Help with tire problem: nervous front end on gravel at 80kph+

TL;DR version: what causes a nervous front end that feels too light, with regards to tire choice?

 

Grab a coffee for this quick back story: last year I had new K760 Trakmaster II front and rear (80/100R21 and 110/100R18) that I used with moderately good success. I'm ok with the wear, but the front tire has an annoying tendency to slide sideways in the mud at low speed, I'm told due to the tread pattern.

 

Last fall, I was prepping for a trip to TX that had me buy new tires. I got another K760 rear, though due to availability I got a wider rear, a 120/100R18. For the front, a friend recommended a K772 Parker DT front tire, which I got in 80/100R21.

 

The first test drive revealed a very nervous front end at 80kph+. I could feel it wobble on pavement, but on a gravel road it was downright dangerous. I came home and remounted the tire in reverse, since the Parker DT has a different tread pattern in each direction; the result was exactly the same. I also checked the steering head bearings for any sign of wear and found none. After two more remounts trying different things, I remounted my old K760 Trakmaster II front tire, in reverse of the previous mount direction. 

 

A test drive showed the bike riding like it use to ride, smooth and predictable.

 

The K772 Parker DT is narrower and lighter than the K760 Trakmaster. Is this the reason for this result? Perhaps it is exaggerated by the heavier and wider rear tire?

 

I'm still in need of a new front tire that is good in mud and DOT approved. Some people have recommended the Pirelli MT-21, but I don't want to buy new tires again until I understand why I had this problem in the first place. I'm also up for experimenting solutions with the Parker DT.

 

Thanks for the help.

Also, thoughts on the AC10 front tire would be appreciated.

Have you checked your sag is correct?  Uneven front to back ride height can have a dramatic effect on how the bike handles in the loose stuff. 

Yep, very carefully set up sag a couple weeks prior. Good suggestion though; probably why it feels like a 'heavy rear end' problem, because it also feels like when the sag is too low.

Also, I removed the rear racks during testing of the Parker DT so see if the additional weight was a problem; but the symptoms were the same, and the bike rides wonderfully on the K760 with lots of rear weight from racks and bags.

 

edit: also tried various air pressures front and rear.

Edited by everlast

My D606 front felt the same way you describe off road until I lowered the tire pressure to ~13 psi. It is better now, but still a bit squirelly in gravel/sand for my taste especially at lower speeds. Ive been wondering about going to a wider front or possibly installing tubliss so I can just run 5psi off road and not worry about it.

The ride was 130 miles total, 40 ish miles of rocks and sand in the mountains behind San Diego then we had 90 miles (45 each way) of a horribly winding road with excellent pavement both ways, it was terrible. I tried I D606 front and a 760 on the rear, very impressive.  That said I ran a variety of rocks and sand and the front never seemed push, at least not like I remember.

 

 The road handling was fantastic with the 606 on the front and the 760 in the rear.  The handling had a real notch when I ran paired Kendas, it would lean, "catch", then lean a little more but was very loose at that point. I was able to ride harder than I expected on the pavement.  Only scared myself a couple times. Both at 20 psi.

 

My DRZ will never set foot in Texas again.

Edited by Plane Dr

Some stiffer fork springs will make a hell of a difference.

Some stiffer fork springs will make a hell of a difference.

 

Along with a stabilizer.........

What is it about the Parker DT that would induce this behaviour when the Trakmaster is so stable, though?

over sized rear tire will make the front push

sounds like a balance problem, check that out

 

 

also Try slightly adjusting the sag and the height of the forks in the yoke! There's no such thing as the correct sag, just rules of thumb and subtle but important differences from small changes

 

I like my DR-Z set a little low in the rear and high in the front ... super stable gravel cruiser, I tend to go wayyyyyy faster than the KTM's etc on the fast gravel sections.

sounds like a balance problem, check that out

 

 

 

 

This!   an out of balance wheel will never feel stable at speed.

I'm leaning towards it being a balance issue, since the other tires that have been on the bike were so perfectly smooth. However, the front end felt very light, as if the bike needed more caster, when I ran the Parker DT vs anything else, so in that case out-of-balance can't really explain that either. Argh, I say.

Have you checked your sag is correct?  Uneven front to back ride height can have a dramatic effect on how the bike handles in the loose stuff. 

 

How does one "check your sag" link to any guides?

I'll take a crack at this, someone correct me if I'm wrong in here..

 

There are three measurements: fully extended, static sag and race sag. 

 

Fully extended = the measurement from the ground to a fixed point above the axle.

Static = difference between "fully extended" minus the amount the bike sags without you on it - ideally 35 to 40mm

Race = difference between "fully extended" minus the amount the bike sags with you on it - ideally 90 to 100mm

 

 

First, measure you "fully extended" height from the ground to a fixed point on the bike above the axle. Mark that point with tape or something to easily use the same spot. Lift the rear of the bike until the suspension tops out, and measure.

 

Next measure this same spot with the bike standing on it's own. Hold the bike by the handlebars to keep it upright. The difference between "fully extended" and this is the static sag.

 

Finally, measure this spot again with you on the bike, sitting normally, with your gear. Have the bike held upright for you by the front end again. The difference between "fully extended" and this is the race sag.

