Drop Spindle Discussion - Page 17

scott.venables · #**241** (**permalink**) 08-09-2007, 01:51 AM

Thanks James. I'll run the calcs again with that formula and see what I get. Your formula is exactly the same except the spring constant is different. Are the units imperial (lbs/in, in)?

gtv27 · #**242** (**permalink**) 08-10-2007, 01:05 AM

re the offset thing, to get you head around the impact on the lever action on the torsion bar, imagine two scenarios (both extreme and hypothetical only for the purposes of understanding the geometry).

One where the offset is 3000mm positive and therefore the centre of the wheel is considerably inboard and another where it is 3000mm negative and the centre of the wheel is therefore considerably outboard

(if I got my negatives and positives mixed up, sorry, but you're smart and will work it out).

One scenario has a huge amount of leverage, the other doesn't. Which do you think has the softest wheel rate? Differences in reality are probably academic only.

In terms of ride height - offset itself shouldn't have an impact - the height of the axle will be the same regardless of the wheel offset (which is a side to side thing, not an up and down thing) - however using the extreme example, lots of negative camber and lots of negative offset (ie wide track) would lower the car. In the real world, it can't possibly have much impact.

Now, where's my calculator, I have some spring rates to work out!

scott.venables · #**243** (**permalink**) 08-10-2007, 05:40 AM

Hi Jason. I've thought and thought about this and I'm still not convinced. My main thing is that although the contact patch will be moved, there is still a force acting vertically under the bottom balljoint. This force is always vertical (or close enough) regardless of where the contact patch is.

I think about a 'T'. If you invert it, and the vertical part (the long bit) is fixed so that it can only move vertically, and you now apply a force under either ends of the top(crossbar?) the force acting vertically on the long part will be exactly the same regardless of where on the crossbar you push. That's how I rationalise it.

Ride height should change with offest if the wheel rate does. (the same weight or a 1m lever will deflect more than a the same weight on a .5m lever). I do agree that practically it probably won't make enough difference to worry about, but if you're trying to figure out torsion bar equivalencies, you need to get the formula right. I'm a practical person so I might try and experiment this weekend with it and see.

Cheers, Scott

gtv27 · #**244** (**permalink**) 08-10-2007, 04:34 PM

Hi Jason. I've thought and thought about this and I'm still not convinced. My main thing is that although the contact patch will be moved, there is still a force acting vertically under the bottom balljoint. This force is always vertical (or close enough) regardless of where the contact patch is.

I think about a 'T'. If you invert it, and the vertical part (the long bit) is fixed so that it can only move vertically, and you now apply a force under either ends of the top(crossbar?) the force acting vertically on the long part will be exactly the same regardless of where on the crossbar you push. That's how I rationalise it.

OK, I think I understand where you are coming from with the upside down T. What changing the offset will do is either shorten or lengthen the cross bar, giving a change to the effective lever ratio. So while the force is acting verticaly on the ball joint (or the vertical bit of the T), it can have a greater impact as a result of the wheel offset giving it more mechanical advantage (hypothetically speaking if the wheels stick out heaps)

Ride height should change with offest if the wheel rate does. (the same weight or a 1m lever will deflect more than a the same weight on a .5m lever). I do agree that practically it probably won't make enough difference to worry about, but if you're trying to figure out torsion bar equivalencies, you need to get the formula right. I'm a practical person so I might try and experiment this weekend with it and see.

Aha, I hadn't thought about the impact of the weight of the car on the ride height, just the geometry. You are right, the big outboard offset wheel would have the car sitting a bit lower due to the increased leverage on the spring - the ride height would reduce (in theory). I think that helps confirm the theory from an independent line of thought!

scott.venables · #**245** (**permalink**) 08-14-2007, 07:07 AM

Hmmm. What I was trying to say with the "T" thing was that if the vertical part was fixed so it could only slide vertically, and you then applied a force on the crossbar, it wouldn't matter where on the crossbar the force was applied- at either end or in the middle right under the vertical support- the vertical force acting on the vertical support would always be the same.

This is the same as with the lower balljoint /upright. The vertical support of the "T" is the spindle and the crossbar is the plane that the contact patch travels along when you change wheel offset. The upright can only move in a (roughly) vertical motion, and vertical force on the upright = vertical force on the lower balljoint. Therefore the lever length is to the balljoint, not the contact patch.

