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Load Transfer / Shock Travel

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When applying the lateral load transfer calculation:

load = w * cgh * g's / tw

Using static scale weights, the calculated lateral load using the above formula is pretty far (i.e. lower) from what the actual shock / spring travel indicates in race conditions.

E.g. a 600 lb/in RF spring compressing 1.5 inches is 900 lbs. That's way more than the static load transfer calculation's result.

I understand track banking and longitudinal load from rear to front (i.e. pitch) will have impacted and contributed to the RF compression in the dynamic state. Is the RF travel / load basically a combination of track surface change, pitch, and roll, or is there other factors / forces to consider?

What are you using to give you the actual travel? If it's a simple bobbin, tie wrap, or whatever, that'll be the maximum and won't reflect bump, braking, or other forces also acting on the suspension.

What are you using for the "G" value - how are you calculating it?

Are taking the affect of the ARBs (sway bars) into account?

If you have actual logging of the travel, do you have the logs available?

How are you measuring the suspension travel? What is the suspension motion ratio (i.e how much does the shock/spring compress for 1" of wheel travel)? What is the suspension roll center height? Are you sure you have all the units correct?

If you are only measuring maximum travel during a session with an o-ring on the shock shaft, then does the car go over any curbs (kerbs), or large track compressions?

Assuming no track issues... Hard braking (or heavy trail braking) and roll-couple (spring/ARB stiffness front to rear) is probably the most likely cause of more suspension travel at maximum cornering than you were expecting. For some cars you have to add aerodynamic downforce as well.

I have a spreadsheet program I've used for years that lets you enter all the parameters and determine suspension travel at various dynamic conditions. But you need a lot of info: unsprung weights at all corners, ARB and Spring motion ratios, front & rear roll center heights, and CG Heights, static weight distribution, etc... But in the end, you can determine different spring / ARB combinations that will keep the same balance, yet roll different amounts (so now the suspension camber gain is important).

Thanks for the replies Gord and David.

For suspension travel, we have a camera directed at the shock travel indicator that we then playback and watch where we reach steady state max travel in the corner. You can visually factor out the bumps with this approach and seems pretty consistent when you watch the effects of spring / ARB changes between different sessions.

For G's we have an AIM device that collects LAT and LNG forces. Now that you mention it, I should be adding in the LON forces.

Is your spreadsheet available to use? :o)

I got the software (I'll find the name on my other computer and update this post) from Roger Kraus Racing about 25 years ago. I don't think he still offers it, but you might try contacting him and asking:


> roll-couple (spring/ARB stiffness front to rear) is probably the most likely cause of more suspension travel at maximum cornering than you were expecting.

Can you elaborate on that? That's where the question is stemming from. That is, we'll make spring / ARB / RC changes on front, spring / panhard bar height changes (RC) in rear to tune handling, as well as move static weight around. Currently, those adjustments are made from driver feedback in conjunction with what we see on shock travel, tire temps, and AIM data.

Following the balanced setup philosophy (Bob Bolles advocates) of getting the car's front and rear to their "desired" roll positions where they are in sync and not out / under rolling each other, I'm thinking that we should be able to "calculate" what the car needs for a given G force. If the shock travel is not close to the calculated load, then I'm thinking we're just tuning an unbalanced setup.

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