 

If your sag values are too high (meaning the bike sits too low while weighted down) then you'll need to increase the preload. Doing this will affect both static and race sag. If you cannot get both numbers in the desired ranges then you'll need to change the spring to a different spring rate to get the numbers where you want them. 

 

For example: 

 

Let's say you weigh 240 with gear, you might find your static sag is 20mm but your race sag is 110mm.  If you increase the preload to get less race sag, to 100mm, then the static sag will also be less, say 15mm.  You'll need a stiffer spring (higher spring rate) to get both sags correct.

 

If you weigh 140 with gear, you might find your static sag is correct, 35mm, but with your and your gear it only sinks to 70mm race sag. In order to get both values correct, you'll need a softer spring. 

 

Personally, if the spring rate isn't correct, I would target getting the RACE sag correct at the expense of static sag in order to take advantage of the full range of suspension. 

With any given spring rate, there is actually a fairly narrow range of weight the bike can accommodate. The stock spring is said to be for a rider around 160lbs however I think it's higher than that. I weight 168lbs nekkid, and with my gear I got my sag values in spec with two turns of preload.
 

Phew! Please let me know if any of this is incorrect!

Edited by everlast

I'll take a crack at this, someone correct me if I'm wrong in here..

 

There are three measurements: fully extended, static sag and race sag. 

 

Fully extended = the measurement from the ground to a fixed point above the axle.

Static = difference between "fully extended" minus the amount the bike sags without you on it - ideally 35 to 40mm

Race = difference between "fully extended" minus the amount the bike sags with you on it - ideally 90 to 100mm

 

 

First, measure you "fully extended" height from the ground to a fixed point on the bike above the axle. Mark that point with tape or something to easily use the same spot. Lift the rear of the bike until the suspension tops out, and measure.

 

Next measure this same spot with the bike standing on it's own. Hold the bike by the handlebars to keep it upright. The difference between "fully extended" and this is the static sag.

 

Finally, measure this spot again with you on the bike, sitting normally, with your gear. Have the bike held upright for you by the front end again. The difference between "fully extended" and this is the race sag.

 

If your sag values are too high (meaning the bike sits too low while weighted down) then you'll need to increase the preload. Doing this will affect both static and race sag. If you cannot get both numbers in the desired ranges then you'll need to change the spring to a different spring rate to get the numbers where you want them. 

 

For example: 

 

Let's say you weigh 240 with gear, you might find your static sag is 20mm but your race sag is 110mm.  If you increase the preload to get less race sag, to 100mm, then the static sag will also be less, say 15mm.  You'll need a stiffer spring (higher spring rate) to get both sags correct.

 

If you weigh 140 with gear, you might find your static sag is correct, 35mm, but with your and your gear it only sinks to 70mm race sag. In order to get both values correct, you'll need a softer spring. 

 

Personally, if the spring rate isn't correct, I would target getting the RACE sag correct at the expense of static sag in order to take advantage of the full range of suspension. 

With any given spring rate, there is actually a fairly narrow range of weight the bike can accommodate. The stock spring is said to be for a rider around 160lbs however I think it's higher than that. I weight 168lbs nekkid, and with my gear I got my sag values in spec with two turns of preload.

 

Phew! Please let me know if any of this is incorrect!

 

That's pretty much it… however if you find the spring rate to be off because the race/static sags don't fall into their respective ranges, then preloading the spring less or more won't make it work any better.  If its off, its off.  Question is whether you want to invest in a new spring (~$100), I would and did.

 

Another method:  if the spring is preloaded between 5-15mm and your race sag is good, the spring rate is good.  Sometimes the static sag measurement is unreliable due to suspension friction, hysteresis and low load (only bikes weight, doesn't compress suspension too much).

 

I target a static around 32 mm and race of 100 mm (per Dwight_Rudder)

recently put new fork and shock springs on my bike to compensate for my weight and over sized tank. once I set sags and confirmed that when I jump up and down on my pegs with bike balanced  the front and back remain level through the compression stroke, adjusting my settings to keep the bike level while jumping. what I thought was a heavy pig that washed out and was very unstable at the front wheel turned into my trusted best friend. night and day. spend time and money if necessary getting your suspension set up correctly. I spent hours doing this and am continuously tweaking my settings. I laid my bike down left and right in the mud thinking it was my poor skills or tire choices prior to this, now I stand up and throttle up and the bike goes right through the scary stuff.  

Oh yeah, I forgot to mention that "shake the bike up and down" part when measuring static sag. I guess since I'm close to the weight the bike was designed for I didn't need a new spring, and only ~2 turns of preload. The bike handles BIG compression better, but the preload also make the bike slightly harsher or something at the same time. In any case, I'm happy with how the bike works in that respect.

 

But I don't think sag or springs are the cause of my wobble, which was the purpose of this thread, since the bike is perfectly in spec and rock solid with the Trakmaster II up front. 

 

I'm ordering a Pirelli XC Mid Hard tomorrow and I'll try that out I guess. I don't know what other choice I have. :(

I have problem as you! Parker DT rear and Southwick from.

 

Have you found what is causing the nervous front end?

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