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LLTD Worksheet

Suspension Tuning & Optimization

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I have a couple questions:

1) The spring stiffness has only one input for front and rear, how do you account for different spring rates left to right?

2) Similar question regarding static weight, how do you account for a car with higher left side weight percentage?

Hi there, the sheet is currently optimised for a road race application where the performance in left and right hand corners is equal, hence the single spring rate entry. What is your application?

Hello Andre, left turn only asphalt.

Given a 500 lb/in RF spring results in 1" of travel at steady state.

Changing to a softer sway bar results in 1 1/4" of travel at steady state (i.e. an increase of 125lbs). Additionally, the LF is likely compressing further due to increased roll.

This is somewhat counter intuitive as it looks like we're increasing the RF load transfer by 125lbs given the additional 1/4" of travel. However, my understanding is the softer bar has reduced the front roll resistance (i.e. more front roll vs rear) and effectively reduced the wheel load at the RF contact patch.

Is my understanding correct?

Apologies for the slow response to your question. I've passed this on to Tim and asked him to reply here. Thanks for your patience.

Jumping in on this RDAF, your understanding is correct - for all other things being equal, if you make a change to the front ARB to reduce its stiffness this will result in more front roll angle and less lateral load transfer on the front axle. This will result in the total Lateral Load Transfer Distribution moving towards the rear. From a tyre vertical load sensitivity perspective only, as we went through in the course this should result in moving the balance moving away from understeer and more towards the oversteer direction.

Obviously, you're going to hurt rear traction at the same time by making the inside rear tyre more lightly loaded - reducing FR vertical load also reduces RL vertical load. But I know you circle track people understand diagonal loading better than most!

In this situation for a circle track car, yes the FR spring deflection increases, which necessarily means it has a higher reaction force in the spring than it had with the stiffer FARB. But the overall front lateral load transfer for the front axle has decreased.

A nice addition to the LLTD calculation spreadsheet I put together might be to show the resulting wheel, spring and ARB deflections and forces as well. Maybe this would help with people understanding the system more completely? At the same time, this can be used to calculate things like bump stop gaps etc which is another nice possible extension.

Lastly, the spreadsheet I put together makes the assumption of a symmetric setup (road course). So, in its current incarnation, it's not designed to deal with an asymmetric setup. This applies both to the spring rates and static weights you mentioned.

Hope that helps, let me know if any of that is unclear!

Hello Tim, thank you for the thorough explanation. LTO is definitely asymmetric and diagonal load conscientious as you noted. Optimal cross % vs front / rear % is yet another topic I need to pick your brain on :o).

Adding the additional calculations to the LLTD sheet sounds great. I believe the increased left side static weight percentage is likely the most significant factor that would be helpful to have accounted for in the sheet. Not sure how you go about that -- I couldn't find much when goggling for calculations. I'm thinking the bicycle model somewhat aligns better with you symmetric based road racers.

In regards to spring, ARB, and R/C changes and seeing the affect on shock travel, are you aware of any approach to guesstimate where the load moves to? In other words, if you take my original post where a change resulted in a 1/4" more travel on a 500 lb/in spring, can we make an assumption that we've moved 125lbs of load from front to rear?

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