Wheel Travel on IFS

[scrimshaw]

Hope you experts can help me once more with the project I am working on.

I am installing an independant front suspension based on the Mustang II. This is an aftermarket unit built for hotrods etc something like this one.

http://www.totalcostinvolved.com/productdet.cfm?prodID=733

After installation I have noticed that when the lower a-arms are parallel to the floor (the suggested ride height) wheel travel on compression is only about 1 and 3/4 inches before the shocks are fully compressed, with about twice that length available below ride height. According to the tech help at the company this is normal but it doesn't seem like anywhere near enough travel for a street car to me.

Can anyone confirm this is OK or am I able to drop the A-arm a little at ride height to give some extra travel on the shock.

Appreciate all comments.

 

[GregLocock]

45 mm of jounce travel is a bit light on for realistic street performance with reasonably soft springs. If the rest of your suspension geometry is good then I'd be inclined to drop it by 3/4".

Incidentally, keeping the lower arm near horizontal is a good idea, generally, at the driven condition, not kerb weight.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[scrimshaw]

Thank you for the reply Greg.
Your point about 'driven condition, not kerb weight' is duly noted, and I assume you mean with driver, passenger, cargo, fuel etc. I can only assume the rest of the suspension geometry is good as my knowledge is pretty limited.

Which leads me to another question which I should ask. When I first planned components, body, chassis etc I calculated a srung to unsprung ratio of about 15% but in reality it is becoming more like 20% and maybe even higher than that. I am assuming a harsher ride will result - but is there anything else I should be aware of?

Thanks once again for your time.

 

[GregLocock]

I don't know how you got it that high, typically an IFS has an unsprung mass of 80 to 110 lb per corner.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[scrimshaw]

I am referring to the complete car not just the IFS

Wheels and Tyres are 80lbs combined - 320lbs
IFS is 50lbs (hubs and brakes) a side.
Rear live axle is 200lbs without leaf springs.
So at least 620 for unsprung.

Chassis is 300, body is about 600 (composite) including fuel tank, seats, battery etc.
Eng/trans/ exhaust and other eng bay stuff will be about 1000.
Can't imagine other parts like bumpers, lights being more than a couple of hundred.

Unless I have made a large error somewhere (always possible!!) I come up with an approximation at most 3000lbs total with at least 620lbs unsprung.

 

[GregLocock]

Ok, you must have some sort of truck, those are all high.

Not much you can do about it cheaply. What are you going to use this thing or?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

"Unless I have made a large error somewhere (always possible!!) I come up with an approximation at most 3000lbs total with at least 620lbs unsprung."

Try working the sprung:unsprung ratios for each corner (or at least each end) separately. One end isn't nearly as bad, the other is much worse.

I can't imagine an 80# wheel/tire assembly that wasn't at least 20" or where the wheels were really heavy, blingy chrome-plated steel ones. I sure wouldn't want to have to change or rotate them very often.

There really isn't a whole lot that you can do about the mass ratio at the live axle end (especially if you retain the leaf springs in steel). The lighter the total car weight (and presumably its rear weight) gets, the worse the ratio will go. 300-ish unsprung out of 1300 or so total . . .


Norm

 

[scrimshaw]

Thanks for replies. I have accepted the large ratio was just making sure my only problem might be a harsh ride, I didn't want any nasty surprises.

It's not a truck just a car on a truck like chassis, not my design and components had to be chosen on what was available not what limitless money could buy. The wheels are daytons 18x8 inch spokes (no bling!) and the tyres for SUV-- the combined weight surprised me as well. My only other option was a very narrow wheel with bias ply tyres which I did not want. I was thinking an unladen truck must have a pretty high ratio and rides fairly well I was hoping for something similar.

Another unrelated question I have is about setting up approximate caster with a digital level. I was told I could do it by measuring the camber angles with steering turned 20 degrees one way then the other then subtracted the smaller number from the other (or if one number is + and the other - then adding them) Then multiplying by 1.5 and that gives castor. Can someone confirm this is correct?

 

[NormPeterson]

For a 20° right to 20° left steering sweep, 1.5 times the algebraic difference in the camber measurements will give you an approximate (but close) value for caster. There is a way of keeping the signs straight if you're close to zero caster.


Norm

 

[GregLocock]

That's a neat trick

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.