Front sprocket position

[Mark/Foggy]

Ok, I will try to explain what I'm talking about. If the Sprocket center is above the swing arm pivot, as the chain transmits load, it is going to try to lift the back wheel.

So as established, we need that output shaft lower than the line that extends through the wheel center to swing arm pivot and this should give some Pro lift to the rear and aid traction.

Only, the forces generated are slightly complicated to calculate. What we can do though is look at the triangle that I described and those lengths help us to get an understanding of the direction and possible magnitude of the forces involved.

So, you set the bike up on it's rear stand with all the sag taken out and have a quick measure.

This will give you a triangle that you can make on graph paper, long skinny one. but still a triangle. This is your pro-drive/ pro-lift condition that is what is most desireable.

Now if you do the same with the shock out and the rear wheel at maximum travel, if it's looking like the triangle starts to turn the other way up, that would be because it has moved into pro-squat/ pro-lift conditions. Which I can't think of many times when this would be helpful, because as you hit the gas over pumps or are accelerating really hard, what will happens is that you will end up with less and less wheel travel and rapidly end up with the shock on it's bump stop and just find that the wheel will spin.

Now consider that most people these days hang a lot of faith on Shock linkages that behave in a linear fashion. So the shock is damping things consistently no matter what position that the rear wheel is in.

Way back in the midst of time, we had sweet handling Europen bikes that Japan could not get their head around. Pursuit of ever faster engines and more power was always what they were looking for.

I'm certain that what went on was that engine development guys would just keep building bigger and madder and then hand it over to the chassis guys and tell them to make a bike out of it.

So, all those rising ate, complex 1980's and longer , Rising Spring Rate rear suspension solutions may well have actually been the result of the guys in the day not realising how essential the output/pivot/swingarm was.

Then of course we see Ducati go watercooled and fuck off into the middle distance with a bike that looks in profile, not dissimilar to bikes 20 years before.

But, what do I know?

 

 

[Mark/Foggy]

Oh and just for the record, I'm certain that the lads at Bimota  and the likes of Nico Bakker had it all worked out, possibly more down to intuition than pure engineering calculation.

By the time Massimo Tamburini came along with his first MV, the japanese had probably figured it out.

[laney]

On 1/3/2021 at 7:03 PM, hawkati said:

Would you be suggesting the centre front sprocket should be slightly below a straight line from swing arm pivot dead centre to rear sprocket dead centre?

That's wot @Superdunc said at the top of this thread 😁

[Gammaboy]

I've got a PDF somewhere from Aprillia that describes what is going on with sprocket placement, swingarm slope and how it effects jacking or squat of the rear end.

Chapter 9 of the Tony Foale book (Motorcycle Handling and Chassis design) covers it as well. 

From memory, values of swingarm angle between 9 and 13 degrees work well, with sprocket centre between level with swingarm pivot and inline with rear axle/swingarm pivot, dependant on front and rear sprocket sizes (if you're running a big front/small rear sprocket to make up for dirtbike internal gearing, you'll need the countershaft lower, and close to inline if you're running a 13 tooth front sprocket just for practical clearance reasons).

 

[Gammaboy]

To clarify though, the chainline is the critical part, not the actual centre of the sprocket.

[TLRS]

I thought squat would allow for slightly more weight on the rear wheel. At cost of consistent geometry..

Maybe that’s old news, when tires weren’t as good..?

Always thought you could see the pro lift and that well known slomo at the jump at what is it.. cadwell park? Just seems to dig in while the whole bike is about to go loose..

[Gammaboy]

Just now, TLRS said:

I thought squat would allow for slightly more weight on the rear wheel. At cost of consistent geometry..

Maybe that’s old news, when tires weren’t as good..?

Always thought you could see the pro lift and that well known slomo at the jump at what is it.. cadwell park? Just seems to dig in while the whole bike is about to go loose..

Squat will slacken the head angle and make the bike run wide on throttle. Too much anti-squat steepens the head angle too much and gives oversteer. 

Pro-squat has never been good, it is trying to pull the rear tyre away from the ground. 

 

[TLRS]

 

1 minute ago, Gammaboy said:

Squat will slacken the head angle and make the bike run wide on throttle. Too much anti-squat steepens the head angle too much and gives oversteer. 

Pro-squat has never been good, it is trying to pull the rear tyre away from the ground. 

 

With you on the geo.. but the rear moving up or down is only temporary if the load is constant?

Would pro squat help them mx boys? Might drop cog a bit?

