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MacPherson: knuckle from another model

Motorsport Wheel Alignment Fundamentals

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Hello,

I'm thinking about replacing my cast-iron front knuckle with aluminum one from another car model. Could you please tell what I need to consider? What should I measure and how to correctly do this? Probably it's bad idea?

Bottom ball-joint connects knuckle to arm like it was with old knuckle.

Outer cv-joint should be taken from driveshaft that goes with alu knuckle. Driveshaft length will be the question.

I need to measure length and height of steering arm.

Brake rotor won't fit because dust-shield is in the way. Probably because of this wheelbase will decrease, which is not good. But I thought about replacing brakes & rotors.

And boot that connects knuckle and coilover-damper should be replaced since thickness and bolt pattern is different.

Any comments & ideas are appreciated )

(pic of cast-iron knuckle attached)

Thanks, Alexander

Attached Files

What car are you sourcing the new knuckle from? Does it have a similar mass and acceleration characteristics as the vehicle that it is going to be used in? Most of the knuckles/hubs like you have pictured are actually cast steel to obtain the best balanced between strength, mass and cost.

Source of the knuckle is lexus ct200h (prius 3) which mass is 1485kg. My celica st205 now ~1300kg. And it should get even lighter )

Acceleration of lexus little bit worse i believe ) But lexus bearing looks stiff enough. Wheel bolt pattern is the same. Driveshaft diameter and number of teeth also the same.

Hi Alexander,

The first thing is to make sure you are doing this for the right reasons. It's quite a big job to retrofit a whole upright from another model, so there needs to be some good gains!

In case you have not seen already, one of our other RC members is currently doing a similar upright swap with some Aluminium Prius uprights into a GT4 Celica, so this will give you a good idea of the amount of work involved:

https://www.racecrafthq.com/forum/motorsport-wheel-alignment/show/choosing-your-ideal-ride-height

I presume the main reason is you wish to save weight? This is valid, but considering the size of the project, I'm not sure this is where I would start, although I don't know anything about the rest of your project. How much lighter is this new upright? How does the stiffness compare to your original one, these are the sorts of things you need to answer before starting this project.

If you do decide to go down this path, the first thing is to understand the geometry differences. How does the location of the connection points compare to your current upright (assuming you keep the wheel hub in the same location). Keeping the wheel hub in the same location may simplify a few thing for you like such as driveshaft length. From there you need to understand the relative position of the steering point attachment, the lower pivot point and the point where the strut will attach to see if this will be easily compadible.

Hello Tim,

Thank you. I saw this upright conversion, but in Instagram.

You right, looks like this project is too difficult and not so profitable at this stage :)

Maybe someday I'll return to this idea.

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