Different sized front and rear for 4WD?

[alvinlwh 5889]

OK so I asked a question about wheels and tyres size and, TBH, I am still none the wiser. I suppose the worst aspect is they are sometimes labelled for the cars they are intended for and I have no idea what are those sizes.

Anyway, as some of you may know, I dropped into a physical shop and bought some correct sized wheels + tyres. Thing is, the ones for buggy often are often sold in front and rear size, which is as expected for a 2WD buggy but what about a 4WD one? The seller said having slimmer ones at the front will make steering more responsive, is that true? For both 2 and 4WD? 

Also, what are the effects on the drivetrain of a 4WD? Uneven load, stress, etc? To take it a little further, what happens if smaller diameter wheels are used on a 4WD vehicle? Will the diff take care of the difference in rotation speed or will it screw everything up? 

PS to play safe, I bought 2 sets of everything, so I can use front front and rear rear at the same time if it really turns out to be bad to have different sized on a 4WD.

[Frog Jumper 3656]

Great question!  I'm curious as well.

Thanks!

 

 

 

4 minutes ago, alvinlwh said:

 

PS to play safe, I bought 2 sets of everything, so I can use front front and rear rear at the same time if it really turns out to be bad to have different sized on a 4WD.

 

Hello, my name is Frog Jumper.  I'm a tire addict too...

 

[Matty36 267]

As far as I'm aware, the rolling radius should be the same on narrow 2wd fronts and wide 4wd fronts. The diff would possibly break if you use smaller radius front tyres, as the diff works on turning so can work with the inner tyre turning at a faster rate than the outer tyre, but not on the rolling radius.  If you do fit lower profile front tyres, it may not break anything if you are on low grip surfaces, i would not try it on high grip. I have used narrow fronts on a 4wd buggy with no issues, it turns better on astroturf. 

On a side note, I have an old losi xxx4 buggy which has the option of using a diff on the front that has one more tooth, or one less for over driving or under driving the front tyres, which I can only assume was for grass tracks, i have never tried in case i break the drive train.

[alvinlwh 5889]

Thanks @Matty36

[Nikko85 3439]

AFAIAA a central diff will allow different rotations of front a rear, whilst the regular diffs just deal with side to side.

If you are running a car with different sized fronts and rears, they'll have to rotate at different rates. This will mean you'll get slippage, either the front or rear will slip unless you run a centre diff, which works in the same way as a regular diff (left-right) allowing for changes in speed between the two.

I know some crawlers use unequal speed in front and rear, but for faster driving, I think it wouldn't be great.

I think for tire width, you'd be fine, it's just perhaps not the most efficient. I am making a 4WD car now, and I'm debating thinner fronts, just for style.

I guess very much thinner front tires would handle more like a RWD car, and wider fronts than rear like a FWD car. The more equal, the more 4WD feel it will be? 

 

 

[hIghQ 149]

^This. And it's called under- or overdrive (usually done in the drivetrain, not the wheels).

[Nikko85 3439]

That being said, this still confuses me..

https://en.wikipedia.org/wiki/Aristotle's_wheel_paradox

 

 

[alvinlwh 5889]

10 minutes ago, Nikko85 said:

I guess very much thinner front tires would handle more like a RWD car, and wider fronts than rear like a FWD car. The more equal, the more 4WD feel it will be? 

I don't know... The observed wear on my rear is much more than on the front, which must mean something. My guess is there are less weigh, stress, contact or something along that line on the front? Which could mean thinner is OK? As there is a less need for contact? Of course I could be completely wrong. But as long as thinner is OK but different diameter is not, my question is answered. Thanks to all. I shall experiment with different thicknesses and see the effects. 

[DeadMeat666 1675]

Just weighing in from my research and experience on the topic:

4WD cars/trucks/buggies must have the same rolling radius on all tires. Most of them do not have a center diff and running different diameters will eventually break the drivetrain.

Most 4WD buggies have more weight bias to the rear, and hence need more grip in the rear to have balanced handling (no oversteer). This is why you see narrower fronts on modern 4WD buggies.

For example, the Avante was first released with equal width tires front and back, which gave it too much grip in the front and resulted in oversteer from losing the rear end on turns.

The Egress and Vanquish later improved upon this by having narrower tires in the front, leading to more neutral handling.

[Juggular 4961]

Sometimes there isn't enough room for the steering arc.  It depends on the tire offset, though.  If the front axles go deep into the wheels, fat wheels could com in contact with A-arms.  Thinner wheels help with that and Tamiya often has thinner front tires.  Thin tires are not so good with braking. But traditionally, with mechanical speed control, you don't want to reverse.  Drive cups could chip and dog bones could bend.   

 

Below is a bonus picture... this makes me wince.  There is no center diff for Konghead.  The front wheels would have to turn a lot more but the gearbox won't allow it.  Something's got to give, and that's not good for the drivetrain.  So either this one runs on loose ground only, or only a show piece.  

 

[silvertriple 3557]

26 minutes ago, alvinlwh said:

I don't know... The observed wear on my rear is much more than on the front, which must mean something. My guess is there are less weigh, stress, contact or something along that line on the front? Which could mean thinner is OK? As there is a less need for contact? Of course I could be completely wrong. But as long as thinner is OK but different diameter is not, my question is answered. Thanks to all. I shall experiment with different thicknesses and see the effects. 

