Why does my rear tyre slide out so often?

[Roggiesmalls]

I own a 2014 KTM RC390 which I’ve been riding for about two years now. For a long time I’ve noticed the rear tyre always seems to want to slide out through the corners. I feel like this is preventing me from getting as low as I’d like too. Could it be attributed to an imbalance in the bikes weight, or maybe tyre pressure? I’m any case I’ve chalked it up to my general lack of skill. But I would like to hear some thoughts. I am also a pretty big guy, weighing close to 90kgs (220 pounds roughly).

 

[John390]

huh. I've never had that. But I am maybe too timid, or am bringing some experience from driving performance cars thru corners and its a good thing?

Don't set tire pressures too high. they are 29 on my '18.

Take some preload out of your rear spring? Tho with your mass, maybe that won't help.

Maybe one of the suspension guys will pipe up?

 

[Chuditch]

Bit hard to say without seeing you in action but I guess the usual cornering rules apply;

Are you in the gear you want to corner in well before you hit the corner, or are you changing down, just as you go into it?
Are you braking predominantly with the front before the corner or are you using rear brake trailing into and through the corner to wash off any 'wobbles' or rear wheel kick-out? If you are really heavy on the front brake and hardly use the rear then that will leave the rear with a lot less stability and smoothness. Google rear brake trailing...

If you were riding a big superduke or like bike I would just say you are in too low a gear and your revs are too high, for the speed you are cornering in. The only time I have experienced what I think you are talking about is when I have done intentionally on a bigger streetfighter, naked style bike like the bigger dukes.

Cornering at speed is kind of the opposite to going hooligan slides..... you really want to have your line set and be committed to it well before you corner, no sudden changes into or mid corner.

What condition are your tyres in? Are you trying to corner on the 'oil' strip on the street or track? I would try to stay well clear of where all that oil and crud is and pick my lines on either side of it.....

I am a pretty big fella too, heavier than you, consider also your balance and body position through the corner, it can affect the way you corner.

Sorry if it comes across like I am telling you what you already know, I admit I am no expert, but I am doing a fair bit of race/road rider training at the moment so the fundamentals are on my mind.

 

[TylerH]

In corners especially, make sure almost all your weight is on the pegs....NOT on the seat. Traction can then be better managed between the outer and inner to correct and adjust traction.

 

[John390]

In corners especially, make sure almost all your weight is on the pegs....NOT on the seat. Traction can then be better managed between the outer and inner to correct and adjust traction.

can you expand on this?

The pegs are still mounted in such a way as to make the weight of the rider compress the rear shock.

Do you mean putting more weight on the right peg or left peg, depending?

and if so, how do you know when and in what situations to add weight to the inside peg or the outside peg?

 

[Formula390]

Peg weighting seems like a stretch for what's going on here. I would FAR more likely suspect tire pressure! Given he's stated he's an inexperienced rider, and this is riding on the street, and he also didn't state that he's upgraded the shock, we've got a heavy guy on the stock shock, with likely with too high of tire pressure. That could easily result in the rear end stepping out, especially if the shock bottoms out on a bump which overloads the tire.

Roggiesmalls, couple of questions for you:
1) what tire do you have on the rear?
2) what tire are you running?
3) how worn is the tire?
4) what shock do you have on the rear?
5) what adjustments have you made on the shock / where is it set to?
6) what is your static sag on the rear?
7) what is your rider sag for the rear?
8) what's your tire pressure when cold?
9) what's your tire pressure once it's up to full heat after riding?

As a GENERAL rule, your tire pressure should come up 10% from cold to hot. So, if you had 30PSI in the rear cold, it should come up to 33PSI once up to temp.

So, now we want to determine what your tire pressure should be, and how to intrepret your changes:
- Cold tire pressure starting at 30 PSI, and once up to temp is only 31 PSI == You've got too much air in the tire. Take it down 2 PSI or so, and try again.
- Cold tire pressure starting at 30 PSI, and once up to temp has shot up to 35 PSI == you've got too LITTLE air in the tire. Add 2 PSI and try again.

NOTE: I am NOT saying set your tire pressure to 30 PSI here. I'm using that number as a means of demonstration to make the math easy to follow. Ask your tire rep for a suggested tire pressure for your tire for your bike. Also, do NOT set the tire pressure to the "Max Pressure" rating which is molded into the tire. That's not the RECOMMENDED tire pressure, that's the MAXIMUM.

If you don't know how to check your static and rider sag, you'll need a friend (or two) to help you with the measurements... and a tape measure

To measure static sag:
Stand the bike straight upright.
Lift the rear tail straight up to is maximum amount of travel.
Measure from the rear axle straight up to a point on the tail section. Record this measurement.
Let the bike settle down under it's own weight. Measure from the same points again and record this measurement. This is your Static Sag.

