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I am getting new tires this weekend and have been looking to give my car a little bit of a "lift". I have seen one or two mentions on reddit of people using 245/50/R18 tires on their B9 Allroads, rather than the stock 245/45R18s. Does anyone have any experience using 245/50R18s on their B9 Allroads? Pics would be great as well. The size difference is +3.4% over stock.
In case anyone considers this tire size in the future, this is what it looks like on stock suspension. On my 2017 B9 Allroad I have not had any rubbing and have taken it on some rough roads and over speed bumps and have not had any issues so far. Once I determine which V Code (V0-VXX) I need to use to get the speedometer and odometer correct, I'll update the post with that info as well.
The biggest problem I see is that big of tire makes the reverse rake these cars have much more obvious.
I'm going in for service at my local independent shop soon, to get the coolant pump/thermostat replaced, and they are going to quote changing out the rear springs with OEM ones that are two load raitings higher. I've read that fixes the issue pretty well, especially if I end up getting a tow hitch and using it for bikes or have a heavy load in the trunk.
I'm going in for service at my local independent shop soon, to get the coolant pump/thermostat replaced, and they are going to quote changing out the rear springs with OEM ones that are two load raitings higher. I've read that fixes the issue pretty well, especially if I end up getting a tow hitch and using it for bikes or have a heavy load in the trunk.
Anything you do that raises the rear of the car relative to the front of the car will alter the front wheel caster. In this case you will be reducing negative caster which reduces the tendency of the steering to self align at highway speeds. Depending on how much the caster changes and how sensitive the B9 is too changes in caster this may or may not be an issue. Reducing negative caster can turn the car into a bit of a squirrel chaser. I haven't checked the service manual to see if caster is adjustable. On some cars (particularly with Macpherson strut suspension) caster is not adjustable. If the B9 caster is adjustable an alignment shop may be able to return you to spec if the change turns out to be noticeable.
With a wheelbase of 110.9" I would expect each inch of raising the rear to change front wheel caster by 0.9%. That is, if it was 4.5° it would then be 0.04° less.
With a wheelbase of 110.9" I would expect each inch of raising the rear to change front wheel caster by 0.9%. That is, if it was 4.5° it would then be 0.04° less.
1" rise in the rear with a 111" wheel base would net you a Tan-1 (1/111) = 0.52 deg change from the starting point. The section of my service manual dealing with alignment (dated 2019) has a whole lot to say about camber; but, doesn't mention the word caster once. As near as I can figure out the caster seems to be around -4.5 deg so a 1" change in elevation would net about an 11% reduction in caster. That still leaves you with a fair amount of negative caster so by most peoples standards would be safe. However, with the power assist on my 2021 alroad I do note that the steering does seem overly sensitive to driver inputs. Reducing the caster is just going to make it a little more sensitive - definitely cause lane wander if you try and text and drive which we know nobody does these days.
Your math is correct, I missed a step. All of the data I can find places the desired camber in the 4° - 5° range, usually closer to the upper limit of that range.
I would think that toe-out would make steering twitchier to a greater amount than caster, but my experience with that is limited to race cars.
01-04-2024, 04:10 PM
2017 B9 Allroad 245/50R18 Tires