On the maths, the conversion from wheel rates to spring rates at the shocker mount I got wrong because it's not a simple ratio on lengths,

Wheel rate = Spring stiffness / (motion ratio)^2

This means that my calculations are even further out.

Scott

gtv27 · #**246** (**permalink**) 08-15-2007, 04:14 AM

Quote:

Originally Posted by scott.venables

it wouldn't matter where on the crossbar the force was applied- at either end or in the middle right under the vertical support- the vertical force acting on the vertical support would always be the same.

This is the same as with the lower balljoint /upright. The vertical support of the "T" is the spindle and the crossbar is the plane that the contact patch travels along when you change wheel offset. The upright can only move in a (roughly) vertical motion, and vertical force on the upright = vertical force on the lower balljoint. Therefore the lever length is to the balljoint, not the contact patch.

Scott

Echo Leader · #**247** (**permalink**) 08-15-2007, 05:03 AM

Motion ratio shouldn't apply to a TB-based wheel rate calculation. You're basically including a motion ratio-esque factor by measuring the distance to the wheel contact patch and including that in your equation as the length of your lever arm. This takes the TB "spring rate" calculated with no lever arm length and reduces it a certain amount based on the distance from the centerline of the TB to the wheel contact patch.

The motion ratio applies when you have a coil spring mounted at a point on the LCA that is closer to the body then the wheel. Thus the spring compresses less then 1 inch per inch of wheel movement.

scott.venables · #**248** (**permalink**) 08-15-2007, 05:33 AM

Yeah James I only used the motion ratio when converting to torsion bar equilvalents ie. amount of coil spring needed at shock mount to give same wheel rate as XXmm torsion bars.

Re offset, I may have to try an experiment and take some photos. Am I all alone on this theory?

Echo Leader · #**249** (**permalink**) 08-15-2007, 01:15 PM

Oooh sorry, you're right. I should have read your previous post more closely. My fault.

Grant · #**250** (**permalink**) 08-15-2007, 08:04 PM

I have a question for anyone else running the drop spindles:

On full lock, did you have problems with the anti roll bar pick up point on the LCA coming in contact with the inside of the rim? I've been rubbing but it's really bad now b/c the wheels were balanced with the knock-on weights, not the glue on type. If I rub, my weights will be whacked off.

Grant · #**251** (**permalink**) 09-14-2007, 10:12 PM

tuoracing.com has actually mapped out the camber and toe curves of their GTV6! I'm emailing the author of the site to see if they have any spindle changes or what not, but it is interesting anyways.

It's important to note however that at the same ride height, the GTV-6 LCA's are pointed even more towards the sky than a Milano in my experiences.

http://www.tuoracing.com/files/GTV6_...ber_curves.pdf

Echo Leader · #**252** (**permalink**) 09-15-2007, 09:58 AM

That's awesome, I need to digest that information now...

Concerning the GTV6 vs. Milano LCA angle: How are you measuring the ride height? If your measuring from the ground the some point on the chassis, did you factor in the diffence in outer diameter (OD) of the tires? If you're OD is larger, in order to get the same 'chassis to ground' ride height, you'll have to cant the LCAs more 'up'.

Just a thought...

Grant · #**253** (**permalink**) 09-15-2007, 10:16 PM

I'm using the same wheels and tires! Lolz.

Seriously, my 15's that I had on when I noticed the LCA height are now on the GTV-6 because 15's don't work with my drop spindles.

Echo Leader · #**254** (**permalink**) 09-17-2007, 07:22 AM

Quote:

Originally Posted by Grant

I'm using the same wheels and tires! Lolz.

Seriously, my 15's that I had on when I noticed the LCA height are now on the GTV-6 because 15's don't work with my drop spindles.

Grant · #**255** (**permalink**) 09-22-2007, 01:59 PM

One more thing, I drove Potenziato's awesome 24valve Milano with RSR coilovers. I knew the spring rate would be quite high, therefore a lack of body roll, but the amazing thing was how smooth the ride was! His car soacked up little to medium bumps with aplomb.

So, my car has far less spring rate, and rides worse! I think this mainly has to do with the shocks (which performed great at the track), but if you can get high wheel rates and a smooth ride it is very hard to ignore for use as a daily driver..