Calculating Suspension Travel
High guys, I'm curious if there is a way to calculate suspension travel on my Elise. I have the non-sport suspension setup. I am planning to put spacers on my car, but I wanted to get a good estimate of how much the wheels/tires (especially in the front) are going to tuck towards the center of the car as the suspension compresses. So, if the wheel/tire is at a certain alignment with the outside of the fender at stand still, as the suspension compresses 1" how for in toward the center of the car has the wheel/tire gone, vs. if the suspension compresses 2" . . . .
Thanks for any help you can give.
Thanks for any help you can give.
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Hey Soks, that sounds like bump steer. I've not looked into it (you're really opening a can of worms when you start tweaking the suspension without consistent testing conditions good data acquisition IMHO) but you should be able to do a search on here for it. With that being said, spacers won't change the bump steer since you aren't changing the angle of the upright
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Yes, I don't plan on changing any geometry besides adding the spacers. So, I just wanted to make sure that my intended setup would not rub. That's why I wanted to know how much the tire is tucking in towards the center of the car under compression to give myself an estimate of how far out I can go.
re: Suspension travel
1. This is not bump-steer. Bump-steer is the change in wheel steering angle due to suspension travel.
2. As the suspension travels the camber changes. It a-arm suspension, this is usually camber gain i.e. the wheel will lean towards the inside of the car more, as it goes up, since the upper a-arm is shorter.
Adding spacers CHANGES GEOMETRY!!! It, also, places a lot of stress on hub bearings. It will affect the track of the car and the Ackerman geometry and scrub radius i.e. the wheels will not point in the same direction when turning... None of this is the case if you get a wider wheel/tire and keep wheel center-line the same i.e. you just increase track and this is generally good for handling.
To figure out how far the tire will tuck in:
1. Take suspension travel. Lotus does not have much. I would say 2" under hard cornering or a severe bump.
2. Estimate or measure camber angle at this 2" travel, in stock car about 2 degrees.
Tuck in from vertical at the top of travel = tire diameter * sin (2 degrees)
or about .75". There are lots of assumptions in this calculation....
I would be more worried about what happens when you turn all the way right or left. the inside of the wheel may hit the fender....
I would not recommend it, but it is your car...
Anton
1. This is not bump-steer. Bump-steer is the change in wheel steering angle due to suspension travel.
2. As the suspension travels the camber changes. It a-arm suspension, this is usually camber gain i.e. the wheel will lean towards the inside of the car more, as it goes up, since the upper a-arm is shorter.
Adding spacers CHANGES GEOMETRY!!! It, also, places a lot of stress on hub bearings. It will affect the track of the car and the Ackerman geometry and scrub radius i.e. the wheels will not point in the same direction when turning... None of this is the case if you get a wider wheel/tire and keep wheel center-line the same i.e. you just increase track and this is generally good for handling.
To figure out how far the tire will tuck in:
1. Take suspension travel. Lotus does not have much. I would say 2" under hard cornering or a severe bump.
2. Estimate or measure camber angle at this 2" travel, in stock car about 2 degrees.
Tuck in from vertical at the top of travel = tire diameter * sin (2 degrees)
or about .75". There are lots of assumptions in this calculation....
I would be more worried about what happens when you turn all the way right or left. the inside of the wheel may hit the fender....
I would not recommend it, but it is your car...
Anton
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3,589 Posts
This isn't really an answer but there are quite a few cup guys running spacers. I highly doubt something hugely detrimental is happening when doing this. Also, the hubs on this car are sourced from another production vehicle, which I believe weighs twice as much. Doubt the hubs will notice the difference, but I'm certainly open to some education otherwise. I'm not debating whether things change at all, I'm just asserting that outside of rubbing various pieces of bodywork,small spacers (lets say 9mm or less) will not have much of any affect.
re: spacers
In race conditions small spacers may help. They can also be compensating for a different offset on the wheels that they run. The trick is that you can have any wheel you like, as long as the offset is the same for same diameter wheel. This keeps the overall geometry same. I do that, too. In this case, right offset on the wheel is lighter than too little offset + spacer.
Small spacers are of little concern i.e. 3mm, 5mm, 10mm.
I think, I answered the question. If one can follow calculations, the amount of 'tuck-in' offered by the camber at Lotus suspension travel is less than the build tolerance on the car i.e. how the body is mounted on the chassis. Hence, if You are that close, .74", then you have to try the wheel, spacer and tire combo on the car to make sure. Also, your combo may not work on another (same make/model) car. Most manufacturers leave 10mm extra clearance. (Again, besides the camber curve, there is flex in the bushings, suspension, tire sidewall, etc., etc.)
Anton
In race conditions small spacers may help. They can also be compensating for a different offset on the wheels that they run. The trick is that you can have any wheel you like, as long as the offset is the same for same diameter wheel. This keeps the overall geometry same. I do that, too. In this case, right offset on the wheel is lighter than too little offset + spacer.
Small spacers are of little concern i.e. 3mm, 5mm, 10mm.
I think, I answered the question. If one can follow calculations, the amount of 'tuck-in' offered by the camber at Lotus suspension travel is less than the build tolerance on the car i.e. how the body is mounted on the chassis. Hence, if You are that close, .74", then you have to try the wheel, spacer and tire combo on the car to make sure. Also, your combo may not work on another (same make/model) car. Most manufacturers leave 10mm extra clearance. (Again, besides the camber curve, there is flex in the bushings, suspension, tire sidewall, etc., etc.)
Anton
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