What is the Best Secondary Gear ratio for MX

[Ji Gantor]

As Brent has already written you can not install larger cogs than the areas allow as chain guards and case covers will be damaged. It is also true that you can not install too small a cog. On the rear the hub dictates the min diameter and on the front it's the chain.
The chain can only curve around a set diameter, under that it is being torn apart.

Ji

[Nathan S]

The 'best' ratio is the one that has you just touching the redline in top gear, once per lap. Everything else is compromising acceleration and consequently lap times.

You may wish to make some other sort of compromise, such as gearing a 125 lower so it pulls up the biggest hill faster (at the expense of top speed), or trying to minimise the number of gearchanges (as Brent has suggested), but that's not the 'best'.

The absolute best that this thread can hope to acheive is some very broad rules of thumb.

[Lozza]

It all depends on gearbox ratios and the spread of power from the engine.Been experimenting with higher a lower primary drives on road race engines lately and have had some interesting findings.

[Ji Gantor]

Hi Nathan,
I hope this thread helps those that need it.
If this thread makes us think about finding performance from something as simple as changing a front cog then it has been worth every word we have typed.
All I ask is that members share what they have found when they have gone up or down a tooth on their sprockets. Members will bennifit from other members that post what was the standard secondry gear ratio set by the bikes manufacturers.

Ji

[Ji Gantor]

Hi Lozza,
Great to hear from you.
I know that 125 and 400cc bikes have different secondary gear ratios but both are still around that 4:1 ratio.
Supercross bikes have a ratio between 4-5:1 because they have to jump triples within a few meters from a corner.
A 125 could have a ratio of 5:1 on a track with a short straight because most or all 125's have a 6 speed gear box and would rarely use 5th or 6th on said track.

Ji

[Lozza]

There has yet to be an engine that is driven through a gearbox that has a higher top speed in 5th than in 6th.The non-use of 5th or 6th gear is a serious compromise in performance and will never be addressed by changing sprockets.In this situation running gearing to pull in 6th and starting in second is much better.

[EML]

2 cents worth from a sidecar crosser, as we almost all use road bike based engines we have to compromise with gearing.
However as we have/build very torquey engines, twins seem better the fours, we can run very wide ratio boxes.
Now back to Euros verses Japs and how the g/boxes spin at different ratios, that's so the gears will change better- ever noticed how on a Euro bike the gears are wider and quite hard to shift at times where as the Japs are usually very nifty to up shift and save time that way. Primary shaft speed.
On a Euro bike you seem to have to shift first then gas on, a very comfortable way to ride but if you try to push it the shifting goes to hell and you seem to lose more time.
Back to the chairs. On the Yamaha 650 based engines they spent alot of time building a bigger bore and stroke, all the way to and past 1000cc. The g/box is way strong enough to take the added hp/torque but then we have trouble with shifting, particularly under load. The answer to this is to buy and fit a special primary gear to speed up the rotation of the gear cluster thus making shifting much easier.
Proof of this was at Conondale where the Wasp from SA was far quicker out the gat than us and in the first race on Fri we must have missed 25 shifts trying to catch him. The harder Popeye tried the more shifts he missed, but on Sat he was more relaxed and we took a second a lap off him.
He also has about 30 more HP than us as we had ours dynoed by Raceline 2 weeks before the Nats and came home with 57 where his builder quotes 84 not 95 as Wally boasts!!
 

[All Things 414]

Too many variables really

Too true. Some guys will run very tall gearing in hard-pack or slick conditions to reduce the rear wheel from breaking traction. You might not get into top gear but you'll be having less drama. 

[Ji Gantor]

See we are learning.
Over gear to prevent wheel spin on hard pack tracks.

The more we share the more we learn.

Ji

[Lozza]

What about a softer rear spring for reducing wheelspin?

[All Things 414]

What about a softer rear spring for reducing wheelspin?

Yeah. It all helps.

[EML]

lozza, there is as much in the suspension settings as there is in hp as you still need to get the drive.

Good to see wally take the bait as he tried all week end to phsych Popeye out with massive hp stories when in sidecar cross you can pretty much "run what ya brung" by way of engine or outfit provided it fits the era.

[EML]

Just spoke to Ronny the owner of Q1, tells me he had the motor freshened up 20yrs ago in Toowoomba and it hasn't been touched till we went looking for a miss before the nats.
The guys at Startline m/c gave it some plugs and stuff and we went racing-as you should in vmx.
Still the racing was a race and come Broadford we'll be there again-hopefully with a real field as we had in Warrnambool.

[Ji Gantor]

Well I raced the YZ125 H on Sunday and with the 51/12 secondary gearing it lacked nothing.
It climbed all the hills and was sideways out of most corners. I still have to get the carby tuned but at least I can say that the 4:1 ratio holds true for 125s.  In one of the races I started very poorly due to the carb and was last behind the National champion Pre 85 125 bike. I followed it for the first two laps then twisted the throttle on a down hill and edged past. Not wanting to stuff the other rider in a corner I took the wide line and blasted out of the corner. Really great racing.

Ji

[Gerard De Ruyter (Twistandshout)]

The 'best' ratio is the one that has you just touching the redline in top gear, once per lap. Everything else is compromising acceleration and consequently lap times. 

when i first read this nath, i thought nah, can't be right - its only on big big grasstracks that i ever go near redline in top.  but then i punted my 06 rm250 round fairbairn park at a round there on the weekend with 13 tooth front cog w 52 rear (4:1).  i'll be blowed if i didn't spend half the first race thinking what's wrong with the gearbox, i can't get past third or fourth.  its was just the gearing tho - i'd actually been near maxing her in top!  it did give it a massive burst in low-rev pickup, which you need at fairbairn these days, and what i did it for, but prob overdid it a bit (like you're out of control half the time).  will try 13/50 next - 3.85:1.  Stock is 14/50, or 3.6:1 - good for big open US tracks.  I then tried 14/52 - 3.7:1, but still not low enough, so hope 13/50 will be goldilocks preferred porridge for me).