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 Post subject: Strut suspension bad?
PostPosted: July 13, 2006, 5:33 pm 
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Joined: July 11, 2006, 5:09 pm
Posts: 21
Would it be a "bad" idea to try and use a Macpherson strut design in front instead of double A-arm like most everyone does? Is the Macpherson a "worse" design? I ask because I have these parts readily available.

Will


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PostPosted: July 13, 2006, 6:15 pm 
I don't know if 'bad' is the right word to use. The biggest problem I see is that a strut is quite tall, it'd be hard to find one that would even look semi right on the front end. But then again if you don't really care about aesthetics, or going with something that's been proven, I say go for it. However, a question that comes to mind is what are you going to mount it to on the bottom?

--JOsh


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PostPosted: July 13, 2006, 7:28 pm 
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Bottom I would mount to the control arm. Mini Cooper struts/shocks are surprisingly small so I thought those *might* work. I dunno. Just brainstorming. What are the pros/cons of struts and double A arm?

Will


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PostPosted: July 13, 2006, 7:53 pm 
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Joined: August 15, 2005, 10:13 pm
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Location: Charleston, WV
Strut style suspensions have little if any camber gain with compression which is why cars with double A arm designs will most always out-handle them. Strut equipped cars also don't have the amount of alignment adjustability offered by a double A arm design.

Stated simply, struts were designed to simplify construction and make it cheaper to make cars. Look at all the true sports cars out there and they don't have struts, but every economy car on the road will. They are also used because they make more room for a front wheel drive drivetrain between the front tires.

Not to mention they would look retarded on a Locost as they require a strut tower up top for mounting.


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PostPosted: July 13, 2006, 7:55 pm 
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Okie doke. Thanks for the info chetcpo.

Will


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PostPosted: July 13, 2006, 8:11 pm 
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My motto is "Just say NO to MacStruts."

Struts require a fairly high Steering Axis Inclination (SAI) to get the strut out of the wheel - in the neighbourhood of 10 to 15°. While this alone isn't bad, when the wheel is turned in (torward the centre of the car), the spindle will go from a horizontal position, to pointing down - producing positive dynamic camber. This is not good. Another option is to move the entire strut further inboard to reduce the SAI, at the cost of Scrub Radius and the resulting high steering effort. ("Cut" struts turned into uprights have the same problem as full struts with respect to SAI and camber loss).

One way to offset this positive camber gain is to add more caster. Generally run have the caster that you have SAI. So, if your struts have 10° SAI, you want to run about 5° caster.

Using camber plates, as so many "tuner" folk do, only exacerbates the problem as sliding the top of the strut inward increases SAI, thus requiring more caster to compensate.

Caster "lifts" (or "unloads") the outside wheel on a turn, which may cause the inside rear to pick up or at least unload to a degree. This may increase wheel spin and other issues. You will want a very compliant rear suspension on a RWD vehicle.

Another issue with struts is on roll there is little to no negative camber gain. If a car is lowered and the lower control arms are no longer horizontal, the suspension can even go positive camber in roll. This prevents the car from using the full contact patch, thus reducing traction.

The solution is twofold: 1) Increase static negative camber such that the wheel is vertical in roll (my FSP Nissan Sentra ran 3.5° negative camber in order to keep the tires flat in a corner) - this costs you in acceleration and braking power. 2) Stop the strut suspension from doing what it shouldn't by running very stiff springs and/or very stiff sway bars. Of course, this can cause other issues. There is no free lunch.

Having said that, struts do work. BMW, Porsche and even Lamborghini use struts. BUT, neither NASCAR nor F1 use them. They are simple, and cheap to produce.

Double wishbone, specifically short-long arm suspensions, allow much more desireable control of the wheel in roll. By using different length arms (short upper, long lower), you can cause the top of the wheel to tuck in in roll, such that the tire remains vertical in a turn, maintaining contact patch for acceleration, turning, braking, smiling, etc.

After having tried to get a MacStrut car (B13 Sentra) perform better than a double-wishbone car (EG Civic), I'll never go strut if I don't ~have~ to. Just say no.

Does that help at all?

G

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PostPosted: July 13, 2006, 8:24 pm 
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Yea that explains a lot. My BMW has struts in the front and a lot of people have the problem of losing camber in hard corners. What is the "desired ball joint rate" ? I am referring to this page that I am playin around with.
http://www.stockcarproducts.com/pstech9.htm

Will


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PostPosted: July 14, 2006, 3:49 am 
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Location: Sunny-Okanagan, Canada, eh?!
I use the term "Wheel Rate" which is the effective rate of the spring acting on the wheel. I run a 320lb spring, yet because of the mounting the spring is acting as if it was 140lbs right on top of the wheel. thus, my wheel rate is 140lbs/in.

Ball Joint Rate is the same thing, only measured at the ball joint.

There are many different philosophies about what wheel rate to run. Some like to shoot for 1/2 the corner weight, for example. I like to calculate it based on suspension frequency, which sets the spring rate for "ride quality," if you will. Then use sway bars to tune the handling.

G

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PostPosted: July 14, 2006, 10:19 am 
I worked through all the mathematics and physics and wrote it up for my webpage. You will need to use internet Explorer to view the equations.

http://members.cox.net/enderw88/springratetheory.htm


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PostPosted: July 22, 2006, 5:54 pm 
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Joined: July 22, 2006, 4:30 pm
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I`ve been messing with Fox bodied Mustangs for a while and can tell you that Strut type suspension can and has quite often been made to handle as well as A-arm suspension in racing situations for short races.
The bad thing is you need so much negative camber to do it that tire wear is extreamly bad.


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PostPosted: July 22, 2006, 5:58 pm 
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"Handle as well" as IRS, maybe, as in cornering. But the large static camber will cause trouble with braking and acceleration due to the tires not having full contact with the road.

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 Post subject: Using strut knuckles
PostPosted: October 1, 2006, 8:10 pm 
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Joined: August 28, 2006, 9:26 pm
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Location: Ontario, Canada
This might be somewhat of a newbie question, but here we go. Most discussion on front suspension seems to revolve around using Fiero/Acadian knuckles to get double A-arm suspension. However, would there be any issue with using the knuckle from a Macstrut suspension using one of the strut bolting points in a manner like the photo below?(same thought applies to front or rear)

Image

Biggest problem I can see is trying to isolate it with a bushing, although I suppose you could potentially use rod ends on either side of the knuckle, and bushings at the inboard side?

Just something I've been curious about.

Thanks,
Steve


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PostPosted: October 1, 2006, 11:53 pm 
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Joined: August 14, 2006, 2:09 am
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Location: Sacramento, CA
I think the only problem with that idea is finding an upright that has the pivot points in the correct positions. If you used the strut connection to make a bracket that allowed you to specify the pivot points. Then that kinda defeats the purpose of using a mac strut knuckle


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PostPosted: October 2, 2006, 1:21 pm 
If you get a chance, look at one of the older Toyota tube frame SCCA race cars. They were fitting rules and ran very short macpherson struts and very long lower control arms. They were very fast cars and worked quite well on a race track, but I see them not having enough suspension travel for street cars. The design was very inventive though.


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PostPosted: October 2, 2006, 7:40 pm 
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Location: Ontario, Canada
THAWA wrote:
I think the only problem with that idea is finding an upright that has the pivot points in the correct positions. If you used the strut connection to make a bracket that allowed you to specify the pivot points. Then that kinda defeats the purpose of using a mac strut knuckle


Ok, yeah, that makes sense. I guess I'll figure out my suspension design, and then see what's available that fits the correct dimensions (and then still probably go back to the Fiero parts)

Steve


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