To measure rider sag:
Stand the bike straight upright.
Have the rider get on the bike and get into their normal riding position. This is where the second friend can come in handy to help you keep the bike upright!
Measure from the same point used for static sag measurement.
Record this measurement. This is your Rider Sag.

Ballpark numbers you should aim for should be something in the middle of:
Static Sag: 4-6mm
Rider Sag: 15-25mm

That said, every rider likes a different setup, but as a REASONABLE starting point on the RC390 which you can aim for as a starting point is in the middle of those values. Some like a firmer setup, some like a medium setup, others want their teeth to fall out if the ride over a pebble. Rider Sag is the more important of the two measurements. If you can manage to get 20mm Rider Sag, and your Static Sag is 2mm (or even 0mm), don't fret about that as much. It's WAY more important that Rider Sag is in the correct ballpark than it is for the Static Sag... especially for a Big Guy on a Little Bike. Static Sag is pretty difficult to get right, especially when the rider mass gets closer and closer to the mass of the bike itself. I say this as a self proclaimed 6'3" Fat Fucking Bastard who likes to ride bikes which others say are "too small for him."

So, what happens if your Rider Sag is > 25mm. You need more spring or more preload on the shock. If you already have the shock at Max Preload and you still have too much Rider Sag... well... it's time for a decent shock. If your Rider Sag is only 10mm, then you probably have too MUCH preload on the shock, and need to back some out because you are using the tire as suspension, and that'll easily put you on your head!!!

If you can't adjust the stock shock to get you into reasonable values, then you can look into my open sourced R6 or R1 shock swap solutions, or pick up a JRi shock and WELD that rear tire to the tarmac.

 

[John390]

excellent info Matt, thanks.

I have been trying to educate myself on these matters, but its tricky when everything you read contradicts everything else.

 

[TylerH]

I have seen many track day riders with the majority of their weight on the seat. Even on the street you will see a noticeable difference in your bump compliance and stability with your weight on the pegs. the pegs being lower will effectively lower you center of gravity, and help lessen weight transfer during hard braking and acceleration. If your rear does step out on corner exit the effect of the weight lower will tend to keep the rear from stepping out as fast and keep driving forward. By weighting the inside or outside pegs you can effect turn in and traction.

This all assumes a bike in proper sound mechanical condition.

A bigger rider on a smaller bike can adversely effect the handling with their weight on the seat and bars. This is the only point I am addressing.
I hope this may be of some service.

 

[Tack Reacher]

There is something that not many riders will actually admit to and don't normally talk publicly about it....it may be a contributing factor here....it's called "premature acceleration". thankfully there is a cure...you need a kind caring understanding motorcycle.

 

[Andy]

Honestly as a larger rider it can be a frightening place getting the 390 low!

I work on maintaining body position, I had issues with front tucking but I tend to ride in a more upright 70s style now. Switching to a 140 section on the rear seemed to help.


Sent from my iPhone using Tapatalk

 

[Roggiesmalls]

Honestly as a larger rider it can be a frightening place getting the 390 low!

I work on maintaining body position, I had issues with front tucking but I tend to ride in a more upright 70s style now. Switching to a 140 section on the rear seemed to help.


Sent from my iPhone using Tapatalk

What do you mean by 140 section? I’m also pretty tall, so I’ve found moving to the front and really leaning off of it to be quite scary yeah. Also if it normal to apply pressure to your outside peg as opposed to your inside peg in some corners?

 

[Roggiesmalls]

This is al very helpful information, I’ll have to go check this stuff out. Thank you for your input

 

[Roggiesmalls]

Thanks very much for all this information! I’ll definitely look into it! I appreciate it

 

[Roggiesmalls]

This is all massively helpful information, thank you. I feel a lot of the time it could be contributed to the weight I’m putting on the seat, a lot of the time out of laziness. But it has happened before even when I’m fully on the pegs so idk. I’ll definitely check my tyre pressures out and look into some other stuff.

Does anyone else have trouble getting their 390 nice and low?
Idk if I’m just afraid to do it? But it just seems very difficult to take it that one step further and get down to the ground.
Sometimes it just feels like it doesn’t have anymore room to give. Looking at my tyres as well, it seems like I’ve taken them just about as far as they can go. It’s not uncommon for me to have my foot hitting the ground. I’ve never once gotten my knee to the ground though. Any thoughts?

 

[Tom]

I dont think it's been said before, so here it goes.... Get a slipper clutch.

 

[John390]

I've got about an inch more on my rear tire. I'm not brave enough. I suspect that if I was on a school bike that had been dropped, and in a large parking lot or race track, I'd go further, but I don't want to damage a brand new bike. I've never purchased or owned a brand new vehicle of any kind. Also, I don't want to wipe out on public roads and either get run over, or slide into a tree or a ditch or a fence etc.