[Gammaboy]

11 minutes ago, TLRS said:

 

With you on the geo.. but the rear moving up or down is only temporary if the load is constant?

Would pro squat help them mx boys? Might drop cog a bit?

That video doesn't really show anything as far as I can see on a 6" scren.

I would guess with MX you want neutral or minimal anti-squat to keep the rear suspension action from being locked out.

The squat or lift will be for as long as the tension is being applied to the chain by the driveline. Which reminded me, pro-squat will "pump" when the bike is cranked over and spinning up as the rear spring drives the suspension into extension.

[DesmoEddy]

15 minutes ago, Gammaboy said:

That video doesn't really show anything as far as I can see on a 6" scren.

I would guess with MX you want neutral or minimal anti-squat to keep the rear suspension action from being locked out.

The squat or lift will be for as long as the tension is being applied to the chain by the driveline. Which reminded me, pro-squat will "pump" when the bike is cranked over and spinning up as the rear spring drives the suspension into extension.

I thought I'd read somewhere that MX needs geometry which extends the rear (anti-squat) on the throttle, pushing the tyre into the ground for increased traction? But I may have made it all up / forgotten / become confused.

Edited January 20, 2021 by DesmoEddy

[Gammaboy]

2 minutes ago, DesmoEddy said:

I thought I'd read somewhere that MX needs geometry which extends the rear (anti-squat) on the throttle, pushing the tyre into the ground for increased traction? But I may have made it all up / forgotten / become confused.

Could be - I do know that if you go too far in the anti-squat direction it can really fuck up the way the suspension responds to bumps.

[2moto]

The whole "driving the tyre into the ground" is a misconception, there is no such thing. The amount of rear traction is affected by the weight on the rear wheel, all else being equal. Or, in other words rearward weight transfer. Also, the amount of pro/anti squat forces are not constant over rear swinging arm travel. As the rear suspension compresses, squat forces will become increasingly pro-squat. The important variable is where in the suspension travel the zero crossover point is placed.

Unless you really want to study this area of chassis dynamics, and Tony Foale's book is a good starting point, the best thing in practice is to copy dimensions from an existing bike that has about the same power and type of use. If you do take measurements from an existing bike, make sure you do it with the rear suspension fully extended to remove any static sag.

Edited January 20, 2021 by 2moto

[4tgp]

5 hours ago, Gammaboy said:

To clarify though, the chainline is the critical part, not the actual centre of the sprocket.

100% agree.  esp if you are not using "normal" size sprockets.

I think it's a bit OTT theoretical, but this is what I used when I made my race bike.  Of course I had no idea where CoG was, so that was a guess........

Edited January 20, 2021 by 4tgp

[Gammaboy]

2 hours ago, 4tgp said:

100% agree.  esp if you are not using "normal" size sprockets.

I think it's a bit OTT theoretical, but this is what I used when I made my race bike.  Of course I had no idea where CoG was, so that was a guess........

Yeah, I spent ages fucking around with this all in AutoCAD (and the front end geometry) to try to mimic the Aprilia 250GP geometry (info from a bloke running one in the US was useful), and then I fucked it all up by forgetting about the downward tilt of the motor and the sprocket wound up lower than planned... Luckily I got a little back with standing the front back up and messing with sprocket sizes so I won't saw through the swingarm pivot when it tops out... 

Turns out stock Mito setup is neutral to slightly pro squat from memory, not that it really was noticeable with 30 hp

[TLRS]

7 hours ago, 2moto said:

The whole "driving the tyre into the ground" is a misconception, there is no such thing. The amount of rear traction is affected by the weight on the rear wheel, all else being equal. Or, in other words rearward weight transfer.

I agree that it doesn't continually add traction. Though, when the bike wants to wheelie/raise it's centre of gravity, the mass of the bike is accelerating up. Which suggests the traction comes from both the mass of the bike as well as the force needed to accelerate the cog upwards.

Seems to me that even though the incline tapers off, the rear wheel sticks to the asphalt in a peculiar way.. maybe adding a little bit more drive. And if the bike rotates a bit backwards mid air around it's cog, the rear wheel might touch down slightly sooner.. possibly resuming drive 'earlier'..?

[2moto]

Unless you have both wheels off the ground, it's physically impossible for it to "rotate around the cog". Once the front wheel is off the ground the location of cog is irrelevant for traction.

[TLRS]

 

[hawkati]

13 hours ago, TLRS said:

 

If only he'd managed the full loop!😛