.
different drivers in the same car may, and usually do, have a totally different setup.
yes they were better as the original Rev A was bad to start with.
Having said that, either type can give the right solution that the designer is looking for.
me personally I prefer a system that can provide control of bump steer. Bump steer is not a bad thing , excesssive can be bad.
Drivers preference, vehicle layout etc etc all go to make up the "package" i.e. you cant just look at one part in isolation.
different drivers in the same car may, and usually do, have a totally different setup.
No I had not cosidered it as I dont run caster on my car but thinking about it would change bump steer due to different heights on the outer lower lateral mounting points and let us assume parallel mounting points on the inner.
Then again you could use it to you advantage.
Always learning.
Jim
Trevor Booth
Lifetime Supporter
true
experience is the adventure from which we gain knowledge
experience is the adventure from which we gain knowledge
The way to visualize the approximate toe change you get with a reversed-A/trailing arm is to imagine that the A-arm and trailing arm are a single rigid unit. Now add an imaginary pivot axis between the chassis attachments. As the suspension travels up and down, the pieces rotate around the axis. Project the moving hub-carrier axis into the horizontal plane, and you will see any toe change.
The extreme visualization is that with the LCA at 90 degrees up or down, the hub carrier axis will be parallel to the inner pivot axis. Lots of toe change if the axis is at an angle to the car's centerline.
Conversely, the closer that you can move the the trailing arm axis to directly in front of the lower A arm axis, the less the toe change. That's why, on the original GT40, the trailing arms were very long and as close to the center of the car (while allowing enough engine room) as they could be. All of this to bring the imaginary (inner) pivot axis as parallel to the car's centerline as possible.
The extreme visualization is that with the LCA at 90 degrees up or down, the hub carrier axis will be parallel to the inner pivot axis. Lots of toe change if the axis is at an angle to the car's centerline.
Conversely, the closer that you can move the the trailing arm axis to directly in front of the lower A arm axis, the less the toe change. That's why, on the original GT40, the trailing arms were very long and as close to the center of the car (while allowing enough engine room) as they could be. All of this to bring the imaginary (inner) pivot axis as parallel to the car's centerline as possible.
Trevor Booth
Lifetime Supporter
Bob,
spot on ! very good description. What some fail to recognise or have difficulty in conceiving is that the arrangement of the suspension is in fact a wide based "wishbone" in effect.
spot on ! very good description. What some fail to recognise or have difficulty in conceiving is that the arrangement of the suspension is in fact a wide based "wishbone" in effect.
Terry Oxandale
Skinny Man
This is where (for my humbling lack of experience) the easy part of this 40 suspension becomes mucked up with anti-dive angles. Does the anti-dive create a toe-out situation on compression? If so, is this why caster in the rear helps reduce this effect?
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By moving the front point for the trailing arm outward (which causes toe-in much like a semi-trailing arm suspension will do) by the same amount that this point has been moved upward (for anti-dive), I’ve in essence canceled the two effects to some degree (shifted the bump steer curve around to benefit the loaded wheel at the expense of increased toe on the unloaded wheel.<o></o>
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></o>This is where (for my humbling lack of experience) the easy part of this 40 suspension becomes mucked up with anti-dive angles. Does the anti-dive create a toe-out situation on compression? If so, is this why caster in the rear helps reduce this effect?
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By moving the front point for the trailing arm outward (which causes toe-in much like a semi-trailing arm suspension will do) by the same amount that this point has been moved upward (for anti-dive), I’ve in essence canceled the two effects to some degree (shifted the bump steer curve around to benefit the loaded wheel at the expense of increased toe on the unloaded wheel.<o
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Assuming that your toe-out prediction is correct (I can understand the headache, which I avoided), you are probably right. They raised the front lower pickup to partially compensate (in jounce only) for the expected toe-in. In most conventional situations, most of the anti-dive is accomplished by lowering the front pivot point of the upper trailing arm. Since the tire loads under rebound are less, the slip angle anomalies effect the car less.
Roll centre will change weight transfer on cornering, Not thinking it would have any effect on acceleration straight line.
That is generally adjusted by the height of trailing links and merge points to adjust squat/ anti squat.
Rear roll centre had a major effect on my 40, for the better.
Bit confused you say a lower a arm , 2 pick up points one for the arm and one for a toe link im thinking.
It will if everything was bang on it may work but it will likely bind because it wont be perfect through the whole sweep.
Your toe link will try to control it but it will bind.
Jim
The bump steer comes from the a arm design when trailing links are to high.
2 lower lateral links fixed mine.
Jim
That is generally adjusted by the height of trailing links and merge points to adjust squat/ anti squat.
Rear roll centre had a major effect on my 40, for the better.
Bit confused you say a lower a arm , 2 pick up points one for the arm and one for a toe link im thinking.
It will if everything was bang on it may work but it will likely bind because it wont be perfect through the whole sweep.
Your toe link will try to control it but it will bind.
Jim
The bump steer comes from the a arm design when trailing links are to high.
2 lower lateral links fixed mine.
Jim
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i'm just considering straight line acceleration at the moment, and braking of course.
the lower a arm is conventional with the toe link attached to the a arm so there is no bump steer. consider no bump steer for this exercise.
also consider rear caster or kingpin inclination to be either positive or negative 10 degreees. what effect if any would either have on squat/anti squat or weight transfer.
the lower a arm is conventional with the toe link attached to the a arm so there is no bump steer. consider no bump steer for this exercise.
also consider rear caster or kingpin inclination to be either positive or negative 10 degreees. what effect if any would either have on squat/anti squat or weight transfer.