Lot of things are impacting this:

• weight partition (more weight at the rear end means more traction at the rear end, and means as well more wear)
• gearing : if too short development, front wheel are very often lifted in the air
• the way the wheels orientation are set (i'm not sure about the words for this in english, but I would translate in english as rear wheels being pinched while front wheel generally with opening)
• the camber of each train

Obviously, the wear of the front and rear train will be different and depending about the compromise chosen. Some cars are having equivalent tyres front and rear (Optima), but when you set them with narrow front and wide at the rear, that doesn't change much the car in terms of behavior - at least from my level of knowledge and abilities to run a rc car)...

[alvinlwh 5889]

14 minutes ago, silvertriple said:

 

• the way the wheels orientation are set (i'm not sure about the words for this in english, but I would translate in english as rear wheels being pinched while front wheel generally with opening)

That will be the toe of the wheel, in English. 😉

[Frog Jumper 3656]

This is the answer I gave to Alvin earlier:

-----

All cars have to balance traction at the front with forward momentum in order to turn at speed. Too much traction and the car will whip around causing it to roll. Too little and the car continues forward no matter how much you try to steer.

In a 2wd car, the front wheels do nothing to in terms of forward momentum, they only change the direction, ie, you turn the wheel and the car wants to go forward, but lateral traction of the tire makes it turn.

In 4wd, the front wheels do change the direction of momentum since the are pulling the car.  The tire is experiencing linear traction the whole time and essentially pulls the car into a turn, ie “straight” is where you are pointing the tires, not where the car is pointed.

In terms of traction, the 4wd needs the extra width to take advantage of the traction available, similarly to why 2wd usually likes a smaller tire up front.

But I have no idea and just pulled all that out of my butt…

-----

[Frog Jumper 3656]

22 minutes ago, Juggular said:

Sometimes there isn't enough room for the steering arc.  It depends on the tire offset, though.  If the front axles go deep into the wheels, fat wheels could com in contact with A-arms.  Thinner wheels help with that and Tamiya often has thinner front tires.  Thin tires are not so good with braking. But traditionally, with mechanical speed control, you don't want to reverse.  Drive cups could chip and dog bones could bend.   

 

Below is a bonus picture... this makes me wince.  There is no center diff for Konghead.  The front wheels would have to turn a lot more but the gearbox won't allow it.  Something's got to give, and that's not good for the drivetrain.  So either this one runs on loose ground only, or only a show piece.  

 

 

Are you able to put a one-way diff in the Konger???  

Terry

 

[SlideWRX 781]

In a turn, the rear wheels (as a set) turn through a smaller circle than the front wheels (as a set), so they go slower than the front wheels, but yet the drivetrain does not explode on buggys without a center diff.

Tire/wheel width shouldn't be a big enough factor to worry about for buggys.

 

[alvinlwh 5889]

40 minutes ago, DeadMeat666 said:

Most 4WD buggies have more weight bias to the rear, and hence need more grip in the rear to have balanced handling (no oversteer). This is why you see narrower fronts on modern 4WD buggies.

Ah! Light bulb just went off. Putting narrow fronts to my stock equal Thunder Dragon might improve its handling (which I feel is quite good to begin with). 

[Matty36 267]

1 hour ago, Juggular said:

Sometimes there isn't enough room for the steering arc.  It depends on the tire offset, though.  If the front axles go deep into the wheels, fat wheels could com in contact with A-arms.  Thinner wheels help with that and Tamiya often has thinner front tires.  Thin tires are not so good with braking. But traditionally, with mechanical speed control, you don't want to reverse.  Drive cups could chip and dog bones could bend.   

 

Below is a bonus picture... this makes me wince.  There is no center diff for Konghead.  The front wheels would have to turn a lot more but the gearbox won't allow it.  Something's got to give, and that's not good for the drivetrain.  So either this one runs on loose ground only, or only a show piece.  

 

Maybe it is only 4wd not 6wd, that makes me want to buy a 6wd now.

[Morinstal 170]

Lol, i was just typing and see Matty36 response, thinking the same too, yea i think its 4wd, only the mid and back using train gears, the front just using steering.

[TurnipJF 9182]

1 hour ago, alvinlwh said:

Ah! Light bulb just went off. Putting narrow fronts to my stock equal Thunder Dragon might improve its handling (which I feel is quite good to begin with). 

I suspect it will. My Thundershot benefited from narrower fronts, and uses the same chassis. 

[Juggular 4961]

10 hours ago, Frog Jumper said:

 

Are you able to put a one-way diff in the Konger???  

Terry

 

I haven't thought of one way... it might work!

 

 

[Wooders28 4718]

Think most of what I thought, has been covered anyway, but just to throw something else in the mix.

Modern tracks are higher grip than bitd, especially carpet.

I've cut spikes off fronts before now, as you grip roll the car otherwise, where on grass/dirt, you struggled to find grip!

The rears wear first, as under power the front end is light, and the weight transfers to the rear, and with modern power, the front end might just be touching the ground (if the slipper is set right), it's only under braking and steering, the fronts do work, and if you're running a one way, they don't even do braking!! (Not a fan of one ways tbh, you have to brake like a 2wd)

[Mrowka 493]

On 10/18/2021 at 1:26 PM, Matty36 said:

On a side note, I have an old losi xxx4 buggy which has the option of using a diff on the front that has one more tooth, or one less for over driving or under driving the front tyres, which I can only assume was for grass tracks, i have never tried in case i break the drive train.

I believe that the OG Optima/Javelin offered something similar. You could probably get away with it on loose dirt or grass, just as a dedicated 1:1 mud bogging truck may have different gear ratios in the front and rear - if you were to drive that on tarmac, you'd quickly blow up the transfer case from windup.